Variable Speed Drives

Operating Principle

Description

For controlling the rotational speed of an alternating current (AC) electric motor, the system which is used is called as Variable Speed Drive. This system controls the electric motor by controlling the frequency of the electrical power that is supplied to the motor. The Variable Speed Drive is also known as Variable Frequency Drive (VFD), Adjustable Frequency Drive (AFD), AC Drive, Micro Drive or Inverter Drive. Along with the variations of frequency of electrical power, the voltage also varies. Therefore, this drive is also called as Variable Voltage Variable Frequency Drive (VVVFD).

Operating principle of Variable Speed Drives
The principle under which the variable speed drives work is “the frequency of the AC supply and number of poles in the stator winding, determine the contemporaneous speed of an AC motor.” The relationship is depicted as follows:

where

• f = AC power frequency (hertz)
• RPM = Revolutions per minute
• p = Number of poles (an even number)

The above equation determines the contemporaneous speed of the contemporaneous motors. The contemporaneous speed is slightly higher than the speed of the induction motor.

Description of Variable Speed drive

VSD motor
A three face induction motor is generally used in variable Speed Drive. Sometime, single face induction motor is also used, but their use is very rare. Different types of synchronous motors are useful in some specific situations, but the three face induction motors are the most suitable type of motors fir the variable speed drive. These motors are also advantageous from economical point of view. These three face induction motors are designed according to the fixed-speed mains voltage operation. In these motors, certain standardized enhancements are implemented, so that these motors can show higher reliability and standard performance for variable speed. First, second and third shift heads are arranged in a sequential order in a Gear Shift Mechanism of a manual transmission. For building first and second gear trains, the first shift head is joined to the first synchronizer, and for building the third and fourth gear trains, the second shift head in connected to the second synchronizer. To build the reverse gear train, the third shift head is being connected (operationally) to a shift fork. For short timing operation, the gear-shift mechanism is arranged to operate the first synchronizer in shifting operation. This shifting operation helps in establishment of the reverse gear train.

VSD controller
Variable Speed Drive controllers are the electronic power conversioning devices and these devices are available in the solid state. The conventional design of such VSD controllers is as follows:

• By using a rectifier bridge, convert the AC input power into DC intermediate power. The rectifier is usually a three-phase diode bridge.
• In this step, by using the inverter switching circuit, the DC intermediate power is again converted into quasi-sinusoidal AC power.
• In most of the Variable Speed Drive inverter circuits, a new kind of semi-conductor switches are used. These switches are called as Insulated Gate Bipolar Transistors (IGBTs).
• Since, the AC power has been converted into DC power, many drives present inside the drive, will accept both three-phase and single phase input power. These units act like the speed controllers as well as phase converters. But, one should remember that single phase input cannot be used as sole part of the rectifier bridge without the participation of the three-phase input.
• The total operations of the VSD controller is monitored by an embedded microprocessor.

VSD operator interface
In starting and stopping the motor and also in the adjustment of the motor-speed, the operator interface is required. From an external process control signals, switching and reversion between the automatic control and manual speed adjustment are the additional operator controlling functions. In order to provide informations regarding the operations of the drive, there is an alphanumeric display or indication lights and meters are present on the VSD operator interface. On the front of VSD controller, a display unit and an operator interface keyboard are always present. To configure the VSD, a serial communication port is also present on the VSD operator interface.

Existing Power Ratings of Variable Speed Drives
The Variable Speed Drives are available in different current and voltage ratings in the market. These ratings are provided, because accordingly suitable 3-face induction motors are manufactured. The controllers of variable speed drive are designed in between of 110 to 690 volts. Controllers of this voltage range are considered as low voltage controllers. For these low voltage controllers, the 3-phase induction motors which are used, belong in the category of 0.2kW or 1/4 horsepower (Hp) to 750kW or 1000Hp. The voltage power for the medium range variable speed drives (VSD) comes in the following categories: 400/4160 volts(60Hz), 3300 volts(50Hz) or up to 10kV. Sometime, a transformer is placed in between of a medium voltage load and a low voltage drive. Medium voltage VSDs are designed specifically for such motors having the capacity of either 375kW or 500Hp and above.