Variable-Frequency Mechanical Drives

GOALS:

• Describe the operating principle of variable-frequency drives.

• State why variable-frequency drives are used.

• State the advantage of variable-frequency drives.

The speed of an AC squirrel cage induction motor depends on the frequency of the supply current and the number of poles of the motor:

RPM = 60 x Hertz / Pairs of Poles

A frequency changer may be used to vary the speed of this type of motor.

Several methods can be used for one or more motors to obtain variable frequency from a fixed-frequency power source. A common method is to drive an alternator through an adjustable mechanical speed drive. The voltage is regulated automatically during frequency changes.

In this system, an AC motor drives a variable cone pulley, or sheave, which is belted to another variable pulley on the output shaft, ill. 1.When the relative diameters of the two pulleys are changed, the speed between the input and the output can be controlled.

As the alternator speed is varied, the frequency varies. In cases where more than one motor is involved, such as in conveyor systems, all of the motors are connected electrically to the power system of the alternator. These motors are generally called slave motors. Each motor thus receives the same frequency and operates at the same speed. If a high-speed motor and a slow speed gear motor are connected to the same variable-frequency circuit, they will change speeds simultaneously and proportionally, ill. 2. To obtain smooth acceleration of the motors attached to the alternator, the frequency is increased. Electric braking with suit able controls can be used in this system.

ill. 1 Variable pitch pulleys; mechanical method of obtaining continuously adjustable speed from constant speed shaft.

ill. 2 Screw conveyors may be controlled individually by variable drive slave motors. Once the motors are set, they rotate at the same proportional rpm, controlled by the variable-frequency power unit supplying them.

ill. 3 Variable power unit used to control one or more motors at coordinated speeds.

ill. 4 Coordinating many drive motors, all of standard AC, squirrel cage construction.

Variable-frequency power drives are used on many different types of conveyor systems, ill. 3. Assembly lines and coordinated screw conveyors for the proportional mixing of ingredients make use of these power drives, ill. 2 and ill. 4. Variable pitch systems are among the lowest-cost methods of achieving variable speed, both in initial cost and maintenance. Remote control isn't an inherent feature. Because the drive uses mechanical means to vary the speed, electrical control signals must be adapted to existing mechanical controls.

QUIZ:

1. What is the formula for finding the speed of an AC squirrel cage induction motor?

2. If several motors driving an overhead conveyor system are connected to a variable frequency supply line and the frequency is changed from 40 hertz to 80 hertz, what effect does this have on the speed of the motors?

3. How is the AC frequency controlled in an alternator?

4. If many feeder conveyor motors of different poles (speed) are connected to a main conveyor and to a variable-frequency power unit, can increased production be achieved by increasing the frequency of the power unit? Explain your answer.

5. What is a slave motor?