Editor's Note: This is the first part of a two part series of how adjustable frequency drives work. To read part two, click here.
In recent years, adjustable frequency AC drives have become increasingly popular as they provide an efficient, direct method of controlling the speed of the most rugged and reliable of prime movers, the squirrel cage motor. They provide a spectrum of benefits for a broad range of applications.
An adjustable frequency AC drive system consists of three basic components: an ordinary three-phase induction motor, an adjustable frequency drive to control the speed of the motor and an operator's control station.
The most common motor used with an AF drive system is a standard NEMA design B squirrel cage induction motor, rated for 230 or 460 volt, three-phase, 60-Hz operation.
The adjustable frequency controller is a solid state power conversion unit. It receives 240 or 480 volt, three-phase 60-Hz power and converts it to a variable frequency supply that can be seamlessly adjusted between 0- and 60-Hz. The controller also adjusts the output voltage in proportion to the frequency so as to provide a nominally constant ratio of voltage to frequency as required by the characteristics of the motor.
The operator's station provides him with the necessary controls for starting and stopping the motor and varying the motor speed. These functions can be performed with a wide variety of automatic control systems.
There are several classifications of adjustable frequency AC drives. Some types of drives include variable voltage input (VVI) or six step drives, current source input (CSI), pulse width modulated (PWM) drives, sensorless vector drives, field oriented drives and closed loop vector drives. The most common AC drives are PWM, sensorless vector and closed loop vector drives. (See Figures 1 through 4.)
Figure 1. A block diagram of a typical VVI drive. The AC/DC converter is an SCR bridge, which receives ac power from the input line and provides adjustable voltage dc power to the dc bus. A voltage regulator is required to preset the dc bus voltage to the level needed to provide the required output voltage amplitude to the motor. The inverter uses either SCRs or transistors as solid-stat switches to convert the dc power to a stepped waveform output. The amplitude of the dc bus voltage determines the amplitude of the output voltage.
Figure 2 shows a typical output voltage and current waveforms for a VVI inverter. The voltage waveform is normally referred to as a "six step" waveform.
Figure 3 is a block diagram of a typical PWM drive. It receives line voltage and converts it to a fixed dc voltage using a three-phase full wave diode bridge. Since the dc bus is a fixed voltage level, the amplitude of the output voltage is fixed. Modulating the output waveform using IGBT inverter switches controls the effective value of the output voltage.
Figure 4 shows the output voltage and current waveforms for the PWM inverter.
Induction Motor Speed Control
Standard induction motors (NEMA design B) have approximately 3 percent slip at full load. If the drive only controls the output frequency, the motor speed will deviate from the set speed due to slip. For many fan and pump applications, precise speed control is not needed. The motor slip can be:
- Ignored
- Compensated for by the drive based on motor current and a programmed speed-torque characteristic of the motor
- Compensated for by the control loop external to the drive. An example would be a pump where a certain flow rate is desired. The "flow control loop" tells the drive to either speed up or slow down to reach the desired flow. The actual speed of the pump has no importance.
Vector controlled drives need speed feedback of the rotor. For Sensorless Vector, the rotor speed is calculated based on a model of the motor stored in the drive. For Closed Loop Vector, a digital encoder is added to the motor to provide actual rotor speed. See Figure 5.
Figure 5 is drawn for a motor operating at a fixed frequency. Changing the frequency of the power applied to the motor changes the slip/torque curve. This figure shows a family of ideal speed/torque curves drawn for a motor operating from an adjustable frequency power source. The value of slip is constant at any given operating torque level, and the normal operating portions of the curves are a series of parallel lines. When a motor is operated from an AC drive, it normally never enters the dotted line portion of the curve.
Next month we will explore motor application performance, including what happens when operating above rated motor speeds or with multiple motors. We will conclude with AC drive applications and AC drive performance.
Pumps & Systems, March 2009
