Sensor-less Vector Control

This sensor-less vector control performs the vector control of the induction motor without use of the speed sensor. Conventionally, there has been the V/f control without the speed sensor. However, this sensor-less vector control provides the simple control feature of the V/f control and the high performance of the vector control. The following describes the features of the sensor-less vector control.

(1) Sensor installation and wiring construction are not required.
(2) This control is applicable to motors, in which the sensor cannot be installed, such as two-axis motors or super high-speed motors, and other motors, which require special sensors, such as explosion-proof motors.
(3) This vector control technology is used for parallel drive of multiple motors, which is difficult to control by the conventional vector control.
(4) This sensor-less vector control provides excellent stability and large start-up torque when compared to the V/f control.
(5) The torque can be limited, ensuring stable rapid acceleration and deceleration.

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Normal and Reverse Winding Operations

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Torque Control

In winding machines, the winding materials are controlled at a specified tension. Therefore, the host PLC calculates the torque (reference) to be output from the motor. Additionally, the drive unit controls to output a torque corresponding to this torque reference. Furthermore, operation is made with speed control when the winding is completed or winding of next materials is started.

On the other hand, if operation based on the torque reference sent from the host PLC continues in case of a fault, such as material breakage, overspeed may result. In such case, the control is automatically changed to the speed control. (Torque control with speed limit function) The following describes how to use the torque control for operation with normal rotation and positive torque.

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Auto Field Weakening Control

Operation shown in Fig. (a) to make the magnetic flux constant is used for general operation method of the induction motor. Operation is performed with the magnetic flux and ID_REF made constant. At this time, the induced voltage is calculated by multiplying the speed by the magnetic flux. The voltage is then increased in proportion to the speed.

In the auto field weakening control, when operating at a higher speed, the induced voltage is controlled at a constant level based on the magnetic flux reference in inverse proportion to the speed feed back after the voltage has reached the rated voltage.
If the speed exceeds the start speed of the field weakening control, the induced voltage becomes constant and the motor output shows the constant output characteristics. (Fig. (b)).

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Braking a drive system (2) - Finish

Braking methods

Brake motors
Motors fitted with mechanical brakes stop the load quickly and efficiently and provide holding torque at standstill. Their disadvantage is that the linings wear and require replacement from time to time. Brake lining wear can be reduced by combining mechanical braking with any of the electrical braking methods listed below.

Countercurrent braking
This involves switching a motor to the opposite rotational direction. After eceleration to standstill, the motor starts in the opposite direction unless the current is disconnected at the right moment. This method creates a very high braking torque, resulting in a large amount of heat being developed in the motor. Temperature monitors should always be used to protect the windings.

DC injection braking
DC braking can be performed with or without a frequency converter. With a frequency converter, a stop command makes the frequency converter switch to supplying the motor with direct current, developing a braking torque. The same effect can also be achieved using suitable DC excitation equipment. This method gives a considerably longer braking time than countercurrent braking, but its heat losses are much lower, so more frequent braking is possible. Still, its use is confined to applications in which braking accounts for a relatively small proportion of the running time.

Flux braking
Flux braking is a method based on increasing motor losses in a controlled way. This method is available in frequency converters based on DTC (Direct Torque Control). When braking is needed, the flux in the motor is increased, which in turn increases the motor’s capability to brake. When braking is not needed, DTC brings the motor flux down to its nominal value. Unlike DC braking, the motor speed remains controlled during braking.

Brake chopper and braking resistor
The braking chopper is an electrical switch that connects the DC bus voltage to a resistor, where the braking energy is converted to heat. During deceleration, the motor changes to generator operation and supplies energy back through the inverter. As brake energy cannot be fed back to the supply via the normal diode bridge, the brake chopper will turn on at a certain level and feed energy out via the brake resistor. Here, the energy is converted to heat and wasted, unless a separate heat recovery system is installed; additional ventilation for the room may be required. The chopper works even during loss of AC supply. This method is used when the braking cycle is needed occasionally, when the amount of braking energy with respect to motoring energy is small, or when braking operation is needed during mains power loss. Other solutions may be considered when the braking is continuous or regularly repeated.

Controlled mains bridge - anti-parallel thyristor solution
The diode rectifier bridges can be replaced by two thyristor controlled rectifiers, allowing the power flow to be reversed, effectively feeding mechanical energy back to the supply network, saving energy. However, the DC bus voltage is always lower than the AC supply voltage in order to maintain a commutation margin, which may cause a drop in torque. Additionally, the cos phi varies with loading, total harmonic distortion is higher than in IGBT regenerative units (see below), and the braking capability is not available during mains power loss.

Controlled mains bridge - IGBT solution
The IGBT, or insulated gate bipolar transistor, is a type of semiconductor power switch that can replace the anti-parallel thyristor. It has a low amount of supply current harmonics in both motoring and regeneration, as well as high dynamics during fast power flow changes on the load side. It also offers the possibility to boost the DC voltage higher than the respective incoming AC supply. This can be used to compensate for a weak network or increase the motor’s maximum torque capacity in the field weakening area. The IGBT solution is useful when the braking is continuous or repeating regularly, when the braking power is very high, when space savings can be achieved compared to the braking resistor solution, when network harmonics limits are critical, or when energy savings are targeted.

Common DC bus
When a process consists of several drives where one motor may need braking capability while others are operating in motoring mode, the common DC bus solution is a very effective way to reuse the mechanical energy. A common DC bus solution uses the DC bus as the channel to move braking energy from one motor to benefit the other motors.

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Braking a drive system (1)

When is braking needed?

In many applications, being able to stop safely and precisely is as important as being able to start and accelerate quickly. Obvious examples include cranes, elevators and ski lifts, but quick and precise stopping is also necessary in machine tools, feed equipment and many other processes.

Design of the braking system

Evaluating the braking requirement goes back to the mechanical fundamentals of the process. Typically, there will be a requirement to brake the mechanical system within a specific time. There may also be sub-cycles in the process where a moving load forces the motor to operate as a generator. Any devices used for braking must be dimensioned for the braking power required. This depends on braking torque and speed; the higher the speed, the higher the power. In electrical braking systems, this power is transferred as a certain voltage and current; the higher
the voltage, the lower the current needed for the same power.

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How DTC Works (2) - Finish

Speed Control

Step 5 Torque Reference Controller

Within the Torque Reference Controller, the speed control output is limited by the torque limits and DC bus voltage. It also includes speed control for cases when an external torque signal is used. The internal torque reference from this block is fed to the Torque Comparator.

Step 6 Speed Controller

The Speed Controller block consists both of a PID controller and an acceleration compensator. The external speed reference signal is compared to the actual speed produced in the Motor Model. The error signal is then fed to both the PID controller and the acceleration compensator. The output is the sum of outputs from both of them.

Step 7 Flux Reference Controller

An absolute value of stator flux can be given from the Flux Reference Controller to the Flux Comparator block. The ability to control and modify this absolute value provides an easy way to realise many inverter functions such as Flux Optimisation and Flux Braking.

Source: www.abb.fi

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How DTC Works (1)

Figure 1, below, shows the complete block diagram for Direct Torque Control (DTC).

Walk around the block

Figure 1: DTC comprises two key blocks: Speed Control and Torque Control

The block diagram shows that DTC has two fundamental sections: the Torque Control Loop and the Speed Control Loop. Now we will walk around the blocks exploring each stage and showing how they integrate together.
Let’s start with DTC’s Torque Control Loop.

Torque Control Loop

Step 1 Voltage and current measurements

In normal operation, two motor phase currents and the DC bus voltage are simply measured, together with the inverter’s switch positions.

Step 2 Adaptive Motor Model

The measured information from the motor is fed to the Adaptive Motor Model. The sophistication of this Motor Model allows precise data about the motor to be calculated. Before operating the DTC drive, the Motor Model is fed information about the motor, which is collected during a motor identification run. This is called auto-tuning and data such as stator resistance, mutual inductance and saturation coefficients are determined along with the motor’s inertia. The identification of motor model parameters can be done without rotating the motor shaft. This makes it easy to apply DTC technology also in the retrofits. The extremely fine tuning of motor model is achieved when the identification run also includes running the motor shaft for some seconds.

There is no need to feed back any shaft speed or position with tachometers or encoders if the static speed accuracy requirement is over 0.5%, as it is for most industrial applications. This is a significant advance over all other AC drive technology. The Motor Model is, in fact, key to DTC’s unrivalled low speed performance.
The Motor Model outputs control signals which directly represent actual motor torque and actual stator flux. Also shaft speed is calculated within the Motor Model.

Step 3 Torque Comparator and Flux Comparator
The information to control power switches is produced in the Torque and Flux Comparator. Both actual torque and actual flux are fed to the comparators where they are compared, every 25 microseconds, to a torque and flux reference value. Torque and flux status signals are calculated using a two level hysteresis control method. These signals are then fed to the Optimum Pulse Selector.

Step 4 Optimum Pulse Selector

Within the Optimum Pulse Selector is the latest 40MHz digital signal processor (DSP) together with ASIC hardware to determine the switching logic of the inverter. Furthermore, all control signals are transmitted via optical links for high speed data transmission.
This configuration brings immense processing speed such that every 25 microseconds the inverter’s semiconductor switching devices are supplied with an optimum pulse for reaching, or maintaining, an accurate motor torque.
The correct switch combination is determined every control cycle. There is no predetermined switching pattern. DTC has been referred to as “just-in-time” switching, because, unlike traditional PWM drives where up to 30% of all switch changes are unnecessary, with DTC each and every switching is needed and used.
This high speed of switching is fundamental to the success of DTC. The main motor control parameters are updated 40,000 times a second. This allows extremely rapid response on the shaft and is necessary so that the Motor Model (see Step 2) can update this information.
It is this processing speed that brings the high performance figures including a static speed control accuracy, without encoder, of ±0.5% and the torque response of less than 2ms.

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DTC Questions & Answers (2)

Operation

What is the difference between DTC and traditional PWM methods?
• Frequency Control PWM and Flux Vector PWM
Traditional PWM drives use output voltage and output frequency as the primary control variables but these need to be pulse width modulated before being applied to the motor.
This modulator stage adds to the signal processing time and therefore limits the level of torque and speed response possible from the PWM drive. Typically, a PWM modulator takes 10 times longer than DTC to respond to actual change.
• DTC control
DTC allows the motor’s torque and stator flux to be used as primary control variables, both of which are obtained directly from the motor itself. Therefore, with DTC, there is no need for a separate voltage and frequency controlled PWM modulator. Another big advantage of a DTC drive is that no feedback device is needed for 95% of all drive applications.

Why does DTC not need a tachometer or position encoder to tell it precisely where the motor shaft is at all times?
There are four main reasons for this:
• The accuracy of the Motor Model.
• Controlling variables are taken directly from the motor.
• The fast processing speeds of the DSP and Optimum Pulse Selector hardware.
• No modulator is needed.
When combined to form a DTC drive, the above features produce a drive capable of calculating the ideal switching voltages 40,000 times every second. It is fast enough to control individual switching pulses. Quite simply, it is the fastest ever achieved. Once every 25 microseconds, the inverter’s semiconductors are supplied with an optimum switching pattern to produce the required torque. This update rate is substantially less than any time constants in the motor. Thus, the motor is now the limiting component, not the inverter.

What is the difference between DTC and other sensorless drives on the market?
There are vast differences between DTC and many of the sensorless drives. But the main difference is that DTC provides accurate control even at low speeds and down to zero speed without encoder feedback. At low frequencies the nominal torque step can be increased in less than 1ms. This is the best available.

How does a DTC drive achieve the performance of a servo drive?
Quite simply because the motor is now the limit of performance and not the drive itself. A typical dynamic speed accuracy for a servo drive is 0.1%s. A DTC drive can reach this dynamic accuracy with the optional speed feedback from a tachometer

How does DTC achieve these major improvements over traditional technology?
The most striking difference is the sheer speed by which DTC operates. As mentioned above, the torque response is the quickest available. To achieve a fast torque loop, ABB has utilised the latest high speed signal processing technology and spent 100 man years developing the highly advanced Motor Model which precisely simulates the actual motor parameters within the
controller.

Does a DTC drive use fuzzy logic within its control loop?
No. Fuzzy logic is used in some drives to maintain the acceleration current within current limits and therefore prevent the drive from tripping unnecessarily. As DTC is controlling the torque directly, current can be kept within these limits in all operating conditions.

A drive using DTC technology is said to be tripless. How has this been achieved?
Many manufacturers have spent years trying to avoid trips during acceleration and deceleration and have found it extraordinarily difficult. DTC achieves tripless operation by controlling the actual motor torque.

The speed and accuracy of a drive which relies on computed rather than measured control parameters can never be realistic. Unless you are looking at the shaft, you are not getting the full picture. Is this true with DTC?
DTC knows the full picture. As explained above, thanks to the sophistication of the Motor Model and the ability to carry out 40,000 calculations every second, a DTC drive knows precisely what the motor shaft is doing. There is never any doubt as to the motor’s state. This is reflected in the
exceptionally high torque response and speed accuracy. Unlike traditional AC drives, where up to 30% of all switchings are wasted, a drive using DTC technology knows precisely where the shaft is and so does not waste any of its switchings. DTC can cover 95% of all industrial applications. The exceptions, mainly applications where extremely precise speed control is needed, will be catered for by adding a feedback device to provide closed loop control. This device, however, can be simpler than the sensors needed for conventional closed loop drives.

Even with the fastest semiconductors some dead time is introduced. Therefore, how accurate is the auto-tuning of a DTC drive?
Auto-tuning is used in the initial identification run of a DTC drive. The dead time is measured and is taken into account by the Motor Model when calculating the actual flux. If we compare to a PWM drive, the problem with PWM is in the range 20-30Hz which causes torque ripple.

What kind of stability will a DTC drive have at light loads and low speeds?
The stability down to zero speed is good and both torque and speed accuracy can be maintained at very low speeds and light loads.

We have defined the accuracies as follows:
Torque accuracy: Within a speed range of 2-100% and a load range of 10-100%, the torque accuracy is 2%.
Speed accuracy: Within a speed range of 2-100% and a load range of 10-100%, the speed accuracy is 10% of the motor slip. Motor slip of a 37kW motor is about 2% which means a speed accuracy of 0.2%.

What are the limitations of DTC?
If several motors are connected in parallel in a DTC-controlled inverter, the arrangement operates as one large motor. It has no information about the status of any single motor. If the number of motors varies or the motor power remains below 1/8 of the rated power, it would be best to select the scalar control macro.

Can DTC work with any type of induction motor?
Yes, any type of asynchronous, squirrel cage motor.

Source: www.abb.fi

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DTC Questions & Answers (1)

General

What is Direct Torque Control?
Direct Torque Control - or DTC as it is called - is set to replace traditional PWM drives of the open- and closed-loop type in the near future.

Why is it called Direct Torque Control?
Direct Torque Control describes the way in which the control of torque and speed are directly based on the electromagnetic state of the motor, similar to a DC motor, but contrary to the way in which traditional PWM drives use input frequency and voltage. DTC is the first technology to control the “real” motor control variables of torque and flux.

What is the advantage of this?
Because torque and flux are motor parameters that are being directly controlled, there is no need for a modulator, as used in PWM drives, to control the frequency and voltage. This, in effect, cuts out the middle man and dramatically speeds up the response of the drive to changes in required torque. DTC also provides precise torque control without the need for a feedback device.

Why is there a need for another AC drive technology?
DTC is not just another AC drive technology. Industry is demanding more and existing drive technology cannot meet these demands.
For example, industry wants:
• Better product quality which can be partly achieved with improved speed accuracy and faster torque control.
• Less down time which means a drive that will not trip unnecessarily; a drive that is not complicated by expensive feedback devices; and a drive which is not greatly affected by interferences like harmonics and RFI.
• Fewer products. One drive capable of meeting all application needs whether AC, DC or servo. That is a truly “universal” drive.
• A comfortable working environment with a drive that produces much lower audible noise.

Who invented DTC?
ABB has been carrying out research into DTC since 1988 foll owing the publication of the theory in 1971 and 1985 by German doctor Blaschke and his colleague Depenbrock. DTC leans on the theory of field oriented control of induction machines and the theory of direct self control. ABB has spent over 100 man years developing the technology.

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Comparison of Variable Speed Drives

Let us now take a closer look at each of these control blocks and spot a few differences.

The first observation is the similarity between the control block of the DC drive (Figure 1) and that of DTC (Figure 4).
Both are using motor parameters to directly control torque.

But DTC has added benefits including no feedback device is used; all the benefits of an AC motor; and no external excitation is needed.

As can be seen from Table 1, both DC Drives and DTC drives use actual motor parameters to control torque and speed. Thus, the dynamic performance is fast and easy. Also with DTC, for most applications, no tachometer or encoder is needed to feed back a speed or position signal.
Comparing DTC (Figure 4) with the two other AC drive control blocks (Figures 2 & 3) shows up several differences, the main one being that no modulator is required with DTC.
With PWM AC drives, the controlling variables are frequency and voltage which need to go through several stages before being applied to the motor. Thus, with PWM drives control is handled inside the electronic controller and not inside the motor.

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DC & AC Drives - PWM, DTC (2)

3. AC Drives - flux vector control using PWM


Features:
• Field-oriented control - simulates DC drive
• Motor electrical characteristics are simulated - “Motor Model”
• Closed-loop drive
• Torque controlled INDIRECTLY

To emulate the magnetic operating conditions of a DC motor, i.e. to perform the field orientation process, the flux-vector drive needs to know the spatial angular position of the rotor flux inside the AC induction motor. With flux vector PWM drives, field orientation is achieved by electronic means rather than the mechanical commutator/ brush assembly of the DC motor.
Firstly, information about the rotor status is obtained by feeding back rotor speed and angular position relative to the stator field by means of a pulse encoder. A drive that uses speed encoders is referred to as a “closed-loop drive”. Also the motor’s electrical characteristics are mathematically modelled with microprocessors used to process the data. The electronic controller of a flux-vector drive creates electrical quantities such as voltage, current and frequency, which are the controlling variables, and feeds these through a modulator to the AC induction motor. Torque, therefore, is controlled INDIRECTLY.

Advantages:

• Good torque response
• Accurate speed control
• Full torque at zero speed
• Performance approaching DC drive

Flux vector control achieves full torque at zero speed, giving it a performance very close to that of a DC drive.

Drawbacks:

• Feedback is needed
• Costly
• Modulator needed

To achieve a high level of torque response and speed accuracy, a feedback device is required. This can be costly and also adds complexity to the traditional simple AC induction motor. Also, a modulator is used, which slows down communication between the incoming voltage and frequency signals and the need for the motor to respond to this changing signal. Although the motor is mechanically simple, the drive is electrically complex.

4 AC Drives - Direct Torque Control

Controlling variables:
With the revolutionary DTC technology, field orientation is achieved without feedback using advanced motor theory to calculate the motor torque directly and without using modulation. The controlling variables are motor magnetising flux and motor torque.
With DTC there is no modulator and no requirement for a tachometer or position encoder to feed back the speed or position of the motor shaft. DTC uses the fastest digital signal processing hardware available and a more advanced mathematical understanding of how a motor works.
The result is a drive with a torque response that is typically 10 times faster than any AC or DC drive. The dynamic speed accuracy of DTC drives will be 8 times better than any open loop AC drives and comparable to a DC drive that is using feedback.
DTC produces the first “universal” drive with the capability to perform like either an AC or DC drive.

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DC & AC Drives - PWM, DTC (1)

Evolution of Drives :
1. DC Motor Drives
2. AC Drives, frequency control, PWM
3. AC Drives, flux vector control, PWM
4. AC Drives, direct torque control

1. DC Motor Drives

Features:

• Field orientation via mechanical commutator
• Controlling variables are Armature Current and Field Current, measured DIRECTLY from the motor
• Torque control is direct

In a DC motor, the magnetic field is created by the current through the field winding in the stator. This field is always at right angles to the field created by the armature winding. This condition, known as field orientation, is needed to generate maximum torque. The commutator-brush assembly ensures this condition is maintained regardless of the rotor position. Once field orientation is achieved, the DC motor’s torque is easily controlled by varying the armature current and by keeping the magnetising current constant.
The advantage of DC drives is that speed and torque - the two main concerns of the end-user - are controlled directly through armature current: that is the torque is the inner control loop and the speed is the outer control loop (see Figure 1).

Advantages:

• Accurate and fast torque control
• High dynamic speed response
• Simple to control

Initially, DC drives were used for variable speed control because they could easily achieve a good torque and speed response with high accuracy.

A DC machine is able to produce a torque that is:

• Direct - the motor torque is proportional to the armature current: the torque can thus be controlled directly and accurately.
• Rapid - torque control is fast; the drive system can have a very high dynamic speed response. Torque can be changed instantaneously if the motor is fed from an ideal current source. A voltage fed drive still has a fast response, since this is determined only by the rotor’s electrical time constant (i.e. the total inductance and resistance in the armature circuit)
• Simple - field orientation is achieved using a simple mechanical device called a commutator/brush assembly. Hence, there is no need for complex electronic control circuitry, which would increase the cost of the motor controller.

Drawbacks:
• Reduced motor reliability
• Regular maintenance
• Motor costly to purchase
• Needs encoder for feedback

The main drawback of this technique is the reduced reliability of the DC motor; the fact that brushes and commutators wear down and need regular servicing; that DC motors can be costly to purchase; and that they require encoders for speed and position feedback.
While a DC drive produces an easily controlled torque from zero to base speed and beyond, the motor’s mechanics are more complex and require regular maintenance.

AC Drives Introduction

• Small size
• Robust
• Simple in design
• Light and compact
• Low maintenance
• Low cost

The evolution of AC variable speed drive technology has been partly driven by the desire to emulate the performance of the DC drive, such as fast torque response and speed accuracy, while utilising the advantages offered by the standard AC motor.

2. AC Drives - frequency control using PWM

Features:

• Controlling variables are Voltage and Frequency
• Simulation of variable AC sine wave using modulator
• Flux provided with constant V/f ratio
• Open-loop drive
• Load dictates torque level

Unlike a DC drive, the AC drive frequency control technique uses parameters generated outside of the motor as controlling variables, namely voltage and frequency. Both voltage and frequency reference are fed into a modulator which simulates an AC sine wave and feeds this to the motor’s
stator windings. This technique is called Pulse Width Modulation (PWM) and utilises the fact that there is a diode rectifier towards the mains and the intermediate DC voltage is kept constant. The inverter controls the motor in the form of a PWM pulse train dictating both the voltage and frequency. Significantly, this method does not use a feedback device which takes speed or position measurements from the motor’s shaft and feeds these back into the control loop. Such an arrangement, without a feedback device, is called an “open-loop drive”.

Advantages:

• Low cost
• No feedback device required - simple

Because there is no feedback device, the controlling principle offers a low cost and simple solution to controlling economical AC induction motors.
This type of drive is suitable for applications which do not require high levels of accuracy or precision, such as pumps and fans.

Drawbacks:
• Field orientation not used
• Motor status ignored
• Torque is not controlled
• Delaying modulator used

With this technique, sometimes known as Scalar Control, field orientation of the motor is not used. Instead, frequency and voltage are the main control variables and are applied to the stator windings. The status of the rotor is ignored, meaning that no speed or position signal is fed back.
Therefore, torque cannot be controlled with any degree of accuracy. Furthermore, the technique uses a modulator which basically slows down communication between the incoming voltage and frequency signals and the need for the motor to respond to this changing signal.

To be continue..........

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Veritron BBC (ABB)

Anyone still use Veritron DC Drive? This is old drive from BBC (now ABB).



For online, we need PAESET2 software or PAESIM2 (offline).

For programming EEPROM, use SYMT1 software. There are some procedures, should be followed for EEPROM programming.

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Quadrants

The word 'quadrant' comes up, as in 2 quadrant controller or 4 quadrant controller. The word 'quadrant' comes from a graph. The graph below shows the blank on which you might start to draw motor voltage and motor current.

At the top right, both motor voltage and current are positive: the motor is driving in the forward direction. To brake the motor, the current must be reversed but the voltage remains positive as the motor is still rotating in the same direction. so forward braking is the top left quadrant.
The bottom two quadrants are for reversing.
So a '1 quadrant' drive gives forward drive but no regen braking and no reversing. A 2 quadrant gives forward drive and braking bit np reversing - yes one which gave drive but no braking , with reversing, would also be 2 quadrant, but this is not usually used.
A 4 quadrant drive gives reversing with regenerative braking, thus operating in all 4 quadrants.

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How to Maintain a Variable Frequency Drive?

Do you know how to maintain Variable Frequency Drives (VFDs)? Doing so is easier than you might think. By integrating some simple, logical steps into your preventative maintenance program, you can ensure your drives provide many years of trouble-free service. Before looking at those steps, let's quickly review what a VFD is and how it works.

A Quick Overview

A VFD controls the speed, torque and direction of an AC Induction motor. It takes fixed voltage and frequency AC input and converts it to a variable voltage and frequency AC output. In very small VFDs, a single power pack unit may contain the converter and inverter.
Fairly involved control circuitry coordinates the switching of power devices, typically through a control board that dictates the firing of power components in the proper sequence. A microprocessor or Digital Signal Processor (DSP) meets all the internal logic and decision requirements.
From this description, you can see a VFD is basically a computer and power supply. And the same safety and equipment precautions you'd apply to a computer and to a power supply apply here. VFD maintenance requirements fall into three basic categories:

• keep it clean;
• keep it dry; and
• keep the connections tight.

Let's look at each of these.

Keep it Clean
Most VFDs fall into the NEMA 1 category (side vents for cooling airflow) or NEMA 12 category (sealed, dust-tight enclosure). Drives that fall in the NEMA 1 category are susceptible to dust contamination. Dust on VFD hardware can cause a lack of airflow, resulting in diminished performance from heat sinks and circulating fans (Photo 1).

Dust on an electronic device can cause malfunction or even failure. Dust absorbs moisture, which also contributes to failure. Periodically spraying air through the heat sink fan is a good PM measure. Discharging compressed air into a VFD is a viable option in some environments, but typical plant air contains oil and water. To use compressed air for cooling, you must use air that is oil-free and dry or you are likely to do more harm than good. That requires a specialized, dedicated, and expensive air supply. And you still run the risk of generating electrostatic charges (ESD).
A non-static generating spray or a reverse-operated ESD vacuum will reduce static build-up. Common plastics are prime generators of static electricity. The material in ESD vacuum cases and fans is a special, non-static generating plastic. These vacuums, and cans of non-static generating compressed air, are available through companies that specialize in static control equipment.

Keep it Dry
In Photo 2 you can see what happened to a control board periodically subjected to a moist environment. Initially, this VFD was wall-mounted in a clean, dry area of a mechanical room and moisture was not a problem. However, as is often the case, a well-meaning modification led to problems.
In this example, an area of the building required a dehumidifier close to the mechanical room. Since wall space was available above the VFD, this is where the dehumidifier went. Unfortunately, the VFD was a NEMA 1 enclosure style (side vents and no seal around the cover). The obvious result was water dripping from the dehumidifier into the drive. In six months, the VFD accumulated enough water to produce circuit board corrosion.

What about condensation? Some VFD manufacturers included a type of "condensation protection" on earlier product versions. When the mercury dipped below 32 degrees Fahrenheit, the software logic would not allow the drive to start. VFDs seldom offer this protection today. If you operate the VFD all day every day, the normal radiant heat from the heatsink should prevent condensation. Unless the unit is in continuous operation, use a NEMA 12 enclosure and thermostatically controlled space heater if you locate it where condensation is likely.

Keep Connections Tight
While this sounds basic, checking connections is a step many people miss or do incorrectly - and the requirement applies even in clean rooms. Heat cycles and mechanical vibration can lead to sub-standard connections, as can standard PM practices. Retorquing screws is not a good idea, and further tightening an already tight connection can ruin the connection (see Sidebar).
Bad connections eventually lead to arcing. Arcing at the VFD input could result in nuisance over voltage faults, clearing of input fuses, or damage to protective components. Arcing at the VFD output could result in over-current faults, or even damage to the power components. Photos 3 and 4 show what can happen.
Loose control wiring connections can cause erratic operation. For example, a loose START/STOP signal wire can cause uncontrollable VFD stops. A loose speed reference wire can cause the drive speed to fluctuate, resulting in scrap, machine damage, or personnel injury.

Re-torquing - A Screwy PracticeAlthough "re-torquing" as a way of checking tightness is common in many PM procedures, it violates basic mechanical principles and does more harm than good. A screw has maximum clamping power at a torque value specific to its size, shape, and composition. Exceeding that torque value permanently reduces the clamping power of that screw by reducing its elasticity and deforming it. Loosening and then re-torquing still reduces elasticity, which still means a loss of clamping power. Doing this to a lock washer results in a permanent 50% loss. What should you do? Use an infrared thermometer to note hot connections. Check their torque. If they have merely worked loose, you can try retightening them. Note which screws were loose, and be sure to give them an IR check at the next PM cycle. If they are loose again, replace them. Finally, don't forget the "tug test." This checks crimps, as well as screw connections. Don't do this with the drive online with the process, though, or you may cause some very expensive process disturbances.

Additional Steps

1. As part of a mechanical inspection procedure, don't overlook internal VFD components. Check circulating fans for signs of bearing failure or foreign objects - usually indicated by unusual noise or shafts that appear wobbly.


2. Inspect DC bus capacitors for bulging and leakage. Either could be a sign of component stress or electrical misuse. Photos 5 and 6 show fan and capacitor stress problems.


3. Take voltage measurements while the VFD is in operation. Fluctuations in DC bus voltage measurements can indicate degradation of DC bus capacitors. One function of the capacitor bank is to act as a filter section (smoothing out any AC ripple voltage on the Bus). Abnormal AC voltage on the DC bus indicates the capacitors are headed for trouble.
   Most VFD manufacturers have a special terminal block for this type of measurement and also for connection of the dynamic braking resistors. Measurements more than 4VAC may indicate a capacitor filtering problem or a possible problem with the diode bridge converter section (ahead of the bus). If you have such voltage levels, consult the VFD manufacturer before taking further action.
   With the VFD in START and at zero speed, you should read output voltage of 40VAC phase-to-phase or less. If you read more than this, you may have transistor leakage. At zero speed, the power components should not be operating. If your readings are 60VAC or more, you can expect power component failure.


4. What about spare VFDs? Store them in a clean, dry environment, with no condensation allowed. Place this unit in your PM system so you know to power it up every 6 months to keep the DC bus capacitors at their peak performance capability. Otherwise, their charging ability will significantly diminish. A capacitor is much like a battery-it needs to go into service soon after purchase or suffer a loss of usable life.


5. Regularly monitor heat sink temperatures. Most VFD manufacturers make this task easy by including a direct temperature readout on the Keypad or display. Verify where this readout is, and make checking it part of a weekly or monthly review of VFD operation. You wouldn't place your laptop computer outside, on the roof of a building or in direct sunlight, where temperatures could reach 115 degrees Fahrenheit or as low as -10 degrees Fahrenheit. A VFD, which is basically a computer with a power supply, needs the same consideration. Some VFD manufacturers advertise 200,000 hours-almost 23 years-of Mean Time Between Failures (MTBF). Such impressive performance is easy to obtain, if you follow these simple procedures.

By: Dave Polka

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DC Drive Fundamentals (3) - Finish

TYPICAL ADJUSTMENTS

In addition to the normal external adjustment such as the speed potentiometer. there are a number of common internal adjustments that are used on simple small analog type SCR Drives. Some of these adjustments are as follows:

• Minimum Speed
• Maximum Speed
• Current Limit (Torque Limit) . IR Compensation
• Acceleration Time . Deceleration Time

The following is a description of the function that these individual adjustments serve and their typical use.

MINIMUM SPEED

In most cases when the control is initially installed the speed potentiometer can be turned down to its lowest point and the output voltage from the control will go to zero causing the motor to stop. There are many situations where this is not desirable. For example there are some machines that want to be kept running at a minimum speed and accelerated up to operating speed as necessary. There is also a possibility that an operator may use the speed potentiometer to stop the motor to work on the machine. This can be a dangerous situation since the motor has only been brought to a stop by zeroing the input signal voltage. A more desirable situation is when the motor is stopped by opening the circuit to the motor or power to the control using the on/off switch. By adjusting the minimum speed up to some point where the motor continues to run even with the speed potentiometer set to its lowest point, the operator must shut the control off to stop the motor. This adds a little safety into the system. The typical minimum speed adjustment is from 0 to 30% of motor base speed.

MAXIMUM SPEED

The maximum speed adjustment sets the maximum speed attainable either by raising the input signal to its maximum point or turning the potentiometer to the maximum point. For example on a typical DC motor the rated speed of the motor might 1750 RPM but the control might be capable of running it up to 1850 or 1900 RPM. In some cases it's desirable to limit the motor (and machine speed) to something less than would be available at this maximum setting. The maximum adjustment allows this to be done. By turning the internal potentiometer to a lower point the maximum output voltage from the control is limited. This limits the maximum speed available from the motor. In typical controls such as our BC140 the range of adjustment on the maximum speed is from 50 to 110% of motor base speed.

CURRENT LIMIT

One very nice feature of electronic speed controls is that the current going to the motor is constantly monitored by the control. As mentioned previously, the current drawn by the armature of the DC motor is related to the torque that is required by the load. Since this monitoring and control is available an adjustment is provided in the control that limits the output current to a maximum value.
This function can be used to set a threshold point that will cause the motor to stall rather than putting out an excessive amount of torque. This capability gives the motor/control combination the ability to prevent damage that might otherwise occur if higher values of torque were available. This is handy on machines that might become jammed or otherwise stalled. It can also be used where the control is operating a device such as the center winder where the important thing becomes torque rather than the speed. In this case the current limit is set and the speed goes up or down to hold the tension 0 the material being wound. The current limit is normally factory set at 150% of the motor's rated current. This allows the motor to produce enough torque to start and accelerate the load and yet will not let the current (and torque) exceed 150% of its rated value when running. The range of adjustment is typically from 0 to 200% of the motor rated current.

IR COMPENSATION

IR compensation is a method used to adjust for the droop in a motor's speed due to armature resistance. As mentioned previously, IR compensation is positive feedback that causes the control output voltage to rise slightly with increasing output current. This will help stabilize the motor's speed from a no load to full load condition. If the motor happens to be driving a load where the torque is constant or nearly so, then this adjustment is usually unnecessary. However, if the motor is driving a load with a widely fluctuating torque requirement, and speed regulation is critical, then IR compensation can be adjusted to stabilize the speed from the light load to full load condition. One caution is that when IR compensation is adjusted too high it results in an increasing speed characteristic. This means that as the load is applied the motor is actually going to be forced to run faster. When this happens it increases the voltage and current to the motor which in turn increases the motor speed further. If this adjustment is set too high an unstable "hunting" or oscillating condition occurs that is undesirable.
ACCELERATION TIMEThe Acceleration Time adjustment performs the function that is indicated by its name. It will extend or shorten the amount of time for the motor to go from zero speed up to the set speed. It also regulates the time it takes to change speeds from one setting (say 50%) to another setting (perhaps 100%). So this setting has the ability to moderate the acceleration rate on the drive.
A couple notes are important: if an acceleration time that is too rapid is called for "acceleration time" will be overridden by the current limit. Acceleration will only occur at a rate that is allowed by the amount of current the control passes through to the motor. Also important to note is that on most small controls the acceleration time is not linear. What this means is that a change of 50 RPM may occur more rapidly when the motor is at low speed than it does when the motor is approaching the set point speed. This is important to know but usually not critical on simple applications where these drives are used.

DECELERATION TIME

This is an adjustment that allows loads to be slowed over an extended period of time. For example, if power is removed from the motor and the load stops in 3 seconds, then the decel time adjustment would allow you'to increase that time and "power down" the load over a period of 4, 5, 6 or more seconds. Note: On a conventional simple DC drive it will not allow for the shortening of the time below the "coast to rest" time.

ADJUSTMENT SUMMARY

The ability to adjust these six adjustments gives great flexibility to the typical inexpensive DC drive. In most cases the factory preset settings are adequate and need not be changed, but on other applications it may be desirable to tailor the characteristics of the control to the specific application. Many of these adjustments are available in other types of controls, such as variable frequency drives.

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DC Drive Fundamentals (2)

REGULATORS (CONTROLS)

The control provides two basic functions:

1. It rectifies AC power converting it to DC for the DC motor.


2. It controls the DC output voltage and amperage in response to various control and feedback signals thereby regulating the motor's performance, both in speed and torque.

RECTIFYING FUNCTION

The basic rectifying function of the control is accomplished by a combination of power semiconductors (Silicon Controlled Rectifiers and Diodes) that make up the "power bridge" assembly.

REGULATING FUNCTION

The regulating function is provided by a relatively simple electronic circuit that monitors a number of inputs and sums these signals to produce a so called "error" signal. This error signal is processed and transformed into precisely timed pulses (bursts of electrical energy). These pulses are applied to the gates of the SCR's in the power bridge thereby regulating the power output to the DC motor.
For most purposes it is not necessary to understand the electronic details of the regulator, however, in order to appreciate the regulator function it is good to understand some of the input signals that are required to give the regulator its capabilities, these are shown diagrammatically in Figure 4.
The AC to DC power flow is a relatively simple straight through process with the power being converted from AC to DC by the action of the solid state power devices that form the power bridge assembly.
The input and feedback signals need to be studied in more detail.

SET POINT INPUT

In most packaged drives this signal is derived from a closely regulated fixed voltage source applied to a potentiometer. 10 volts is a very common reference.

The potentiometer has the capability of accepting the fixed voltage and dividing it down to any value of from, for example, 10 to zero volts, depending on where it is set. A 10 volt input to the regulator from the speed adjustment control (potentiometer) corresponds to maximum motor speed and zero volts corresponds to zero speed. Similarly any speed between zero and maximum can be obtained by adjusting the speed control to the appropriate setting.

SPEED FEEDBACK INFORMATION

In order to "close the loop" and control motor speed accurately it is necessary to provide the control with a feedback signal related to motor speed.
The standard method of doing this in a simple control is by monitoring the armature voltage and feeding it back into the regulator for comparison with the input "set point" signal.
When armature voltage becomes high, relative to the set point, established by the speed potentiometer setting, an "error" is detected and the output voltage from the power bridge is reduced to lower the motor's speed back to the "set point". Similarly when the armature voltage drops an error of opposite polarity is sensed and the control output voltage is automatically increased in an attempt to re-establish the desired speed.
The "Armature Voltage Feedback System" which is standard in most packaged drives is generally called a "Voltage Regulated Drive".
A second and more accurate method of obtaining the motor speed feedback information is called "Tachometer Feedback". In this case the speed feedback signal is obtained from a motor mounted tachometer. The output of this tachometer is directly related to the speed of the motor. Using Tachometer Feedback generally gives a drive improved regulation characteristics. When "tach feedback" is used the drive is referred to as a "Speed Regulated Drive". Most controls are capable of being modified to accept tachometer signals for operation in the tachometer feedback mode.
In some newer high performance "digital drives" the feedback can come from a motor mounted encoder that feeds back voltage pulses at a rate related to motor speed. These (counts) are processed digitally being compared to the "set point" and error signals are produced to regulate the armature voltage and speed.

CURRENT FEEDBACK

The second source of feedback information is obtained by monitoring the motor armature current. As discussed previously, this is an accurate indication of the torque required by the load.

The current feedback signal is used for two purposes:

1. As positive feedback to eliminate the speed droop that occurs with increased torque load on the motor. It accomplishes this by making a slight corrective increase in armature voltage as the armature current increases.
2. As negative feedback with a "threshold" type of control that limits the current to a value that will protect the power semiconductors from damage. By making this function adjustable it can be used to control the maximum torque the motor can deliver to the load.

The current limiting action of most controls is adjustable and is usually called "Current Limit" or "Torque Limit".

In summary, the Regulator accomplishes two basic functions:

1. It converts the alternating Current to Direct Current.
2. It regulates the armature voltage and current to control the speed and torque of the DC Motor.

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DC Drive Fundamentals (1)

UNDERSTANDING DC DRIVES

DC motors have been available for nearly 100 years. In fact the first electric motors were designed and built for operation from direct current power.

AC motors are Rowand will of course remain the basic prime movers for the fixed speed requirements of industry. Their basic simplicity, dependability and ruggedness make AC motors the natural choice for the vast majority of industrial drive applications.
Then where do DC drives fit into the industrial drive picture of the future?
In order to supply the answer, it is necessary to examine some of the basic characteristics obtainable from DC motors and their associated solid state controls.

1. Wide speed range.
2. Good speed regulation.
3. Compact size and light weight (relative to mechanical variable speed).
4. Ease of control.
5. Low maintenance.
6. Low cost.

In order to realize how a DC drive has the capability to provide the above characteristics, the DC drive has to be analyzed as two elements that make up the package. These two elements are of course the motor and the control. (The "control" is more accurately called the "regulator").

DC MOTORS

Basic DC motors as used on nearly all packaged drives have a very simple performance characteristic the shaft turns at a speed almost directly proportional to the voltage applied to the armature. Figure 1 shows a typical voltage/speed curve for a motor operating from a 115 volt control.

From the above curve you can see that with 9 volts applied to the armature, this motor would be operating at Point 1 and turn at approximately 175 RPM. Similarly with 45 volts applied, the motor would be operating at Point 2 on the curve or 875 RPM. With 90 volts applied, the motor would reach its full speed of 1750 RPM at point 3.
From this example a general statement can be made that DC motors have "no load" characteristics that are nearly a perfect match for the curve indicated in Figure 1.
However, when operated at a fixed applied voltage but a gradually increasing torque load, they exhibit a speed droop as indicated in Figure 2.

This speed droop is very similar to what would occur if an automobile accelerator pedal was held in a fixed position with the car running on level ground. Upon starting up an incline where more driving torque would be needed, the car would slow down to a speed related to the steepness of the hill. In a real situation, the driver would respond by depressing the accelerator pedal to compensate for the speed loss to maintain a nearly constant speed up the incline.

In the DC drive a similar type of "compensation" is employed in the control to assist in maintaining a nearly constant speed under varying load (torque) conditions.
The measurement of this tendency to slow down is called Regulation and is calculated with the following equation:

%Regulation = [(No Load Speed - Full Load Speed)/No Load Speed] x 100

In DC drives the regulation is generally expressed as a percentage of motor base speed.
If the control (regulator) did not have the capability of responding to and compensating for changing motor loads, regulation of typical motors might be as follows:

One other very important characteristic of a DC motor should be noted. Armature amperage is almost directly proportional to output torque regardless of speed. This characteristic is shown by Figure 3. Point 1 indicates that a small fixed amount of current is required to turn the motor even when there is no output torque. This is due to the friction of the bearings, electrical losses in the motor materials and load imposed by the air in the motor (windage).


Beyond Point 1 through Point 2 and 3, the current increases in direct proportion to the torque required by the load.

From this discussion and Figure 3 a general statement can be made that for PM and Shunt Wound motors load torque determines armature amperage.

In summary, two general statements can be made relative to DC motor performance.
1. Motor Speed is primarily determined by Applied Armature Voltage.
2. Motor Torque is controlled by Armature Current (amperes).

Understanding these two concepts of DC motors provides the key to understanding total drive performance.

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DC or AC Drives ?

We often have think about this, which drives should be used ?

DC or AC Drives ?

Which one cheaper ?

What is advantage/disadvantage between them ?

Which one easier for maintenance, troubleshooting ?

ABB has very good document to compare between DC and AC Drive.

Free for you. Download here:

http://kewlshare.com/dl/8b2aba8f8bcd/DC_or_AC_Drives.pdf.html

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