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Alternating current motor drives

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Many applications require variable-speed motors. The easiest way to vary the speed of an Alternating current (AC) induction motor is to use an AC drive to vary the applied frequency. AC drives commonly are known as variable frequency drives (VFDs). VFDs are microprocessor-based controllers that incorporate an electronic control section, an electromagnetic and semiconductor power section, and typical components that are used with standard motor controllers. VFDs can provide voltage to motors at frequencies of from < 1 Hz to approximately 120 Hz. Currently, they are available for motors ranging from 0.33 hp to thousands of horsepower. Operating a motor at other than the rated frequency and voltage affects motor current and torque. The following sections provide further discussion on this subject.

Volts per hertz ratio

The output torque for a motor is determined on the basis of the ratio of the motor’s applied voltage and applied frequency, known as the volts per hertz (V/Hz) ratio. A typical AC motor manufactured for use in the U.S. is rated for 460 VAC and 60Hz, and thus has a 7.67 V/Hz ratio. Failure to maintain the proper V/Hz ratio will affect motor torque, temperature, speed, noise, and current draw.

For example, increasing the frequency without increasing the voltage will cause an increase in speed and a decrease in air-gap flux density. The air-gap flux density decrease causes motor torque to decrease because torque is directly proportional to the magnetic flux density in the motor’s air gap.

Thus, for a motor to produce its rated torque at variable speeds, it also is necessary to control the voltage and frequency supplied to the motor. A VFD maintains a preset V/Hz ratio in supplying power to a motor at the variable speeds.

Constant torque load

AC motors running on an AC line operate with a constant flux because voltage and frequency are constant. Motors operated with constant flux are said to have constant torque. An AC drive can operate a motor with a constant flux of from zero to the motor’s rated nameplate frequency (typically 60 Hz), which is the constant-torque range. As long as a constant V/Hz ratio is maintained, the motor will generate constant torque. AC drives change the frequency to vary the speed of the motor and change voltage proportionately to maintain constant flux. The V/Hz ratio can be kept constant for any speed up to 60 Hz. See Fig. 1.

Some examples of constant torque loads are:

  • Conveyors
  • Positive-displacement pumps
  • Extruders
  • Hydraulic pumps
  • Packaging machinery

Constant horsepower load

Some applications require a motor to be operated at above base speed. Such applications need less torque at higher speeds, yet require voltage to be no higher than the rated nameplate voltage because the motor insulation deteriorates exponentially at higher-than-rated voltage. VFDs are designed to maintain a constant V/Hz ratio and torque up to 60 Hz. As Table 1 shows, the V/Hz ratio decreases at above 60 Hz because VFDs are designed to maintain constant voltage above 60 Hz. When the V/Hz ratio decreases, the air-gap flux decreases, causing a decrease in the torque. Because the motor horsepower is directly proportional to the torque and speed of the motor, it remains constant while torque decreases in proportion to the increase in frequency. As such, a motor that is operated above its rated frequency is operating in a region known as constant horsepower (see Fig. 1).

Reduced voltage and frequency starting

A National Electrical Manufacturers Association (NEMA) B motor that is started by connecting it to the power supply at full voltage and full frequency will develop approximately 150% starting torque and 600% starting current (Fig. 2). The same motor started with a VFD at reduced voltage and frequency develops approximately 150% torque and current. Fig. 3 shows that the torque/speed curve shifts to the right as frequency and voltage are increased. The dotted lines on the torque/speed curve represent the portion of the curve not used by the drive. The drive starts and accelerates the motor smoothly as frequency and voltage are gradually increased to the desired speed. A VFD drive that is properly sized to a motor is capable of delivering 150% torque at any speed up to the speed that corresponds to the incoming line voltage.

Some applications require a starting torque > 150%. A conveyor, for example, might require a 200% rated torque for starting. If a motor is capable of 200% torque at 200% current, and the drive is capable of 200% current, then 200% motor torque is possible. Typically, drives are capable of producing 150% of the drive nameplate rated current for 1 minute. A load that needs more starting torque than a drive can deliver requires a drive with a higher current rating. It is appropriate to supply a drive with a higher continuous horsepower rating than the motor when high peak torque is required.

Selecting a motor

AC drives often have more capability than the motor. Drives can run at higher frequencies than might be suitable for an application. At frequencies above 60 Hz, for example, the V/Hz ratio decreases and the motor cannot develop 100% torque. Drives also can run at lower speeds than might be suitable. For example, a self-cooled motor might not develop enough air flow for cooling at reduced speeds and full load. Each motor must be evaluated according to its own capability before selecting it for use on an AC drive.

Harmonics, voltage spikes, and voltage rise times of AC drives are not identical. Some AC drives have more sophisticated filters and other components that are designed to minimize undesirable heating and insulation damage to the motor. This must be considered when selecting an AC drive/motor combination. Motor manufacturers generally will classify certain recommended motor selections on the basis of experience, required speed range, type of load torque, and temperature limits.

Distance between drive and motor

The distance between the drive and the motor must also be considered. All motor cables have line-to-line and line-to-ground capacitance. The longer the cable, the greater the capacitance. Some types of cables (e.g., shielded cable or cables in metal conduit) have greater capacitance. The charging current in the cable capacitance causes spikes in the output of AC drives; higher voltage and higher capacitance cause higher current spikes. Voltage spikes caused by long cable lengths can shorten the life of the AC drive and motor. When considering an application in which distance might be a problem, contact the VFD manufacturer for its recommendations.

Service factor on AC drives

A high-efficiency motor with a 1.15 service factor is recommended when used on an AC drive. The 1.15 service factor is reduced to 1.0 because of heat associated with harmonics of an AC drive.

References

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See also

Electrical systems

Electrical distribution systems

Alternating current motors

Induction motors

Synchronous motor

Motor specifications

NEMA motor characteristics

Motor enclosures

PEH:Electrical Systems