AC drives, such as the one shown in Figure 5-54, connect to standard AC induction motors, and have capabilities of adjustable speed, torque, and horsepower control similar to those of DC drives. Adjustable-speed drives have made AC squirrel-cage induction motors as controllable and efficient as their DC counterparts. AC induction motor speed depends on the number of motor poles and the fre- quency of the applied power. The number of poles on the stator of the motor could be increased or decreased, but this has limited usefulness. Although the AC frequency of the power source in the United States is fixed at 60 Hz, advances in power electronics make it practical to vary the frequency and resulting speed of an induction motor.
Variable-Frequency Drive
A variable-frequency drive (VFD) system, also known as a variable-speed drive system, generally consists of an AC motor, a controller, and an operator interface.
Three-phase motors are usually preferred, but some types of single-phase motors can be used. Motors that are designed for fixed-speed main voltage operation are often used, but certain enhancements to the standard motor designs offer higher reliability and better VFD
Figure 5-54 AC motor drive.
Photo courtesy Rockwell Automation, www.rockwellautomation.com.
performance. A simplified diagram of a VFD controller is shown in Figure 5-55. The three major sections of the controller are as follows:
Converter —Rectifies the incoming three-phase AC power and converts it to DC.
Reversing switch
Variable-speed trigger
Figure 5-53 Motor speed and direction controls.
observed when you apply varying pressure on an uni- versal drill motor.
Both the speed and direction of rotation of a univer- sal motor can be controlled, as illustrated in Figure 5-53.
Reversing is accomplished just as in a DC series motor by reversing the current flow through the armature with respect to the series field. Varying the voltage that is applied to the motor controls the speed.
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PART 5 Alternating Current Motor Drives 115 adjusting the motor voltage is called pulse width modu- lation (PWM). With PWM voltage control, the inverter switches are used to divide the simulated sine-wave out- put waveform into a series of narrow voltage pulses and modulate the width of the pulses.
With a standard AC across-the-line motor starter, line voltage and frequency are applied to the motor and the speed is solely dependent on the number of motor stator poles (Figure 5-57). In comparison, an AC motor drive delivers a varying voltage and frequency to the motor, which determines its speed. The higher the frequency sup- plied to the motor, the faster it will run. Power applied to the motor through the drive can lower the speed of a motor below the nameplate base speed, or increase the speed to synchronous speed and higher. Motor manufacturers list
DC fi lter (also known as the DC link or DC bus)—
Provides a smooth, rectified DC voltage.
Inverter —Switches the DC on and off so rapidly that the motor receives a pulsating voltage that is similar to AC. The switching rate is controlled to vary the fre- quency of the simulated AC that is applied to the motor.
AC motor characteristics require the applied voltage to be proportionally adjusted by the drive whenever the frequency is changed. For example, if a motor is designed to operate at 460 Volts at 60 Hz, the applied voltage must be reduced to 230 Volts when the frequency is reduced to 30 Hz, as illustrated in Figure 5-56. Thus the ratio of volts per hertz must be regulated to a constant value (460/60 = 7.67 in this case). The most common method used for
Figure 5-56 The ratio of volts per hertz is regulated to a constant value.
Volts
Frequency Volts/Hz = 7.67
0 30 60
460
230 7.67
V/Hz
Current 0
Motor output waveforms
Voltage 0
Figure 5-55 Variable-frequency drive controller.
Photo courtesy Computer Controls Corporation, www.versadrives.com.
Motor Three-phase
power input L1 L2 L3
T1 T2
T3 DC
bus Converter
⫹
⫺
DC filter Inverter
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116 Chapter 5 Electric Motors
Figure 5-58 Inverter-duty AC induction motor.
© Baldor Electric Company. Reprinted with their permission. Photo Baldor, www.baldor.com.
drives can produce high voltage peaks in the windings of standard AC motors that exceed their insulation break- down voltage. Also, operating motors for an extended time at low motor rpm reduces the flow of cooling air, which results in an increase in temperature. NEMA-rated inverter- or vector-duty motors use high-temperature insu- lating materials that can withstand higher voltage spikes and operating temperatures. This reduces the stress on the insulation system.
AC motors frequently drive variable loads such as pumps, hydraulic systems, and fans. In these applica- tions, motor efficiency is often poor due to operation at low loads and can be improved by using a VFD in place of speed controllers such as belts and pulleys, throttle valves, fan dampers, and magnetic clutches. For example, a pump or fan, controlled by a variable speed drive, running at half-speed consumes only one-eighth of the energy com- pared to one running at full speed, resulting in consider- able energy savings.
the maximum speed at which their motors can safely be operated.
Inverter Duty Motor
Inverter duty and vector duty describe a class of AC induc- tion motors that are specifically designed for use with variable-frequency drives (Figure 5-58). The high switch- ing frequencies and fast voltage rise times of AC motor
Figure 5-57 AC motor starter and drive control.
Motor starter
Operator interface L1 L2 L3
T1 T2 T3
Line voltage and frequency
Two poles—3,600 rpm Four poles—1,800 rpm
Motor drive L1 L2 L3 Fixed line voltage
and frequency
Fixed line voltage and frequency
0 0 0 0 Operator interface
Varying voltage
and frequency Base
speed Speed setting Motor
RPM
⫺ ⫹
T1 T2 T3
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