Application examples of AC servo

In addition to the characteristics explained in the previous section, using a servo motor with a servo amplifier realizes the positioning control which is not provided by other speed variators.

The following shows application examples of a servo motor, based on the servo-specific positioning

control and the characteristics.

1) Machinery that needs positioning control When used with a controller that is exclusively used for positioning control, AC servo system becomes capable of providing highly accurate positioning.
For example, in general Mitsubishi AC servo system, the motor shaft can be divided into 4000 to 262144 for positioning purposes. This is enough to provide positioning control of 1 m, if the motor is driving a machine of 24 to 8m/min.

2) Machinery that needs a wide speed control range AC servo system can be used for applications that need highly accurate speed-variable control, such as production line control. This is because the AC servo's speed control range is 1:1000 to 5000, and its speed fluctuation ratio is 0.01% or less, and also because AC servo motors can rotate with constant output torque, which is a feature not provided by other speed-variable motors.

3) Frequent positioning operation AC servo system can provide up to 300% of the rated torque, allowing rapid acceleration and deceleration. More precisely, with a motor alone, AC servo system takes just several 10ms to accelerate the speed from 0 to the rated speed. Also, it can respond to more than 100 times of positioning commands in one minute. For positioning control, refer to (1). Having no mechanical contacts is another advantage of AC servo system compared to other positioning systems (e.g. clutches, brakes, DC motors). With no mechanical contacts, the maintenance is not required and the operation is not affected by ambient temperature as much as the other positioning systems.

4) Torque control There are models that feature torque control in addition to speed and position controls. Such models can be used for tension control purposes (e.g. winding and unwinding machines).

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What is AC servo system?

A servo mechanism is a system where first a controller gives a unit a target value (e.g. value that represents position or speed), secondly the unit that received the target value detects the current value (e.g. value that represents position or speed), and then these two values are compared to make their difference as small as possible. JIS defines a servo mechanism as "A control system where the position and direction of objects are treated as controlled variables that follow changes in the target."

A servo mechanism consists of three elements, called servo elements, which are drive amplifiers (AC servo amplifiers), drive motors (AC servo motors) and encoders.

Features and functions of AC servo system
One of the features of servo motors is that they provide higher performance than general motors in inertia moment (referred to as J or GD2) of the rotator and electrical response level. This is because servo motors must respond to changes in voltage and current, which are applied by a servo amplifier, more quickly and accurately than general motors. For the same reason, servo amplifiers, which are used to drive servo motors, are designed so that they can give servo motors speed/position commands quickly and accurately.
Focusing on the above, the following explains main characteristics provided by servo motors when used with a servo amplifier. This explanation is made in comparison with a motor that is driven by a general-purpose inverter, which is a commonly used speed variator.

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Exploring Current Transformer Applications (Part 2)

When choosing the burden resistor, the engineer can create any output voltage per amp, as long as it doesn't saturate the core. Core saturation level is an important consideration when specifying current transformers. The maximum volt-microsecond product specifies what the core can handle without saturating. The burden resistor is one of the factors controlling the output voltage. There's a limit to the amount of voltage that can be achieved at a given frequency. Since frequency = 1/cycle period, if the frequency is too low (cycle period too long) so that voltage-time product exceeds the core's flux capacity, saturation will occur. The flux that exists in a core is proportional to the voltage times cycle period. Most specifications provide a maximum volt-microsecond product that the current transformer can provide across the burden resistor. Exceeding this voltage with too large a burden resistor will saturate the transformer and limit the voltage.

What happens if the burden resistor is left off or opens during operation? The output voltage will rise trying to develop current until it reaches the saturation voltage of the coil at that frequency. At that point, the voltage will cease to rise and the transformer will add no additional impedance to the driving current. Therefore, without a burden resistor, the output voltage of a current transformer will be its saturation voltage at the operating frequency.

There are factors in the current transformer that affect efficiency. For complete accuracy, the output current must be the input current divided by the turns ratio. Unfortunately, not all the current is transferred. Some of the current isn't transformed to the secondary, but is instead shunted by the inductance of the transformer and the core loss resistance. Generally, it's the inductance of the transformer that contributes the majority of the current shunting that detracts from the output current. This is why it's important to use a high-permeability core to achieve the maximum inductance and minimize the inductance current. Accurate turns ratio must be maintained to produce the expected secondary current and the expected accuracy. Fig. 2 shows the current transformed is smaller than the input current by:

ITRANSFORMED=IINPUT-ICORE-jIMAG (1)

What about the effect the transformer will have on the current it's monitoring? This is where the term burden enters the picture. Any measuring device alters the circuit in which it measures. For instance, connecting a voltmeter to a circuit causes the voltage to change from what it was before the meter was attached. However minuscule this effect may or may not be, the voltage you read isn't the voltage that existed before attaching the meter. This is also true with a current transformer. The burden resistor on the secondary is reflected to the primary by (1/N2), which provides a resistance in series with the current on the primary. This usually has minimal effect and is usually only important when you are concerned about the current that would exist when the transformer isn't in the circuit, such as when it's used as a temporary measuring device.

Notice the four loss components in the circuit of Fig. 2. The resistance of the primary loop (PRIDCR), the core loss resistance (RCORE), the secondary DCR (RDCR) is reduced by 1/N2, and the secondary burden resistor RBURDEN is also reduced by a factor of N2. These are losses that affect current source (I). The resistances have an indirect effect on the current transformer accuracy. It's their effect on the circuit that they are monitoring that alters its current. The primary dc resistance (PRIdcr) and the secondary DCR/N2 (RDCR/N2) don't detract from the Iinput that is read or is affecting the accuracy of the actual current reading. Rather, they alter the current from what it would be if the current transformer weren't in the circuit. With the exception of the burden resistor, these loss resistors are the components that contribute to the loss in the transformer and heating.

This wasted energy is usually small compared with the power in the circuit it's monitoring. Usually, the design of the transformer and choice of the burden resistor will be within the maximum energy loss the end user can allow. As battery-operated devices come into wider use and power consumption contributes to the energy crisis — even this power may be of concern. Under these circumstances, it may require special design attention to power consumption.

Current transformers are an efficient way to measure current. Since the burden resistor is reflected to the primary by 1/N2, the resistance seen in the circuit being monitored can be very small. This allows a larger voltage to be created on the output with minimal effect on the circuit being measured. A simpler and lower-cost method to measure current is to use a sense resistor connected in series with the current. However, this method can only be used when power consumption is of secondary concern. With the more frequent use of battery-powered devices and the prevailing need to reduce power consumption, the extra expense of a current transformer can soon be recovered with use. Also, with high current or when a voltage of any magnitude is required, a sense resistor would be impractical.

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Exploring Current Transformer Applications (Part 1)

For a variety of applications, using current transformers is an efficient way to sense current with minimum insertion loss.

Current transformers can perform circuit control, measure current for power measurement and control, and perform roles for safety protection and current limiting. They can also cause circuit events to occur when the monitored current reaches a specified level. Current monitoring is necessary at frequencies from the 50 Hz/60 Hz power line to the higher frequencies of switchmode transformers that range into the hundreds of kilohertz.

The object with current transformers is to think in terms of current transformation rather than voltage ratios. Current ratios are the inverse of voltage ratios. The thing to remember about transformers is that Pout = (Pin — transformer power losses). With this in mind, let's assume we had an ideal loss-less transformer in which Pout = Pin. Since power is voltage times current, this product must be the same on the output as it is on the input. This implies that a 1:10 step-up transformer with the voltage stepped up by a factor of 10 results in an output current reduced by a factor of 10. This is what happens on a current transformer. If a transformer had a one-turn primary and a ten-turn secondary, each amp in the primary results in 0.1A in the secondary, or a 10:1 current ratio. It's exactly the inverse of the voltage ratio — preserving volt times current product.

How can we use this transformer and knowledge to produce something useful? Normally, an engineer wants to produce an output on the secondary proportional to the primary current. Quite often, this output is in volts output per amp of primary current. The device that monitors this output voltage can be calibrated to produce the desired results when the voltage reaches a specified level.

A burden resistor connected across the secondary produces an output voltage proportional to the resistor value, based on the amount of current flowing through it. With our 1:10 turns ratio transformer that produces a 10:1 current ratio, a burden resistor can be selected to produce the voltage we want. If 1A on the primary produces 0.1A on the secondary, then by Ohm's law, 0.1 times the burden resistor will result in an output voltage per amp.

Many voltage transformers have adjusted ratios that produce the desired output voltage and compensate for losses. The turns-ratios or actual turns aren't the primary concern of the end-user. Only the voltage output and possibly regulation and other loss parameters may be of concern. With current transformers, the user must know the current ratio to use the transformer. The knowledge of amps in per amps out is the basis for use of the current transformer. Quite often, the end users provide the primary with a wire through the center of the transformer. They must know what secondary turns are to determine what their output current will be. Generally, in catalogues, the turns of the transformers are provided as a specification for use.

With this knowledge, the user can choose the burden resistor to produce their desired output voltage. The output current of 0.1A for a 1A primary on the 1:10 turns ratio transformer will produce 0.1 V/A across a 1Ω burden resistor, 1V per amp across a 10Ω burden and 10V per amp across a 100Ω burden resistor.

Fig. 1 shows an ideal transformation ratio. In this analysis, the secondary dc resistance (RDCR) doesn't become part of the calculation. When considering the secondary current, only the actual current affects V. How well that current can be determined controls the accuracy of the prediction of V. The secondary dc resistance is best analyzed by reflecting it to the primary by RDCR/N2.

 

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Current Tranformers (Part-3 Finish)

Note:

General

CTs should be specified as follows:

RATIO: input / output current ratio

VA: total burden including pilot wires.

CLASS: Accuracy required for operation

DIMENSIONS: maximum & minimum limits

Metering CTs In general, the following applies:
CLASS • 0.1 or 0.2 for precision measurements
• 0.5 for high grade kilowatt hour meters for commercial grade kilowatt hour meters
• 3 for general industrial measurements
• 3 or 5 for approximate measurements
BURDEN (depending on pilot lead length) • Moving iron ammeter 1-2VA
• Moving coil rectifier ammeter 1-2.5VA
• Electrodynamic instrument 2.5-5VA
• Maximum demand ammeter 3-6VA
• Recording ammeter or transducer 1-2.5VA

Protection CTs In addition to the general specification required for CT design, protection CT’s require an Accuracy Limit Factor (ALF). This is the multiple of rated current up to which the CT will operate while complying with the accuracy class requirements.
In general the following applies:
• Instantaneous overcurrent relays & trip coils - 2.5VA Class 10P5
• Thermal inverse time relays - 7.5VA Class 10P10
• Low consumption Relay - 2.5VA Class 10P10
• Inverse definite min. time relays (IDMT) overcurrent - 15VA Class 10P10/15
• IDMT Earth fault relays with approximate time grading - 15VA Class 10P10
• IDMT Earth fault relays with phase fault stability or accurate time grading required - 15VA Class 5P10

Class X CTs Class X CTs are special CTs used mainly in balanced protection systems (including restricted earth fault) where the system is sensitively dependent on CT accuracy. Further to the general CT specifications, the manufacturer needs to know:
• Vkp - Voltage knee point
• Io - Maximum magnetising current at Vkp
• Rs - Maximum resistance of the secondary winding

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