Speed Control of AC Induction Motors

Traditionally, DC motors have been employed when variable speed capability was desired. By controlling the armature (rotor) voltage and field current of a separately excited DC motor, a wide range of output speeds can be obtained.

 DC motors are available in a wide range of sizes, but their use is generally restricted to a few low speed, low-to-medium power applications like machine tools and rolling mills because of problems with mechanical commutation at large sizes. Also, they are restricted for use only in clean, non-hazardous areas because of the risk of sparking at the brushes. DC motors are also expensive relative to AC motors. Because of the limitations of DC systems, AC motors are increasingly the focus for variable speed applications. 

Both AC synchronous and induction motors are suitable for variable speed control. Induction motors are generally more popular, however, because of their ruggedness and lower maintenance requirements. 

AC induction motors are inexpensive (half or less of the cost of a DC motor) and also provide a high power to weight ratio (about twice that of a DC motor). An induction motor is an asynchronous motor, the speed of which can be varied by changing the supply frequency. The control strategy to be adopted in any particular case will depend on a number of factors including investment cost, load reliability and any special control requirements. Thus, for any particular application, a detailed review of the load characteristics, historical data on process flows, the features required of the speed control system, the electricity tariffs and the investment costs would be a prerequisite to the selection of a speed control system. 

The characteristics of the load are particularly important. Load refers essentially to the torque output and corresponding speed required. Loads can be broadly classified as either constant power or constant torque. Constant torque loads are those for which the output power requirement may vary with the speed of operation but the torque does not vary. 

Conveyors, rotary kilns, and constant-displacement pumps are typical examples of constant torque loads. Variable torque loads are those for which the torque required varies with the speed of operation. Centrifugal pumps and fans are typical examples of variable torque loads (torque varies as the square of the speed). Constant power loads are those for which the torque requirements typically change inversely with speed. 

Machine tools are a typical example of a constant power load. The largest potential for electricity savings with variable speed drives is generally in variable torque applications, for example centrifugal pumps and fans, where the power requirement changes as the cube of speed. Constant torque loads are also suitable for VSD application.

Motor Speed Control Systems
Multi-speed motors
Motors can be wound such that two speeds, in the ratio of 2:1, can be obtained. Motors can also
be wound with two separate windings, each giving 2 operating speeds, for a total of four speeds.
Multi-speed motors can be designed for applications involving constant torque, variable torque, or
for constant output power. Multi-speed motors are suitable for applications, which require limited
speed control (two or four fixed speeds instead of continuously variable speed), in which cases
they tend to be very economical. They have lower efficiency than single-speed motors.

Adjustable Frequency AC Drives
Adjustable frequency drives are also commonly called inverters. They are available in a range of
kW rating from fractional to 750 kW. They are designed to operate standard induction motors.
This allows them to be easily added to an existing system. The inverters are often sold separately
because the motor may already be in place. If necessary, a motor can be included with the drive
or supplied separately.
The basic drive consists of the inverter itself which coverts the 50 Hz incoming power to a
variable frequency and variable voltage. The variable frequency is the actual requirement, which
will control the motor speed.
There are three major types of inverters designs available today. These are known as Current
Source Inverters (CSI), Variable Voltage Inverters (VVI), and Pulse Width Modulated Inverters
(PWM).

Wound Rotor AC Motor Drives (Slip Ring Induction Motors)
Wound rotor motor drives use a specially constructed motor to accomplish speed control. The
motor rotor is constructed with windings which are brought out of the motor through slip rings on
the motor shaft. These windings are connected to a controller which places variable resistors in
series with the windings. The torque performance of the motor can be controlled using these
variable resistors. Wound rotor motors are most common in the range of 300 HP and above.