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Variable Frequency Drives - Energy Efficiency Reference Guide

2 Classifications of Drives

a. AC Drives

Variable Frequency Drives

Electronic VFDs are speed control devices which vary the voltage and frequency to an induction motor using a technique called Pulse Width Modulation (PWM). VFDs have become the preferred way to achieve variable speed operation as they are relatively inexpensive and very reliable.

VFDs use power semiconductor devices called insulated-gate bipolar transistors (IGBT). Using PWM, the speed of the motor and torque characteristics can be adjusted to match the load requirements. They convert the fixed frequency AC supply voltage to a variable frequency, variable voltage AC supply to the motor and can regulate the speed of an induction motor from about 10% to 200% with wider ranges possible depending on the model and options selected.

The speed accuracy is affected by the slip of the motor, resulting in slightly slower operation than the synchronous speed for a given frequency. The accuracy can be increased greatly by using tachometer feedback. Extremely precise speed and position control of the motor shaft can be achieved by using a VFD with Vector Control.

The VFD can provide many solutions depending on the required application. For example, a VFD can provide the following:

  1. Energy savings on fan and pump applications
  2. Better process control and regulation
  3. Speeding up or slowing down a machine or process
  4. Inherent power-factor correction
  5. Bypass capability in the event of an emergency
  6. Protection from overload currents
  7. Safe acceleration.

b. Other AC Drives

Wound Rotor Motor Control

Wound rotor motors are a special type of induction motor with copper windings on the rotor rather than typical aluminum, squirrel cage rotor bars.

Connections to these windings are available through a slip ring assembly on the shaft.

If the windings are connected as a short circuit, the motor operates like a fixed speed squirrel cage motor, but if resistance is added to the circuit the slip of the motor increases thus allowing the speed of the motor to be adjusted.

The energy removed from the rotor circuit during the starting procedure is wasted in the resistors as heat.

As an alternative, an electronic circuit can be used instead of resistors, to reduce energy wasted. This circuit recovers the energy and feeds it back to the AC supply system, increasing the overall efficiency of the motor operation.

This motor control technique was once a popular method of speed control, but has largely being replaced by electronic VFDs.

Multi-Speed Motors

Multi-speed motors are induction motors with specially wound stators that allow the number of magnetic poles to be changed by reconnecting the windings of the motor.

Single winding multi-speed motors allow a speed ratio of 2:1. Pole changing is accomplished by reconnecting the windings which doubles the number of poles by reversing the current in each alternate coil group. This is known as consequent pole changing.

Two winding motors can be configured for any number of poles, so other speed ratios are possible. Three speeds are possible by configuring one of the two windings for consequent pole changing. Four speeds are possible by configuring each of the two windings for consequent pole changing.

Because two winding multi-speed motors have to accommodate a second set of windings, they are often larger for a given horsepower than their single speed counterparts.

Multi-speed motors are a relatively inexpensive option where fixed and limited discrete operating speeds are acceptable.

Variable Voltage Speed Controllers

These controllers typically use Silicon Controlled Rectifiers (SCRs) to control the voltage to the motor.

Under reduced voltage, a motor will “slip” more and thus its speed will be reduced.

This control scheme is generally limited to fan applications and requires a motor with high slip rotors.

The control is imprecise and has limited application to single phase Permanent Split Capacitor (PSC) motors. These are typically found in agricultural applications up to several HP.

Variable voltage speed controllers are no longer used in industrial and commercial applications.

c. DC Drives

Direct current (DC) motors are inherently variable speed machines. Speed and torque control is achieved by varying the armature voltage, the field excitation, or both.

Traditionally, speed control for a DC motor came from a motor-generator, or M-G set. In an M-G set, an AC motor drives a DC generator to provide variable voltage DC for motor operation. M-G sets are large, inefficient and require a lot of maintenance.

M-G sets have now been replaced with microprocessor controlled rectifier sets, which permit simple and accurate speed control, high efficiency and reliability.

However, due to the complexity, cost and maintenance associated with a DC motor, they are seldom used in new applications. Many existing DC drive applications are being replaced with AC motors and VFDs.

New applications using DC motors and drives are usually engineered applications where AC motors and drives cannot fulfill a load requirement. An example is for traction drives where the starting torque requirements exceed that available from AC motors.

d. Eddy Current Clutches

Eddy current clutches can be used to control standard AC squirrel-cage induction motors. However, they are low efficiency compared to VFDs and have limited applications.

An eddy current clutch has essentially three major components:

  • a steel drum directly driven by an AC motor,
  • a rotor with poles,
  • windings on the poles that provide the variable flux required for speed control.

A voltage is applied to the pole windings to establish a flux and thus relative motion occurs between the drum and its output rotor.

By varying the applied voltage, the amount of torque transmitted is varied and therefore the speed can be varied.

e. Advanced Motors

Advanced motors are generally classified as a group of motor designs which require power electronics and microprocessor control for operation. This concept was formulated many years ago, but is only now practical with today’s electronics. All advanced motors allow for variable speed operation.

They are now starting to be used in original equipment manufacturer applications, for instance as blower motors in high-end heat pumps and air compressors. Some advanced motors have become available as general purpose motors with ratings up to about 600 HP. Examples include Switched Reluctance motors, Electronic Commutated motors, and Written Pole motors.

f. Mechanical Speed Control

Variable speed operation of machines can be achieved by using a fixed speed motor with a mechanical speed control device. Examples include fluid couplings, adjustable pulley systems, magnetic speed control, and mechanical transmissions such as belt drives, chain drives, gear boxes, etc.

Mechanical methods of speed control require the motor to operate at a constant speed and the choice of coupling alters the speed for the applied load. The efficiency of the systems is dependant on a number of factors including belt tension, type and number of belts/chains, etc. Typical mechanical methods have constant and preset speeds which cannot be dynamically adjusted to variable loads. Mechanical speed control devices typically have low efficiencies at low loads.

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