Pumps and Systems, April 2009
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
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
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
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:
Figure 9. Speed vs. AC Drive + Motor Torque