Load Characteristics

Introduction

Motor loads are classified into two main groups depending on how their torque requirement varies with operating speed. The following paragraphs deal with the various load types found in industry.

Constant Torque LoadThe torque demanded by the load is constant throughout the speed range. Loads of these types are essentially friction loads. Figure 2.1 shows the constant torque and it’s effect on horsepower demanded by the load.

Since HP is a product of Torque times speed, and torque remains constant in this type of load, horsepower is a function of speed.

Examples of this type of load are conveyors and extruders. Constant torque is also used when shock loads, overloads or high inertia loads are encountered.

Variable Torque Load

With this type of load, the torque demand increases with speed, usually speed squared (Speed^2).

Torque Constant x (Speed)^2

Horsepower is typically proportional to speed cubed (Speed^3).

Figure 2.2 shows the variable torque and it’s effect on horsepower demanded by the load.

 

Examples of loads that exhibit variable load torque characteristics are centrifugal fans, pumps and blowers. This type of load requires much lower torque at low speeds than at high speeds.

Constant Horsepower Operation

This is a function of the motor being operated above base motor speed. The horsepower demanded by the load is constant within the speed range. The speed and torque are inversely proportional to each other. Figure 2.3 shows the constant horsepower and variable torque demanded by the load.

Examples of this type of load are center-driven winders and machine tool spindles. Read more...

AC Drives and Soft Starter (8) - Finish

Sample Applications

Provided here are four sample applications. Two will be for pumps, and two will be for conveyors. These examples do not require variable speed or precise speed regulation, so a VFD or soft starter could be used.

Application 1) A pump is being started on full voltage. There is significant water hammer and the pipe bracing needs constant maintenance.
Answer: A soft starter will fit the application. It provides controlled torque during acceleration and has been shown to minimize and in many cases eliminate water hammer. There is no concern about current limitations as the application is now being started on full voltage.

Application 2) A new irrigation pump is being installed in a rural location. Because of this, the maximum current draw from the utility line without significant voltage drop has been calculated as 200 percent of the motor nameplate reading.
Answer: An inverter is preferred over a soft starter. In some instances soft starters can accelerate pumps with as little as 200 percent current. Application experience indicates that more often 250 – 300 percent current is required. The VFD can provide the torque required to accelerate the pump within the current limit restrictions of the distribution system.

Application 3) An overland conveyor requires 100 percent torque to accelerate when starting fully loaded. The maximum current draw from the utility is limited to 500 percent of the motor full load amperes. The conveyor will normally be started unloaded; however, on occasion it may need to be started when it is loaded. Rate of acceleration is critical to prevent the conveyor belt from being damaged

Answer: Initially a soft starter seems to be the correct choice. The soft starter can provide 101 percent torque with 450 percent current (table 1). However the rate of acceleration, which equates to starting time is critical. The load also varies from unloaded to fully loaded. In this case a VFD would be the correct solution.

Application 4) A 20 horsepower motor drives an overhead plastic chain conveyor through a gearbox. It starts and stops frequently. Full voltage starting could be used, but if the conveyor starts too quickly the product will swing and may be damaged or the chain may break.
Answer: A soft starter would fit the application. There is no time constraint and no limitation on current. Ramp start would typically be used to allow for minor load variations reflected back to the motor. If the gear reduction is high enough, a current limit start could provide a smoother start.

Conclusion
These examples were designed to show how slight application variations can change the type of motor starting that is required. Each application must be evaluated on its on merits. Neither soft starters nor VFD’s are the perfect solution for all situations.

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AC Drives and Soft Starter (7)

Application Differences

With the knowledge of VFD and soft starter principles of operation and motor performance with each, application differences can be reviewed. With the list of applications being very similar, the general application parameters will be covered along with several application examples.
Motor speed is a parameter where a VFD has an advantage over soft starters. First, and most obvious, is where the speed of the motor needs to be varied from 0 to line frequency and sometimes higher than line frequency. The soft starter applies line voltage and frequency; therefore, the operating speed is fixed.

The second speed-related advantage to which an inverter relates is processes that require a constant speed. If a fixed frequency is applied to a motor, the actual speed of that motor is not precisely regulated by the input frequency. The output speed is actually regulated by the load applied to the motor. So if a process requires very tight speed regulation, the frequency applied to the motor must be changed in relation to the load that is applied. With the use of feedback to the VFD this can be accomplished. Again the soft starter only applies line frequency so any speed regulation is not possible.
On applications where acceleration time needs to be consistent, an inverter should be used. This is due to the fact that acceleration time for a soft starter is more dependent on the load than the selected ramp time. If acceleration time is not an issue and controlling the torque or current is that is needed, then a soft starter is a good candidate for the application. (Note: some soft starters use feedback, such as tachometers. These units can provide timed acceleration with varying loads. It should be noted that current during feedback acceleration could reach the same level as starting at full voltage – 600 – 800 percent of full load).
With regard to stopping, a VFD will bring the motor to a rest in a specified time. This may be built into an inverter or may require a dynamic braking optional function for high inertia and overhauling type loads. The soft starter with a soft stop feature can only extend the stopping time, and just like acceleration, the stopping time is dependent on the load. If stopping time and stopping characteristics are not critical then a soft stop may fit the application.
Some specially designed soft starters can also provide braking. These are designed to reduce stopping time where coast to rest is very long. If the load is not a pure inertia and can vary, the stopping time will also vary.
Where limiting current is the prime reason for not starting at full voltage, the first method to be considered today is usually soft starters. This is due to the cost differential between a soft starter and a VFD at the ampere ratings that current limiting becomes a factor. In most instances the soft starter is an appropriate choice.
There are applications where the additional cost of an inverter is appropriate. These cases are where the motor cannot provide sufficient torque to start the load with the ampere limitations imposed by the distribution system.
Table 1 shows the motor torque provided at various levels of soft starter current limit. Unlike soft starters, drives can accelerate a motor to full speed at full load torque with line current that does not exceed the full load amperes of the motor. Keep in mind that the power into the VFD is equal to the power out plus the losses. Therefore, for those loads that require higher torque than the soft starter can provide with the limits imposed by the distribution system, an inverter may be the required solution.
If starting torque is a concern when selecting a drive or starter, keep in mind the drastic difference in the amount of torque that can be developed for a given amount of line current. The drive has a much higher torque per ampere ratio.

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AC Drives and Soft Starter (6)

Motor Characteristics Using Soft Starters

Unlike the ac drive, the line current and motor current for a soft starter is always the same. During starting the current varies directly with the magnitude of the applied voltage. The motor torque varies as the square of either the applied voltage or current.
The most critical factor when evaluating a soft starter is the motor torque. Standard motors produce approximately 180 percent of the full load torque at starting. Therefore, a 25 percent reduction in voltage or current will result in the locked rotor torque equal to the full load torque (180%*(.75)2 = 101%). If the motor draws 600 percent of the full load current on starting, then the current in this example will reduce the normal 600 percent starting current to 450 percent of the full load current.

Table 1 below gives more examples of the effects of reducing the voltage or current on a motor’s locked rotor torque. This data is valid for soft start and series impedance starting. They do not apply to other types of reduced voltage starting such as autotransformer and wye-delta starting.
When applying soft starters, the same constraint as electromechanical reduced starters applies. That constraint is “will the motor be able to produce enough torque to get the load started with the current the soft starter is allowing to flow to the motor?”
Soft starters do have an advantage over conventional reduced voltage starting. They are able to adjust voltage, current, and, therefore, torque over a wide range instead of single or a few fixed values. This can be seen in Figure 10. When voltage or current is held to a constant value, the speed-torque curve labeled “Current Limit” is produced. This curve would move up or down depending on the current limit setting. The upper boundary of this adjustment is the “Full Voltage” curve.
The soft starter can also ramp the voltage from an adjustable initial value up to full voltage over an adjustable time frame. This is represented by the “Soft Start” curve. A stepless transition, which is designed to eliminate current/torque transients, is produced by this ramp.
The operating speed of the motor cannot be varied because the soft starter only adjusts the voltage to the motor and not the frequency. The frequency applied to the motor is always the line frequency. Because of this, the acceleration time is more dependent on the load than the ramp time.

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AC Drives and Soft Starter (5)

Motor Characteristics Using VFD’s

During acceleration, the inverter applies different frequencies to the motor. It also changes the voltage but in direct proportion to the frequency. This is know as constant volts per hertz and provides constant torque while the motor accelerates.
A series of speed torque curves is shown in figure 9. These relate to speed torque curves at various frequencies. The “constant torque” line represents the full load or rated torque of the motor.

This “constant torque” line is actually the full load point on a locus of curves representing the speed torque curves of the motor from 0 to full speed. The inverter produces rated motor torque from 0 to rated speed. It will produce full load torque while drawing much less than full load current from the power line during starting. This is due to the fact that the motor is effectively always running at speed for the applied frequency.
When full voltage starting, the slip of the motor at 0 speed is 100 percent and the motor is highly inductive. This results is the very high inrush current, 600–800 percent, and relatively low starting torque, 150–180 percent of full load torque, compared to the current draw. Almost all of the motor current here is reactive. Reactive current, by nature, does not produce torque.
When a motor runs at speed the slip is typically in the area of one to three percent. Under this condition the reactive current is much less and the motor produces rated torque at rated current. With a VFD the motor runs virtually at speed during acceleration. Since the voltage is reduced at low speeds, the input current can be 10 percent or less with more than 150 percent torque.
Since the motor always runs at speed, or within rated slip, the acceleration time is dependent on the ramp time setting. This assumes that the drive has been properly selected for the load.

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AC Drives and Soft Starter (4)

Operation of Soft Starters

Timing of when to turn on the SCR’s is the key to controlling the voltage output of a soft starter. During the starting sequence the logic of the soft starter determines when to turn on the SCR’s. It does not turn on the SCR’s at the point that the voltage goes from negative to positive, but waits for some time after that. This is known as “phasing back” the SCR’s. The point that the SCR’s are turned on is set or programmed by what is called either initial torque, initial current or current limit setting.

The input voltage to the soft starter is the same as the VFD shown in figure 3. The result of phasing back the SCR’s is a nonsinusoidal reduced voltage at the terminals of the motor which is shown in figures 7. Since the motor is inductive and the current lags the voltage, the SCR stays turned on and conducts until the current goes to zero. This is after the voltage has gone negative. If compared to the full voltage waveform in figure 3, it can be seen that the peak voltage is the same as the full voltage wave. However the current does not increase to the same level as when full voltage is applied due to the inductive nature of motors.
When this voltage is applied to a motor, the output current looks like figure 8. As the frequency of the voltage is the same as the line frequency, the frequency of the current is also the same. As the SCR’s are phased on to full conduction, the gaps in current fill in until the wave form looks the same as applying the motor directly across the line.

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AC Drives and Soft Starter (3)

Variable Speed Drive Operation

The ac line voltage, figure 3, is rectified with a passive diode bridge. This means that the diode(s) conduct whenever the line voltage is greater than the voltage on the capacitor section. The resulting current waveform has two pulses during each half-cycle, one for each diode conduction
window.
The waveform, figure 4, shows some continuous current when the conduction transitions from one diode to the next. This is typical when a reactor is used in the dc link of the drive and some load is present. Inverters use pulse width modulation to create the output waveforms. A triangle waveform is generated at the carrier frequency where the inverter IGBT’s will switch.
This waveform is compared with a sinusoidal waveform at the fundamental frequency that is to be delivered to the motor. The result is the voltage waveform shown in figure 5.
Figure 6 shows the resulting current waveform at the motor with a PWM signal applied.
The inverter output can be any frequency below or above the line frequency up to the limits of the inverter and/or the mechanical limits of the motor. Note that the drive is always operating within the motor slip rating

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AC Drives and Soft Starter (2)

Soft Starters

The soft starter operates on a different premise. This principle is that by adjusting the voltage applied to the motor during starting, the current and torque characteristics can be limited and controlled.

For induction motors, the starting torque (LRT) is approximately proportional to the square of the starting current (LRA) drawn from the line. LRT ∞ I2. This starting current is proportional to the applied voltage (V). So the torque can also be considered to be approximately proportional
to the applied voltage. LRT ∞ V2.. By adjusting voltage during starting, the current drawn by the motor and the torque produced by the motor can be reduced and controlled.
By using six SCR’s in a back-to-back configuration as shown in figure 2, the soft starter is able to regulate the voltage applied to the motor during starting from 0 volts up to line voltage. Unlike the VFD, line frequency is always applied to the motor. Only the voltage changes.
Feedback from the motor to the logic circuit controlling the SCR firing is required to stabilize motor acceleration.

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AC Drives and Soft Starter (1)

Abstract:

There are usually several choices for starting motors. Two of these, ac variable frequency drives (VFD’s) and soft starters, seem to have similar characteristics. Terms and descriptions used in product literature are nearly the same. Even the list of possible applications is similar. However, the technology and performance are significantly different. When these differences are understood, it becomes clear when and where to properly apply each of them.

Introduction
The objective of this paper is to provide the basic technical information to understand the differences. First covered are the operating principles of the VFD and soft starter. How motor performance is affected is the other key to selection of the proper starting method. Finally, guidelines will then be presented.

Variable Speed Drives
The VFD works on the principle that the ac line voltage is converted to a dc voltage. This dc voltage is then inverted back to a pulsed dc whose rms value simulates an ac voltage.
The output frequency of this ac voltage normally varies for 0 up to the ac input line frequency. On certain applications the frequency may actually go above the line frequency.
Though high performance current regulated ac drives capable of operating in “torque mode” are available, the more prevalent volts per hertz drive is addressed here. The most common VFD’s manufactured today work using pulse width modulation to create the output sine wave. The conducting components used in drives are diodes, SCR’s, transistors and IGBT’s. These inverters have three distinct and different sections to their power circuits as shown in the typical inverter block diagram figure 1 below.
The first section uses a diode or SCR full-wave bridge to convert the ac line voltage to dc. Filtering of this dc is done in the second section with a capacitor to supply the inverter bridge with a stable dc power source. A dc link choke is normally present on 10 horsepower and larger drives. The final section uses a transistor or IGBT bridge to deliver a pulse width modulated (PWM) dc voltage to the motor. The effective rms voltage delivered to the motor is dependent on the fundamental output frequency that the inverter bridge is commanding. This is what leads to the term “volts per hertz drive.”
The control or logic section of the inverter and user programmed settings determine the frequency output of the inverter. During acceleration, the frequency will vary according to a predetermined algorithm such as linear ramp or s-curve, from minimum or 0 Hz up to commanded speed.
The drive can also be programmed to skip over certain frequencies that may cause a mechanical resonance.

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Basic Operation of AC Induction Motors (6) - FINISH

Voltage and Current Waveforms

Today’s AC variable speed drive systems (up to 600 Volts and about 1500 HP) are dominated by PWM configurations. The current waveforms seen today (Figure 14) are much closer to the ideal sinusoid, thanks mostly to higher switching rates of transistors. The availability of low switching loss devices has allowed this to occur.

One of the negative aspects of the newer devices is that the low switching loss is typically accompanied by a very short transition time. This short transition between on and off states implies a high dV/dt output of the inverter. The high dV/dt results in capacitively coupled current flow according to Equation 4.

In addition to the capacitively coupled current, the high dV/dt also results in a higher peak voltage (ringup) due to cable-to-load mismatch. Finally, the high dV/dt also results in an instantaneous high voltage across the first windings within the ac motor. A companion paper, “AC Induction Motor Insulation Issues in High dV/dt Environments,” addresses this in greater detail.

Conclusions
AC induction motors are likely to continue to be increasing sources of variable speed rotating power. Their successful use in variable speed applications is a function of the collective understanding of the various parties involved in the specification, design, application, and integration of the system.

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Basic Operation of AC Induction Motors (5)

Constant Power Operation

The prior discussions regarding voltage boost and field oriented control as a means to maintain motor flux have been presented in regard to "constant torque" operation. This can also he thought of as operation "below base speed" (Figure 10).

Above the speed at which the output voltage of the controller is maximum, the controller can no longer maintain constant flux as speed is increased further (since the voltage cannot be increased to keep pace with the frequency). This is equivalent to where a DC motor begins to be "field weakened" to achieve higher speeds. Both for AC as well as DC machines, voltage (armature voltage for DC) remains constant, so for constant load current, constant output power
is available.
As the frequency supplied to an AC induction motor is increased (with voltage held constant), the resultant "field weakening" causes a reduction in the motor peak torque capability as seen in Figure 11.
This family of curves can alternatively be drawn as speed - power, rather than speed - torque curves (Figure 12). The fact that the peak power decreases as speed is increased by field weakening is the most "inherent" limitation to the "constant power speed range" of an AC induction motor drive.

A technique which is commonly employed to achieve wider speed ranges above base speed (constant power) is to utilize some of the “constant flux” speed range to augment the inherent constant power capability. By selecting a motor winding which does not require full voltage until some speed already into the desired constant power speed range, the constant power speed range can be extended as seen in Figure 13. The plots of Figure 13 show an example where the application demands a constant 100 HP from 650 RPM to 3200 RPM. By utilizing this technique, the motor size does not have to be increased in order to satisfy the wide constant power speed range.

This same technique is also used to extend the constant power speed range of DC systems as well. In the case of DC, it is to avoid commutation limits to the top speed at which constant power can be provided. For both AC and DC systems, the “price” which is paid to use this technique is an oversized source of power (higher kVA inverter or DC supply). Wind and unwind applications, along with machine tool spindles employ this technique quite commonly.

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Basic Operation of AC Induction Motors (4)

"Field Oriented" Control

In order to obtain even better yet control of AC motor torque, adjustable frequency controls often can make use of a regulation scheme known as "field-oriented" or "vector" control. This technique is intended to control the motor flux, and thereby be able to decompose the AC motor current into "flux producing" and "torque producing" components. These current components can be treated separately (in the control), then recombined to create the actual motor phase currents. This results in a solution to the boost adjustment problem, plus provides much better control of the motor torque - which allows much higher dynamic performance.

One way of looking at field oriented control is that the inverter would like to be able to have the same sort of simple, direct control of both flux and torque that is enjoyed with separately-excited dc motors. With dc motors, the flux level is controlled by simply regulating the field current, while the torque is controlled by regulating the armature current. By using field oriented control, the inverter can treat the ac induction motor as if it had the same sort of independently regulated flux and torque characteristic. When the actual induction motor phase currents are decomposed into flux and torque producing components (in the control, not in the motor), this gives the opportunity to “decouple” these two and achieve better system performance as a result.
In order to accomplish field-oriented control, the controller needs to have an accurate “model” of the motor. Over the last several years a large number of different schemes have been proposed to accomplish the “flux and torque control” desired. Many provide this control without the use of a speed feedback (tachometer) signal. These are typically referred to by the generic term of “sensorless” vector control. Many of today’s techniques also involve some sort of self-tuning at startup in order to obtain information which helps to more accurately model the motor – and thereby produce more optimal control. In addition, there are also techniques by which the models can adaptively adjust to changing conditions, such as the motor temperature going from cold to warm (which impacts the slip).

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Basic Operation of AC Induction Motors (3)

Adjustable Frequency, Variable Speed Operation

For steady-state (as opposed to starting) operation, AC induction motors offer a reasonably linear torque per amp and high power factor characteristic. This is seen in Figure 5 as the part of the speed torque curve between "breakdown RPM" and "synchronous (no load) RPM." It is this portion of the AC induction motor range of operation within which adjustable frequency drives function.

By varying both the frequency and voltage supplied to an AC motor, the controller can cause the motor to operate on a continuum of speed torque curves which allows operation in the "linear" region between breakdown and synchronous speeds (Figure 6).

This then allows the motor to operate near its optimal torque per amp or maximum efficiency point for a given load and speed.
As long as the motor flux is maintained constant while the frequency and voltage are varied, the basic "shape" of the speed torque curve will remain unchanged. The motor flux is proportional to the internal "counter-emf" divided by the frequency of that generated voltage. This can be described as:
where F is the motor flux,

Eg is the internally generated voltage due to motor rotation, f is the stator frequency, and k is a motor constant related to the winding turns, etc.
The flux paths for a four pole configuration are as seen in Figure 7 (for an instant in time). This pattern rotates at an effective speed given by Equation 1.

The motor counter-emf (Eg) can also be thought of as the voltage across the magnetizing reactance (Xm) in the equivalent circuit of Figure 3.
Maintaining constant flux while the speed (frequency) is varied can then be seen as requiring constant ratio of Eg / f (or constant Im).
Since Eg is a motor internal voltage, this needs to be related to the terminal voltage of the motor. From the AC motor equivalent circuit, it can been seen that the voltage drops across the stator resistance and leakage reactance represent the difference between Eg and the terminal voltage Vt.
If a controller were to maintain a constant ratio of TERMINAL voltage to frequency (Vt / f), rather than Eg / f, this would result in a noticeably decreasing flux level at lower speeds (frequencies).
The curves of Figure 8 demonstrate the effect of this failure to maintain the motor flux. It can be seen that the peak value of torque falls off at the reduced flux levels. In fact, the peak torque is approximately proportional to the square of the flux level, so the drop-off can be significant. The torque per amp is also (directly) proportional to the motor flux, so increased current draw for a given load (torque) will also result from reduced flux.

As a means to improve the system characteristics (beyond the curves of Figure 8), controllers often compensate for the difference between Vt and Eg in order to select the correct voltage for a given frequency. This compensation is often referred to as "voltage boost." Since the major detrimental effect of constant Vt / f is at low voltages, low frequencies (low speeds), the voltage drop across the stator leakage reactance is usually ignored (as the impedance of an inductor is proportional to frequency). This leaves the drop across the stator resistance as the major source of a discrepancy between Vt and Eg at these low speeds.
Many controllers use a value of voltage boost which compensates for the IR drop of the stator at a current equal to the motor full load amps.

Vb is the per phase (line-to-neutral) voltage boost,
Rl is the per phase stator resistance,
IFL is the motor full load current.

This would result in a voltage versus frequency characteristic as shown in Figure 9. A weakness in this technique of boosting voltage is that the value of Vb is only "correct" for a single value of load current. If the full load current is used to set the voltage boost, then the motor will be overfluxed for lighter loads, and underfluxed for overload conditions. Depending on the low speed performance required by a given application, this may or may not be a problem.
It is now common to provide a “more intelligent” voltage boost function in many controllers. This can provide a closer to optimal operating condition at low speeds, resulting in better low speed torque delivery from the system.

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Basic Operation of AC Induction Motors (2)

Speed / Torque Curves

As an AC induction motor is started, the values of resistance and reactance offered by the motor (or seen by the power source) will vary. At the instant of applying power to a stopped motor, the magnetic field is rotating much faster than the (stationary) rotor. This implies 100% slip, so R2/s is minimized. As a result, the current drawn at starting (locked rotor) conditions is quite high. Also, it is common to design rotor slots which have dramatically different impedance at high slip (say 60 Hz for starting) versus at normal running where slip is typically in the range of 0.2 - 2 Hz. This changes the values of both X2 and R2 from starting to running conditions.

As a motor accelerates to speed from a standstill, the changing impedances result in a unique characteristic developed torque and current drawn during the time of acceleration. Depending on the design of the motor, a torque / current characteristic such as one of those shown in Figure 4 would typically result. The NEMA Design B motor is considered the most "general purpose" of these characteristic shapes, with Design C and D typically used for more "difficult to start" loads. Table 2 gives some ranges of characteristics for integral HP, 1200
and 1800 RPM motors.

As can be seen from all of these speed / torque / current curves, the current drawn by an AC motor in accelerating a load up to speed can be dramatically higher than the nominal running current. At the same time, the developed torque (during acceleration) may in some cases be less than the rated full load torque. Various methods exist to control the starting current drawn by an AC motor but the torque per amp seen during (fixed frequency) starting is always much lower than at running conditions.
The nature of an AC induction motor’s acceleration to running speed is such that it can impose high stresses on both the stator and the rotor. The high current draw also stresses the upstream power system, including cabling, transformers, switchgear, etc. For this reason, there is often significant effort made to "control" AC motor starting and acceleration - both in terms of motor design as well as application.

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Basic Operation of AC Induction Motors (1)

Terminology and Equivalent Circuits

Before trying to understand the operation of AC induction motors on adjustable-frequency power (variable-speed), it will be useful to briefly review the basic fixed-frequency (constant speed) operation of AC induction motors. The fundamental electromagnetic components are the stator and rotor.
Examples of typical laminations which comprise the basic magnetic path in the stator and rotor are shown in Figure 1. In the most common configuration, the stator has three interconnected phase windings, and the rotor winding is a set of short circuited bars known as a "squirrel cage." A wound stator and an aluminum die cast (squirrel cage) rotor are seen in Figure 2.

With balanced three phase voltages applied to the windings of the stator, balanced currents flow in the three interconnected phase windings. These currents produce a magnetic field which can be thought of as "rotating" within the stator at a speed given by Equation 1.

N1 = 120 x f /P (1)
N1 = rotational speed of stator magnetic field in RPM (synchronous speed)
f = frequency of the stator current in Hz
P = number of motor magnetic poles

For various numbers of motors poles, Table 1 shows the synchronous speeds based on 60 Hz and 50 Hz frequencies.
The natural tendency is for the rotor to "follow" the rotating magnetic field, and at no-load the rotor will turn at a speed virtually equal to Nl. Any difference in the rotational speed of the magnetic field and that of the rotor will result in a voltage being induced in the rotor squirrel cage winding. The resultant rotor current interacts with the magnetic field to produce torque. The difference in rotor mechanical speed versus magnetic field rotational speed is what is known as "slip."
The equivalent circuit for an AC induction motor can help visualize some of the motor characteristics.

Figure 3 shows a typical equivalent circuit for AC induction motors. The variable resistor “R2/s”
represents the way slip causes increased current and corresponding increased torque. The greater the slip, the lower this value of resistance, and the more current is going to flow in this branch of the circuit. When the slip is virtually zero at a “no-load” condition, this resistor is seen to be a very high value. As a result, the current can be thought of as all going through the “XM” or magnetizing branch of the circuit.

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Troubleshooting Drives

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Inverter Duty Failures

It has been documented that some electric motors fail in inverter applications. This has often been attributed to inverter voltage “spikes.” While this is relatively correct, it misses some important aspects to the mode of failure.
The number of pulses that a PWM drive fires in order to control the current waveform to the drive is known as the carrier frequency. The carrier frequency tends to run from 2 to 18 kHz in most modern PWM drive. In addition, each voltage pulse is not a square waveform. They have a tendency to overshoot on startup, causing a “ringing” effect at the peak voltage of the pulse. Insulation systems are designed, not only for temperature, but also for “rise time,” how fast the voltage increases over time.

Initially, it was thought that inverter duty failures occurred only on the first few turns of the electric motor winding. It was later found that this was not correct for all cases. Instead, it was discovered, a phenomenon normally seen in electric motors rated at 6,000 VAC, and above, known as Partial Discharge, was now occurring in motors rated as low as 460VAC. This phenomenon is similar to a lightning storm within the windings themselves. Within voids in the winding insulation, charges build up, then discharge (much like a capacitor). The end result is ozone, which begins to break down the insulation on the wires, eventually causing a current path, or short.
The mode of failure for motors in this environment is as follows:
· The motor and drive are placed a distance apart and the carrier frequency is set high (ie: above 8kHz) in order to keep the motor quiet. The lower the carrier frequency the louder the motor noise. No filtering is put in place.
· The pulses from the drive travel out to the motor. Based upon the impedance of the cable and motor, a reflection of the pulse travels back to the drive. This cycles through the “free-wheeling” diodes of the inverter and travel back out with the normal pulses. This adds on to the peak voltage, causing a greater peak (as much as 2 to 4 times, usually 2) with an extremely fast rise time. (ie: less than .1 u-sec per 500 V versus the 1 u-sec per 500 V recommended by NEMA).
· In some cases, the voltage spikes will cause the weakest part of the winding insulation to fail and the motor shorts.
· In other cases, small voids in the insulation begin to have partial discharge problems, the ozone eats away at the insulation, until, finally, the insulation becomes weak enough for the spikes to break through.

It should be pointed out that this tends to be a rare problem. Following are measures to avoid the chance of this problem occurring to you:
· Check with the motor manufacturer to ensure that the motor can operate in an inverter environment.
· Use filters in the inverter system (ie: from line reactors to spike arrestors, designed for inverter use).
· Read the VFD operators manual. It will often state the minimum distances and frequency settings.
· Use proper wire sizes.

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Power Quality

Harmonics and electrical noise are potential problems when power electronics are utilized. As more AFD's are put into use, utilities may force users to install harmonic filtering from entering their systems. IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems; IEEE Std. 519 - 1992; is written to attend to this issue. The standard has been written to limit the harmonic content introduced into the system by either the utilities or the customer.

(Note: The limits are, generally, 5% voltage distortion and 3% current distortion at the Point of Common Connection (PCC) or the point at which the utility power enters the customer plant.)
Harmonic content has attracted quite a bit of attention when discussing power quality and power electronics. Harmonics, created by the load, generally come from feedback into the line from electronic power supplies. Voltage and current harmonics tend to create alternate fields within motors and rotors, cause transformers to overheat, and interfere with other electronic systems. Odd harmonics of the fundamental frequency are generally found in power electronic systems.
In motor systems the following fundamentals of 60 Hz can be recognized:
Harm: 1st 3rd 5th 7th etc.
Rot: pos. zero neg. pos. etc.

Voltage and Current Harmonics

Positive harmonics rotate in the direction of the rotor. Other than the fundamental frequency, this type of harmonic causes heating within the stator. Negative rotating harmonics rotate against the rotor causing overheating of the rotor and reducing torque.
Zero rotating harmonics generally cause system neutrals to overheat. In the case of electronic drives, in general, the predominant harmonics are the 5th and 7th.

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Variable Speed Concerns

Whenever load speeds are varied, there are many considerations which must be taken into account. These concerns are both electrical and mechanical in nature.

The electrical problems associated with electronic drives generally concern the insulation. Because of the type of output generated by the inverter, there is great stress placed upon the insulation and the temperature rise of the windings may increase. In other cases, the motor may be run below its minimum self-cooling speed. The main trouble is that for every 10 degrees C, the insulation life of the windings are reduced by half. If the temperature rise is allowed to climb too high, the motor will overload and burn-up in a very short time. An additional problem, which is rare, is inverter resonance.
These difficulties can be avoided through the following means:
· Rewind or replace the motor - Rewind the motor to a higher insulation class, or replace it with a new motor. the new motor may be of the energy efficient or inverter duty type.
· Provide external cooling - This is especially important in cases where the self-cooling ability of the motor is compromised.
· Re-set the parameters - inverter resonance is found in cases where the drive parameters are not properly set. If this is not the case, the drive should be programmed to by-pass those frequencies where the problems are found.

Mechanical considerations include mechanical resonance and driven load incompatibility. Mechanical resonance can be defined as the speed of the driven load that matches its natural frequency. If this speed is found and maintained, the equipment will develop extremely high levels of vibration and may shake itself apart. Load incompatibility can be defined as loads which may not be operated at speeds lower than their design speed. For instance, many gearboxes have a minimum speed at which the lubricating oil may not be properly moved over the contacting parts.
Mechanical resonance can be avoided by programming the drive to avoid the appropriate frequency(s). The resonance levels may be determined by using a vibration analyzer and operating the machine through the entire speed range. Another way is by performing a "ring-test" using vibration analysis equipment. Load incompatibility can only be avoided by not allowing the drive to operate below a minimum speed.

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Variable Frequency Drives (4) - FINISH

4. Basic Operation of a PWM Inverter (VFD)

In this section we will discuss how the five basic drive system components work together. After this discussion we shall include a detailed, component level, discussion of operation.

The rectifier circuit of a pulse width modulated drive normally consists of a three phase diode bridge rectifier and capacitor filter. The rectifier converts the three phase AC voltage into DC voltage with a slight ripple. This ripple is removed by using a capacitor filter. (Note: The average
DC voltage is higher than the RMS value of incoming voltage by: AC (RMS) x 1.35 = VDC)
The control section of the AFD accepts external inputs which are used to determine the inverter output. The inputs are used in conjunction with the installed software package and a microprocessor. The control board sends signals to the driver circuit which is used to fire the inverter.
The driver circuit sends low-level signals to the base of the transistors to tell them when to turn on. The output signal is a series of pulses, in both the positive and negative direction, that vary in duration. However, the amplitude of the pulses are the same. The sign wave is created as the average voltage of each pulse, the duration of each set of pulses dictates the frequency.
By adjusting the frequency and voltage of the power entering the motor, the speed and torque may be controlled. The actual speed of the motor, as previously indicated, is determined as:

Ns = ((120 x f) / P) x (1 - S)

where: N = Motor speed; f = Frequency (Hz); P = Number of Poles; and S = Slip.

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