Need for Industry Standards for AC Induction Motors Intended for Use with Adjustable-Frequency Controllers

Abstract

With the increased use of ac induction motors on adjustable-frequency controls (AFC) offered in a wide variety of types, there is a need to develop industry standards to provide the guidance necessary for the proper selection of the motor, which will function as intended with the control. These new standards should establish requirements for both electrical and mechanical characteristics of the motor necessary to establish uniformity within the industry. This paper discusses the reasons why certain topics must be covered by such standards and presents proposals for the content.

INTRODUCTION

DURING the 1980's the use of ac adjustable-frequency controls (AFC) with squirrel-cage induction motors has increased significantly in both the industrial and the commercial sectors. To a great extent, this has been due to the tremendous gains in technology and packaging of both signal and power electronics. The result has been smaller controller sizes and lower controller costs. Most of the initial applications have been on pumps and fans-where flow control achieved by speed control has provided a reduction in total system energy losses over the use of traditional flow-control valves, dampers, or inlet guide vanes. In many cases, the efficiency and performance of the overall process was increased as well.

Ten years ago, AFC's were looked on with casual, if not critical, interest by the petroleum and chemical industry. Today they are viewed as just another one of the standard components in the designer's tool kit. As we enter the 1990s, applications for AFC's and induction motors will expand to higher performance applications. Applications will require speed operation over wider speed ranges than the typical fan and pump control of 4:1. Operation speeds above the traditional 60 Hz sinewave motor speeds of 1200, 1800, and 3600 r/min may provide advantages in size reduction, gear elimination, and increased process flexibility. Increased utilization of special, nonstandard motors will require development of a formal means of communication between the user and motor specifier and the motor supplier.

The application of NEMA standards developed for ac constant speed motors [1] falls short in specifying many important considerations and motor requirements for adjustable speed applications. New standards are required. To this end, NEMA has formed the Task Force on Inverter-Fed Motors. One purpose of this motor industry task force is to develop standards and application guidelines for motors designed specifically for use on adjustable-frequency power (AFP) supplies.

This paper introduces the user to the many items that must be covered by such industry standards in order to assure successful applications on adjustable frequency.

II. MOTOR OPERATING CURVES ON AFP

The performance of an induction motor on AFP is not the same as that on 60 Hz sinewave power. This is particularly evident during starting and acceleration and is best described by the motor speed-torque and speed-current curves that apply in each case.

The speed-torque and speed-current curves for a typical induction motor operating from a constant 60-Hz power supply are shown in Fig. 1. When the motor is started, it requires an inrush of current equal to more than six times its rated full-load value in order to produce a torque level slightly greater than full-load torque. During the time that the motor accelerates from zero speed to the point of maximum, or breakdown, torque the current remains near this high level. The current then drops off rapidly as the motor continues to accelerate until the motor output torque matches the load torque.

Similar to Fig. 1, a family of speed-torque curves based on changes in the frequency and voltage of the power applied to the motor can be constructed. Such a group is shown in Fig. 2 for the case where the ratio of voltage to frequency, or volts/Hertz, is assumed to be constant for frequencies below 60 Hz and where, the voltage is assumed to be constant for frequencies above 60 Hz. Power is initially applied to the motor at a frequency near 1 Hz. As a result, the motor exhibits a performance characteristic similar to that when operating at low slip levels with 60-Hz power applied. In short, the motor operates between the point of synchronous speed and breakdown speed. As the frequency is gradually increased from 1 Hz the slip r/min and torque increases. When the torque is sufficient to break the load away from rest, the motor and load will begin to accelerate. The percentage of full-load current necessary to produce this breakaway torque will be only slightly greater than the percentage of full-load torque that the breakaway torque represents. The high inrush associated with the across-the-line starting and the shock of the sudden application of torque are eliminated. The motor is thus soft started.

Fig. 1 Speed-torque current curves - 100 HP energy-efficient full-voltage starting	Fig. 2 Speed-torque curves: Constant volts/Hertz below base speed, constant voltas above base speed, no voltage boost.

As shown in Fig. 2, the amount of torque produced by the motor is dependent on the slip r/min at each frequency. The amount of current required is a function of the slip and the applied voltage. Since the current required to produce a torque level equivalent to the breakdown torque is several times larger than the full-load current, the maximum level of torque that the motor can actually produce at any frequency is dependent on the maximum current rating of the AFC. The rate at which the motor driven equipment can be accelerated is dependent on the difference between the motor torque and the load torque. This acceleration rate can then be controlled by controlling the rate at which the applied frequency is increased, within the limits of the peak current rating of the controller. This controlled acceleration contrasts with the uncontrolled acceleration of the across-the-line starting.

Also illustrated in Fig. 2 is the breakdown torque decreasing and the slip at constant-torque increasing as the frequency is decreased when the volts/Hertz is held constant. This occurs as a result of the voltage drop across the stator resistance and leakage inductance reducing the active voltage available at the motor air gap. This active air gap voltage determines the torque/amp ratio for the motor. To compensate for the stator voltage drop, a voltage boost is used at frequencies below 15 Hz. With an appropriately chosen level of voltage boost, it is possible to produce the same level of torque anywhere in the constant torque speed range using a constant level of current. Fig. 3 illustrates the change in the speed-torque curves of Fig. 2 that can be achieved by proper selection of the voltage boost. For further enhanced performance, some adjustable-speed controllers are actually capable of varying the voltage boost as a function of load to control the amount of current required from the current rated controller as well as to control the peak torque and acceleration rate.

At frequencies above 60 Hz, the motor depicted in Fig. 2 is generally operated at a constant horsepower level. The speed at which the type of operation changes from constant torque to constant horsepower is often referred to as the base rating point. For constant horsepower the torque level varies inversely with the speed. With the voltage held constant, the current required to produce constant horsepower over a small speed range remains relatively constant. The value of peak torque decreases at a rate proportional to the square of the volts/Hertz ratio, which is faster than the rate at which the torque corresponding to constant horsepower decreases. Therefore, there is a maximum speed at which the motor will no longer be able to carry the rated horsepower. For operation above this speed, the load on the motor must be further reduced.

When the motor is designed specifically for operation with an AFC, it is not necessary to restrict the speed and performance range depicted as in Figs. 2 and 3. A wider speed range can be achieved by moving the base rating point to a higher speed. Another method is to provide a motor capable of producing a higher level of peak torque at the base rating point to extend the maximum speed at which the peak torque falls below that for rated horsepower. The volts/Hertz ratio could he reduced to increase the frequency at which the voltage reaches its maximum. This would increase the speed at which the peak torque starts to decrease.

Fig 3 Speed-torque curves: Voltage boost below base speed,
constant volts above base speed

Industry standards presently exist to cover the expected performance of constant speed motors used on 60-Hz power supply systems, as depicted in Fig. 1. These standards establish limits on such items as permissible in rush current, locked rotor torque, and base voltage rating. Although motors have been successfully applied with AFC's in various applications, the limitations imposed by the existing motor standards make it difficult to obtain the optimum performance that could be achieved in many areas. It is time that a separate standard be established for motors designed directly for use with AFC's to take advantage of the many features that the controller offers.

III. CURRENT AND VOLTAGE WAVEFORMS

AFC's for use with induction motors can be classified as one of two types of power sources-voltage or current. In the voltage source, the magnitude of the voltage applied to the motor is directly controlled and the current is a function of the motor characteristics and the applied voltage. Such voltage control is typically performed in an open-loop fashion with the magnitude of the voltage for any frequency predefined by a volts/Hertz curve. In the current source, the level of current flowing in the motor is controlled directly and the voltage is a function of the motor characteristics and the current. Current control can be achieved by using a closed loop to monitor the output current from the AFC.

A six-step variable voltage input (VVI) inverter is one example of a voltage source inverter. The output voltage waveform for this type is shown in Fig. 4, together with the resulting current waveform. The six-step output voltage can be represented by a Fourier series consisting of all of the odd harmonics, except for those which are a multiple of 3. The current at each harmonic frequency is equal to the level of the harmonic voltage divided by the impedance of the motor at that harmonic frequency. The higher the impedance of the motor, the lower the value of the harmonic current and the lower the peak value of the distortions in the current from a sinewave at the fundamental frequency.

The output current waveform for a six-step current source input (CSI) inverter is shown in Fig. 5 together with the resulting voltage waveform. In this type of inverter, it is the current waveform that is defined by the inverter. The six-step output current can also be represented by a Fourier series. The voltage at each harmonic frequency is now equal to the product of the harmonic current and the impedance of the motor at that harmonic frequency. In this case, it is preferred that the impedance of the motor be low to minimize the peak value of the harmonic voltages.

Fig 4. VVI waveforms

Fig. 5 CSI waveforms

Fig. 6 PWM waveforms

Fig. 6 shows the output voltage and current wave shapes associated with a pulse width modulated (PWM) inverter. This type of modulation can be employed in either a voltage source or current source type of inverter. In the simpler type of PWM inverters, the modulation scheme for the voltage to be used over the operating range is predefined as a fixed pattern. In the more complex type of PWM inverters, the modulation of the voltage may be accomplished based on monitoring of the output current for the purpose of achieving a particular current waveform or magnitude.

IV. WHY A DEFINITE-PULSE MOTOR DESIGN?

To date, the most commonly applied motor when powered by an adjustable frequency power source has been a type of constant speed motor termed to be energy efficient. This has been the choice of the application engineer since it tended to offer some inherent advantages over standard efficient motor designs. These advantages are summarized in Table 1.

Table I

Various application and performance trade-offs must be made when the only motor choice is a standard NEMA B design. As noted in previous discussions, it is preferred that the impedance at harmonic frequencies of the with the voltage source inverter be as high as possible to reduce the harmonic currents. However, in the case of current source inverters, a low value of impedance is preferred. One motor design cannot satisfy both of these applications. One compromise that has been used is to recommend voltage source inverters with motors below a certain horsepower rating, such as 150 hp, and use current source inverters with motors above that rating since the impedance of a motor decreases as the motor rating and size increases.

As we enter into higher performance applications, those requiring still wider speed ranges in the area of constant torque and constant horsepower, the desired goals are more difficult and/or costly to accomplish with a standard NEMA design B energy efficient motor. Table H lists some of the various objectives that one might choose from for a particular application and some of the parameters within the motor which might be changed to meet the objective. The problem is that changing one parameter to meet one objective may have an adverse effect on another or the primary objective itself for example, consider the case where the motor to be used must have a low value for the primary time constant L/R. This could be achieved by decreasing the number of turns in the stator winding. The wire size is usually increased when the number of turns is reduced to maintain the same level of slot fill and reduce the power losses. However, both steps of decreasing the number of turns and increasing the wire size will decrease the resistance and increase-not decrease-the primary time constant L/R.

V. PROPOSED MOTOR STANDARDS FOR A.F. OPERATION

A. Temperature Rise

For constant speed ac motors operated on sinewave power, the temperature rise limits are well established and represented in Table IH. These limits are based on the assumption that the motor would be operated continuously at its rated load at a constant temperature. As we select values for use on adjustable frequency, variable speed applications, we should consider that the motor is operated at the particular speed and load that result in maximum temperature only part of the time, not continuously. This duty cycle method of operation is characteristic of that of dc motors that have a history of successful operation at higher temperature limits than constant speed ac motors. It is proposed that the temperatures given in Table IV be used for adjustable-frequency variable speed motors.

B. Defining the Motor Speed Torque Performance Capability

Given an induction motor of a certain physical size having appropriately selected core and winding materials, the speed-torque capability is then established by limiting the temperature of the various components for a specified time rating and the peak torque that the motor can produce.

Table II

Changing Motor Parameters To Meet Performance Objectives

Table III

Maximum NEMA Temperature Rises for Continuos-Duty AC Motors
Temperature Rise Measured by Resistance Method
Temperature Rise with Ambient

Table IV

Proposed Temperature Rise Limits for Definite-Purpose
Inverter Duty AC Induction Motors

For example, curve (a) in Fig. 7 shows the maximum level of torque that a particular motor can carry continuously at any speed point. This maximum value of torque results in a temperature rise equal to the limiting value for the materials used in that motor. Likewise, curve (b) illustrates the maximum level of torque that can be carried for 30 min before the temperature rise increases to the allowable limit. Curve (c) illustrates the maximum level of torque that the motor can produce at each speed point based on the defined voltage versus frequency curve. The required current for each of the three torque curves are shown in Fig. 8.

Fig. 7 Torque capability of TEFC AC induction motor on adjustable voltage-frequency control.	Fig. 8 Current requirements for TEFC AC induction motor on adjustable voltage-frequency control.

Given this type of information for several motors, the user, after much effort, might be able to determine the motor that would fit the needs of the speed-torque curve of his or her particular application and determine the current rating of the control required. However, such a system of describing the capability of all motors used with inverters can become very cumbersome and confusing. For instance, consider the problems of stating the rating of the motor and how the information required to adequately describe the motor could be included on the motor nameplate. Consider how difficult it would be to compare one motor to another or standardize on a uniform system of motor ratings. To overcome these obstacles, it is proposed that the motors be defined on the familiar basis of a. constant torque and a constant horsepower range as commonly used to describe the speed-torque curve of the load. The motor would be rated in terms of the torque or the equivalent horsepower at the maximum speed in the constant torque range. Based on the minimum and maximum speeds for a desired level of torque in Fig. 7, such a motor capability rating could be adequately described by the four points indicated on the curve in Fig. 9. The voltage and current required to produce the indicated method of operation should be included for use in determining the size of and for setting up the control.

C. Breakaway Torque and Voltage Boost

In the IEEE Standard Dictionary of Electrical and Electronics Terms [2], breakaway torque is defined as the torque that a motor is required to develop to break away its load from rest to rotation. In the case of a constant speed motor used with a 60-Hz power system, this torque is commonly referred to as the locked rotor torque. For a NEMA design B motor, the locked rotor torque is generally in the range of 110-175% of full-load torque. The level of starting current can exceed six times the rated full-load current.

Fig. 9 Motor-SpeedTorque capability

In a typical adjustable speed application, the amount of current that a control can supply to the motor is limited to 110-150% of rated torque. This is of sufficient magnitude to produce a torque at zero speed equal to 100-140% of rated torque when the applied frequency is between I and 3 Hz. To eliminate the confusion between the two methods of starting the motor and the large difference in the current required, it is preferred that the term "breakaway torque" be used when referring to the torque supplied by an adjustable speed drive for starting the load.

The amount of torque produced in the motor is related to the level of flux and the stator current. The level of flux is determined by the air-gap voltage, which is the difference between the applied voltage and the stator voltage drop that results from current flow in the stator winding. At the full-load current level, the voltage drop across the stator resistance can be equal to as much as 5 % of the rated voltage at 60 Hz. Although the effect of this voltage drop may not be significant at frequencies above 15 Hz, it can become very significant at a frequency of 3 Hz where the value of the applied voltage based on constant volts per hertz would also be equal to 5 %.

Thus, in order to have the equivalence of 5% voltage for the air-gap voltage at 3 Hz, the applied voltage must be equal to 10% voltage when full-load current and torque is required. This additional voltage is the voltage boost. Obviously, the amount of voltage boost required to maintain a constant level of air-gap voltage is dependent on the amount of current needed to produce the required torque.

On some adjustable frequency controls, such as those based on vector control logic, the level of voltage boost is automatically adjusted as a function of the load current. However, on many controls the level of voltage boost is manually adjusted the first time the drive is operated until the motor is capable of starting the load to rotate and is then left at that level, regardless of any changes in the load. It is possible for the voltage boost to be set to provide the maximum level of current that the control can provide, particularly if the load breakaway torque is greater than expected. This can correspond to a high level of voltage boost, particularly in those cases where the control may oversized for the full-load rating of the motor.

If the torque required to rotate the load at the low speed is significantly reduced once the load starts to rotate, then the current will drop. This will result in an increase in the air-gap voltage, an increase in the saturation level of the motor, and a decrease in the motor inductance. It is then possible for the current drawn at no load to be greater than that at full load or overload because of the constant voltage boost. This condition could result in the unexpected overheating of the motor when it is operated unloaded at the low speed for an extended period of time. Such overheating has been known to occur in those cases where the control rating is greater than the motor and the voltage boost is adjusted to provide greater dm 150% of the rated torque.

From the above discussion, it becomes clear that the proposed standard on adjustable frequency motors must establish a limit on the maximum level of breakaway torque that the motor must be capable of producing in the general case. Such a limit should be based on a current equal to 150% of the motor full-load current. Assuming a reasonable setting for the voltage boost, the expected level of breakaway torque should be at least 140% of rated torque. Second, the standard must restrict, or at least caution against, the use of fixed voltage boost in those cases where the motor may be operated at light loads at low speeds for an extended period of time. This can be accomplished by requiring that the voltage boost not be adjusted to exceed that value based on the resistive voltage drop at a level of current equal to 150% of rated torque and that operation below 6 Hz for an extended period of time should require special consideration and consultation with the motor manufacturer.

D. Peak Torque

"Peak torque" and "overload torque" are two terms that become confusing depending on whether one is referring to the capabilities of the motor itself or to its performance when the motor is under adjustable frequency control. To the motor designer the term "peak torque" refers to the maximum level of torque that the motor is capable of producing with a specified frequency and voltage applied. The more common name for this is "breakdown torque." "Overload" is interpreted to refer to a load on the motor in excess of the rated load, usually for a specified period of time. To the systems analyst, the term "peak torque" refers to the maximum level of torque that the motor can produce within the limitations of the current rating of the control. This is synonymous with "overload."

Fig. 10 Torque-current curves, 100 HP energy-efficient motor

The motor described by the performance curves in Fig. 1 is capable of producing a peak torque in excess of 200% rated torque at the base-rated frequency and voltage. The current required to produce this torque exceeds 300% of the rated current. The general-purpose AFC is only capable of providing a momentary overload current equal to 150% of rated torque. This is obviously less than that needed to produce the peak value of torque that the motor is actually capable of. Expanding the curves of Fig. 1 near full load as shown in Fig. 10, one can determine that at 150% rated current, the motor is capable of producing 150% rated torque. The peak-torque output capability of the adjustable-speed drive-is then limited to 150% rated torque. A control with a greater current rating must be used if a higher level of peak torque is required. Based on Fig. 10, it can be determined that the required rating of the control cannot be based on an assumption of a linear torque-per-amp relationship. The motor exhibits a nonlinear relationship above approximately 150% of rated torque because of the change in the motor impedance as a result of the increase in slip. To obtain a peak torque level of 250 % out of the drive, the control must be capable of supplying 330% of rated current. This is equivalent to tripling the size of the control. This level of peak torque can be obtained without a change in the motor.

Fig. 2 indicates that the peak-torque capability of the motor decreases as the frequency increases above the base rating point when the voltage is held constant. There will be a frequency at which the peak-torque capability of the drive based on 150% current will equal the peak-torque capability of the motor. Above that frequency, or speed, the peak torque of the drive will be determined by the breakdown torque of the motor.

Assuming that the control is selected to match the motor rating and that the typical motor is capable of producing 150% torque at 150% current at the base rating point, then it becomes important that information be established that will indicate what the expected peak torque capability of the motor will be or should be in the higher speed range. The case where the control is oversized to obtained a higher level of peak torque capability should be addressed by consultation with the manufacturer of the motor since no standard could cover all cases.

E. Peak-to-Root-Mean-Square Current Ratio

To protect the electronic components in the control, there are several self-protection features that may be incorporated within the adjustable frequency control. The detection of an error condition and the subsequent action of disconnecting the control from the motor is generally referred to as an "instantaneous electronic trip" or IET. One of the most common types of IET's is based on an over-current condition.

The power-switching devices are given a continuous, a 1-min, and a peak-current rating. The continuous and 1-min ratings are based on the heating caused by the total rms. current through the device. These ratings can be directly related to the total rms. current required by the motor and are used to select the size of the control. The peak current limit is a limit on the instantaneous value of current that can pass through the electronic device. The waveform of the output current from the inverter is dependent on the type of the inverter as shown in Figs. 4, 5, and 6 and the characteristics of the motor or motors being controlled by the inverter.

If the peak-to-rms. current is greater than the rating for the control selected based on the rms. current rating, then an inverter of higher rms. current rating may have to be used to ensure that nuisance IET's on over-current do not occur. It is necessary, therefore, that consideration be given to the instantaneous peak value of current as well as the rms. current when selecting the inverter for a particular motor or motors. Although it may not be possible to set a limit for the peak-to-rms. current ratio in a standard that deals only with the motor, general information and guidance regarding the subject can be included as a part of the application information in the standard.

F. Peak Voltages

The nominal voltage rating of insulation systems typically used with random-wound low-voltage motors is 600 V. The peak value of a 600-V sinewave is 848 V. The slow rate of change of a 60-Hz sinewave results in the full voltage of each phase being distributed over all of the turns of wire in the phase group. The voltage stress across the insulation between adjacent turns in the coil, therefore, is low. The primary voltage stress is between the three phases and between each phase and ground (i.e., the stator core).

The output current waveform of a CSI control or the voltage waveform of a PWM control is not a pure sinewave function of a single frequency, rather it consists of various step functions. When these rapid step-function changes are applied to the inductive motor through a cable with distributed inductance and capacitance, the result in impulse voltage s at the motor terminals is different than that of the output of the control. For example, the impulse phase voltage measured at the output of a PWM control and that measured at the motor is shown in Fig. I 1. The results in Fig. 12 demonstrate the effect of changing the number of motors connected to the same PWM control.

The peak value of the impulse voltages can be several times greater than that of the rated voltage of the motor. Also, the short rise time of the impulse voltage may result in all of the peak voltage being initially applied to the first turn in the phase coil. This will result in a voltage stress across the insulation of the first turn and the adjacent turn several times greater than that experienced for sinewave power.

The voltage-withstand capabilities of enameled wire used in the random wound motor are set forth in NEMA MW- 1 000 [3]. It is not reasonable to put a level of voltage-withstand capability in a motor standard that would be greater than that for which the wire itself is designed. Instead, application guidance for the AFC manufacturer can be put in the motor standard to provide better information on the limitations that must be placed on the peak voltages created by the inverter. In instances where the expected level of impulse voltages will exceed the permissible level for enameled wire, then considerations should be given to the use of other insulation systems, such as glass coated wire.

Fig 11 PWM voltage output at control and at motor	Fig 12 Line-to-line voltage between motor leads at no load with PWM power

G. Bearing Insulation

Irregularities in the magnetic circuit in the stator and rotor may cause some flux to link with the frame and shaft resulting in induced voltages being generated in various components. As a result of these induced voltages, a current can flow through the path formed by the frame, bearing supports, bearings, and shaft.

The magnitude of any current is dependent on the level of the induced voltage and the resistance of the path. Historically, the level of current for most NEMA integral horsepower motors operating under sinewave power has not generally been great enough to result in any damage. When the motor is used on adjustable frequency power, the harmonics in the voltage and current waveforms are known to result in an increase in the level of voltage induced in the frame and shaft. Experience has also indicated that the level of currents resulting from the control induced voltages in large-horsepower motors have been great enough to cause deterioration of ball bearings.

When it is known that such a motor is to be used with an inverter, the motor can be constructed using insulated bearings to interrupt the path for the currents. It is too early to determine yet the horsepower level or frame size above which insulated bearings should be used. The proposed standard can, however, alert the manufacturers to this concern and offer some practical guidance while work is carried out to determine if there are some absolute guidelines that can be adopted.

H. Overspeed

The maximum speed at which the motor rotor can be rotated without failure of any component must be greater than the highest operating speed at which the motor will be used. To provide this margin of safety on present constant speed squirrel-cage induction motors, NEMA MG1-12.48 requires that the motor be designed so that it can withstand an overspeed equal to as much as 20-50%, depending on the motor rating, of the rated synchronous speed for a period of time not to exceed 1 min. Similarly, a standard for adjustable frequency motors should contain an overspeed capability based on a percentage of the maximum rated operating speed.

L. Dynamic Balance of the Motor

In some applications, the level of dynamic balance of the motor can be more important than in others. A motor used to drive a precision grinding machine tool may require a low limit on acceptable vibration to prevent imperfections in the finished surface of the machined part. Such demands on the motor may not be needed if the motor is to be used to drive a fan or pump. Special consideration will also have to be given to motors for which the maximum rated operating speed is significantly greater than 3600 r/min. It is necessary that any proposed standard for inverter-fed motors address the difference between these requirements.

This can be accomplished by establishing a table of values of vibration levels in the standard. The minimum level that a motor meets can then be identified for the motor either on the nameplate or in the software data accompanying the motor.

J. Torque Ripple

Torque ripple is generally defined as the variation of torque -within one shaft revolution expressed as the ratio of peak-to-peak torque amplitude to average torque. It is a function of both the design of the motor and the type of control being used. Placing limits on torque ripple in a standard that only deals with the motor is impractical.

K. Sound Level

The sound level of induction motors will increase when the motors are used with an inverter over that level when the motor is used on sinewave power. An example of this is illustrated in Fig. 13 for a motor with a skewed or a nonskewed rotor where the sound power level was measured with the motor running at no load in an anechoic chamber for the motor operated from a variable frequency motor-generator set and from an adjustable frequency control. The increase in the sound level due to the inverter will be dependent on the type of control and the harmonics present in the voltage and current waveforms. It will be difficult to evaluate the effect of all types of inverters on motors to determine an overall reference value of sound power level as provided in NEMA MGI-12.49.3 for sinewave power. A method of measuring the sound intensity of equipment when operating on site is under development and is likely to become the preferred technique for measuring sound levels in the near future.

Another concern regards the possible presence of pure tone sound generated when the motor is operated at some frequency. In this case, the sound power level may be below acceptable limits, but the continual beat or rhythm of the sound can be annoying, particularly in areas where the ambient sound is generally quite low, such as offices, hospitals, etc.

Fig 13 Sound power levels: (a) Skewed Motor; (b) nonskewed motor

Because the sound level is a result of a combination of the design of the motor and the control, it is difficult to develop any limit in a standard that only covers the motor. However, application guidance can be provided to make the user aware of the potential effects an inverter can have on the sound level of the motor, the recognition of pure tone sounds, and methods that have been used successfully to lower or mask the sound level.

L. Test Standards

Presently available test standards, such as IEEE 112 [4], are sufficient for verifying that the motor meets the performance requirements that are based on sinewave power. However, there is no existing standard that addresses the issue of testing an induction motor and inverter together. This is complicated by the fact that, under the concept of the off the-shelf motor, the motor manufacturer does not know what control type the motor may be used with. An issue that must be addressed is to determine who is responsible for testing the motor with the control and who has responsibility for the overall drive package. It is important that a test standard for the drive include temperature testing over the rated speed torque range of the drive.

VI. CUSTOMER INFORMATION

The information that describes the motor, its performance, and its power requirements is provided by the motor manufacturer in the form of the nameplate data attached directly to the motor and in the form of printed, or software, data.

A. Nameplate

Fig 14 Nameplate for an inverter-fed motor

Fig 15 Sample speed-torque-current curves

The nameplate describes the motor sufficiently so that the user can be certain that the motor is actually the motor that he or she has specified and that it is correct for the intended power supply. To achieve this, the information that is put on the nameplate of any motor can be separated into three general areas.

The first is related to the physical size and characteristics of the motor. Such items as frame size, type of enclosure, and method of cooling fit into this category. The second general area describes rating and performance of the motor. In the case of an adjustable-frequency motor whose intended application is over a defined speed range, items of importance are the speed and torque data shown in Fig. 9, necessary to adequately describe the operating range for each type of duty, such as continuous or 30 min, for which the motor is rated. The third general area is the application information necessary for the user to select and set up the inverter to be used as the adjustable frequency source. Items such as the volts/Hertz curve, voltage boost, amps, and motor equivalent circuit data fall into the latter category. Because of the limited physical size of the nameplate, care must be taken to select just those items that will adequately accomplish the intended goal for the nameplate. An example of a nameplate for an inverter-fed motor is shown in Fig. 14. A standard on inverter-fed motors should provide a list of the minimum amount of information that must be on the nameplate to ensure that all motors are adequately defined and to provide some uniformity in the information listed.

B. Software Information

Additional information beyond that which can be placed on the motor nameplate is provided in the form of printed tabular data or curves. This information is generally referred to as software information.

An example of speed-torque curves and the fundamental voltage curve for an adjustable frequency motor is shown in Fig. 15. Such information can be used to determine if a motor can be applied in a particular application by overlaying the speed-torque curve for the load and determining if the motor is of sufficient size, or oversized, for the application. The voltage boost necessary at low frequencies to overcome the stator impedance voltage drop is clearly shown.

The curves in Fig. 16 illustrate the variation of input current and slip r/min as a function of the output torque for the portion of the operating speed range below 1800 r/min in Fig. 15 where the volts/Hertz curve is held relatively constant.

Sometimes the proper selection of a motor for an adjustable-speed application requires knowledge about the efficiency of the motor at various operating points. The description of the efficiency is complicated by the fact that some of the losses are a function of the voltage, some a function of a frequency, some a function of the temperature, and some a function of the load. It is proposed that this information be provided in a tabular form. Such information can then be used directly in various analytical techniques used to determine the performance of the motor over a specified duty cycle.

C. Basis for Motor Data

The input data required by the motor as a function of speed and load based on operation from a particular adjustable frequency power source can be supplied when the motor manufacturer and the inverter manufacturer work together to develop the drive package. However, this is not possible when the motor is to be designed as an off-the-shelf definite-purpose motor without any knowledge of the type of inverter that will be used. For this reason the data supplied by the motor manufacturer should be based on how the motor performs on standard sinewave power.

The various industry groups representing inverter manufacturers and motor manufacturers should be encouraged to then develop guidelines covering the changes in motor power requirements and temperature that will result when the motor is used on different type of controls.

VII. SUMMARY

To satisfy the increasing application demands for ac adjustable-frequency drives, definite-purpose nonstandard motors will be required. These motors will be selected and specified along with nontraditional guidelines and specifications. We hope that this paper provided the user with an introduction to many of the items that should be detailed by a standard for ac induction motors used with AFC'S.

REFERENCES

[11 NEMA Publication No. MGI-1987, "Motors and generators," National Electrical Manufacturers Association, Washington, DC, 1987.

[2] ANSI/IEEE Std. 100- 1994, IEEE Standard Dictionary of Electrical and Electronics Terms, New York: IEEE, 1984, 3rd ed.

[3] NEMA Standards Publication, No. 1000- 198 1, " Magnet wire, " National Electrical Manufacturers Association, Washington, DC, 1981.

[4] IEEE Std. II 2-1994, IEEE Standard Test Standards for Polyphase Induction Motors and Generators. New York: IEEE, 1994.

Paper PID 91-05, approved by the Petroleum and Chemical Industry Committee for presentation at the 1990 Petroleum and Chemical Industry Committee Technical Conference, Houston, TX, Sept. 10-12, 1990.

IEEE Log Number 9102124.

May 1996