Adjustable Frequency Drive Basics (Part Two)

Editor's Note: This is the second part of a two part series of how adjustable frequency drives work. To read part one, click here.

Last month, we looked at adjustable frequency AC drive system, induction motor speed control and motor application and performance. This month we will explore specific operation conditions, applications and performance.

Motor Application and Performance

Motor Sizing

In sizing a drive, the torque/speed capabilities of the motor need to be matched to the requirements of the driven load, then the inverter can be matched to the motor.

When a drive is used in a constant torque application it is important to remember that as motor speed is reduced below base speed, motor cooling will become less effective. The minimum speed that is allowable for continuous operation under constant torque conditions is affected by this limitation. In variable speed application, this limitation does not apply as motor load is lower at low speeds.

Figure 6 shows typical motor performance curves. These curves demonstrate that by using a high efficiency motor or by oversizing the motor, a wider constant torque speed range can be realized. Operation above 60-Hz will also give a wider speed range.

Figure 6. Typical motor performance curves

Load Characteristics

Most loads are divided into two categories:

• Variable torque: Pumps and centrifugal fans
• Constant torque: Conveyors, hoists, etc.

See Figure 7. The current drawn by an AC motor is proportional to the load torque.

Figure 7. Speed versus torque constant and variable torque

Extended Motor Performance

Extended motor performance can be obtained by operating a motor above its base speed to 90-Hz. If an application was sized by using an 1800-rpm motor, a 1200-rpm motor of the same size can be employed and operated at 1800-rpm by increasing the maximum frequency to 90-Hz. The percentage torque ratings are based on 100 percent torque equal to the rated torque of a four-pole motor. The rated torque of a six-pole motor is 150 percent of the rated torque of a four-pole motor of the same rated horsepower.

The motor voltage is held constant between 60-Hz and 90-Hz; therefore, the available torque follows a constant HP curve.

This mode of operation increases the continuous and intermittent torque available over most of the speed range. It increases the breakaway torque to 225 percent. Continuous constant torque speed range is also increased. Since the operating horsepower is not increased, it is typically unnecessary to oversize the drive to obtain extended motor performance. To be sure, check the motor current.

Operating Below Rated Motor Speed

Most motors have an internal cooling fan. Operation below rated speed reduces the effectiveness of the fan, possibly causing the motor to overheat. Since the load is typically small at light loads, this is usually not a problem for pump or fan applications. For constant torque applications-conveyors, hoists, cranes, etc.-forced cooling of the motor may be necessary.

Figure 8 shows typical torque derating for a fan cooled motor operated below rated frequency. Mechanical resonance may be presented below the rated speed. Continued operation at these speeds can affect the performance of the driven equipment, and lead to premature failures. Most AC drives allow certain speeds or frequencies to be "skipped," avoiding operating at the mechanical resonance speeds or critical speeds.

Figure 8. Motor Torque De-Rating

Operating Above Rated Motor Speeds

Speed/Torque Considerations

Most AC drives can have output frequencies of 120-Hz or greater. However, the output voltage is limited to the magnitude of the line voltage. A drive supplied by 460 volts cannot output more than 460 volts. Therefore, as frequency is increased above 60-Hz, the output voltage remains constant and the volts per hertz ratio decreases. This reduces motor torque.

Figure 9 shows a plot of an AC drive and motor torque versus speed. The thick line is the drive plus motor torque curve. The thin line is a typical speed torque curve for a centrifugal fan or pump. No overspeed is possible for this type of load, since the load torque exceeds the motor torque. Operating above rated speed requires either:

• A load with low torque, such as an unloaded crane
• The motor to be oversized

Figure 9. Speed vs. AC Drive + Motor Torque

Mechanical Considerations

Operating above the motor's rated speed should be carefully reviewed. The NEMA MG-1 Standard gives typical overspeed capabilities of induction motors. Small motors can typically run at 200 percent speed, whereas large motors can typically run at 125 to 150 percent speed. The mechanical vibration of a system will increase as speed increases. The rotating equipment mounting, alignment and balance is more critical as the speed increases. Mechanical resonance may be present above rated speed. Some speeds (frequencies) may have to be skipped.

Multiple Motor Operation

Any number of motors can be connected in parallel across a single AC drive. Since the frequency of the motors will be the same, all motors will be operated at the same speed. With NEMA B motors, motor speed will be matched within 3 percent, depending on load variations. If it is necessary to have exact speed matching, synchronous AC motors need to be used. If an adjustable speed ratio is desired between the motors, individual AC drives need to be used.

The simplest multiple motor application is where all motors are started and stopped together and are permanently connected to the drive. In this case, it is easy to size the drive to provide an output current equal to the sum of the individual motors.

If motors are to be started and stopped separately, the highest intermittent current that will be required for the worst case combination of motors running and motors starting needs to be determined. Stopping individual motors may cause difficulty in certain situations. If two or more motors are to be mechanically coupled together, load-sharing requirements need to be considered. Individual motor overload protection must be provided when using a multiple motor application.

AC Drive Application

PWM and Vector AC drives are designed for use with any standard squirrel cage motor. Sizing the drive is simply a matter of matching the drive output voltage, frequency and current ratings to the motor ratings.

Most modern AC drives are designed for use with various voltages and frequencies. By adjusting the volts/hertz properly, almost any three-phase motor can be used.

AC drive full load currents are matched to typical full load motor current ratings as listed in the NEC Table 430-150. Usually an AC drive can be matched to an AC motor by their horsepower ratings; however, actual motor current required under operating conditions is the determining factor. If the motor will be run at full load, the drive current rating must be as high as the motor current rating. If the drive is to be used with multiple motors, the sum of all the full load current ratings must be used, and adding the horsepower ratings of the motors will usually not provide an accurate estimate of the drive needed.

Motor Protection

Motor overload protection needs to be provided as required by the applicable codes. Motor protection is not automatically provided as part of all AC drives. It may be provided as a standard feature on one model or it may be an optional feature on another.

The best means of motor protection is a direct winding over temperature protection such as an over temperature switch imbedded in the motor windings. Direct over temperature protection is preferred because overheating can occur at normal operating currents at low speeds.  Most AC drives are equipped with electronic overcurrent protection.

In multiple motor applications, individual motor overload protection needs to be provided even where electronic protection is provided by the drive. In some cases, short circuit protection may be required.

Motor Winding Damage

The voltage ouptut of AC drives contains voltage steps. In modern PWM drives, the dV/dt of a motor can cause large voltage spikes. Voltage spikes of 1,500 volts or more are typical for a 460 volt motor. This can cause the end windings of a Non-Inverter Duty or standard induction motor to fail. This problem gets worse as the cable length from the drive to the motor gets longer. Corrective action is normally required for cables longer than 150-ft.

Load side reactors, installed at the drive output terminals, will reduce the voltage spikes at the motor terminals. Most drive manufacturers have load side reactors available as an option.

AC Drive Performance

A means must be provided to start and stop the drive, and provide a speed reference. This can be accomplished with a simple run/stop switch and a speed potentiometer, or by more elaborate means. Most modern drives provide for control by a fieldbus, such as Modbus, DeviceNet or Ethernet-based protocols like Modbus/TCP and EtherNet/IP. Additional functions that may be required include reversing, lights or relays to indicate drive status, and meters to indicate operating speed, load, etc.

Speed range is usually determined by the characteristics of the motor, as the AC drive output frequency range is usually wider than the motor range.

Independently adjustable acceleration and deceleration rates are usually provided with a drive. Actual field conditions determine the optimum acceleration and deceleration rate of the drive.

As most AC drives to do not use encoder feedback, speed regulation is determined by the slip of the motor. Typical slip for a NEMA B motor provides for 3 percent regulation. Slip compensation circuits can be used to improve this to about 1.0 percent regulation. In extreme cases, where very close speed regulation is essential, a motor encoder can be supplied to give 0.01 percent speed regulation.

AC drives are equipped with current limit circuits. If current limit is not provided, the overcurrent trip circuits will shut you down in the event of an overload or attempting to accelerate too fast.

Regeneration Limit and Braking

During deceleration or in the event of an overhauling load, a motor will produce braking torque. When a motor produces braking torque, it is operating as an induction generator. In other words, this means that the drive is being fed power from the motor, and it cannot pass current back out to the line; this excess power is sent to the bus capacitors. If enough power is regenerated, the bus capacitors will charge to the trip level for the drive, causing the bus voltage to rise. If the voltage rises above a preset level, the drive will trip.

When the drive is provided with some type of dynamic braking circuit, it will allow the motor to produce rated torque as braking torque. A full regenerative drive will allow the drive to feed this excess power back onto the line.

Conclusion

AC drives have become popular because they provide an efficient, direct method to control the speed of squirrel cage motors, and provide a wide array of benefits for a range of applications. By employing AC drives, users are able to realize a host of benefits including:

• High degree of efficiency with low operating costs
• Minimized motor maintenance costs
• Controlled acceleration and deceleration
• Uses existing AC motors for adjustable speed operation

Pumps & Systems, April 2009