Variable Frequency Drives

Introduction

What Is a Variable Frequency Drive?

Adding a variable frequency drive (VFD) to a motor-driven system can offer potential energy savings in a system in which the loads vary with time. VFDs belong to a group of equipment called adjustable speed drives or variable speed drives. (Variable speed drives can be electrical or mechanical, whereas VFDs are electrical.) The operating speed of a motor connected to a VFD is varied by changing the frequency of the motor supply voltage. This allows continuous process speed control.

Motor-driven systems are often designed to handle peak loads that have a safety factor. This often leads to energy inefficiency in systems that operate for extended periods at reduced load. The ability to adjust motor speed enables closer matching of motor output to load and often results in energy savings.

How Does a VFD Work?

Induction motors, the workhorses of industry, rotate at a fixed speed that is determined by the frequency of the supply voltage. Alternating current applied to the stator windings produces a magnetic field that rotates at synchronous speed. This speed may be calculated by dividing line frequency by the number of magnetic pole pairs in the motor winding. A four-pole motor, for example, has two pole pairs, and therefore the magnetic field will rotate 60 Hz / 2 = 30 revolutions per second, or 1800 rpm. The rotor of an induction motor will attempt to follow this rotating magnetic field, and, under load, the rotor speed "slips" slightly behind the rotating field. This small slip speed generates an induced current, and the resulting magnetic field in the rotor produces torque.

Since an induction motor rotates near synchronous speed, the most effective and energy-efficient way to change the motor speed is to change the frequency of the applied voltage. VFDs convert the fixed-frequency supply voltage to a continuously variable frequency, thereby allowing adjustable motor speed.

A VFD converts 60 Hz power, for example, to a new frequency in two stages: the rectifier stage and the inverter stage. The conversion process incorporates three functions:

• Rectifier stage: A full-wave, solid-state rectifier converts three-phase 60 Hz power from a standard 208, 460, 575 or higher utility supply to either fixed or adjustable DC voltage. The system may include transformers if higher supply voltages are used.
  
  
• Inverter stage: Electronic switches - power transistors or thyristors - switch the rectified DC on and off, and produce a current or voltage waveform at the desired new frequency. The amount of distortion depends on the design of the inverter and filter.
  
  
• Control system: An electronic circuit receives feedback information from the driven motor and adjusts the output voltage or frequency to the selected values. Usually the output voltage is regulated to produce a constant ratio of voltage to frequency (V/Hz). Controllers may incorporate many complex control functions.

Converting DC to variable frequency AC is accomplished using an inverter. Most currently available inverters use pulse width modulation (PWM) because the output current waveform closely approximates a sine wave. Power semiconductors switch DC voltage at high speed, producing a series of short-duration pulses of constant amplitude. Output voltage is varied by changing the width and polarity of the switched pulses. Output frequency is adjusted by changing the switching cycle time. The resulting current in an inductive motor simulates a sine wave of the desired output frequency (see Figure 1). The high-speed switching of a PWM inverter results in less waveform distortion and, therefore, lower harmonic losses.

The availability of low-cost, high-speed switching power transistors has made PWM the dominant inverter type.

Figure 1. Inverter's Pulse Width Modulation Output

Applications

Variable speed drives are used for two main reasons:

• to improve the efficiency of motor-driven equipment by matching speed to changing load requirements; or
• to allow accurate and continuous process control over a wide range of speeds.

Motor-driven centrifugal pumps, fans and blowers offer the most dramatic energy-saving opportunities. Many of these operate for extended periods at reduced load with flow restricted or throttled. In these centrifugal machines, energy consumption is proportional to the cube of the flow rate. Even small reductions in speed and flow can result in significant energy savings. In these applications, significant energy and cost savings can be achieved by reducing the operating speed when the process flow requirements are lower.

In some applications, such as conveyers, machine tools and other production-line equipment, the benefits of accurate speed control are the primary consideration. VFDs can increase productivity, improve product quality and process control, and reduce maintenance and downtime. Decreasing cost and increasing reliability of power semiconductor electronics are reasons that VFDs are increasingly selected over DC motors or other adjustable speed drives for process speed control applications.

Motors and VFDs must be compatible. Consult the manufacturers of both the VFD and the motor to make sure that they will work together effectively. VFDs are frequently used with inverter-duty National Electrical Manufacturers Association (NEMA) design B squirrel cage induction motors. (Design B motors have both locked rotor torque and locked rotor current that are normal.) De-rating may be required for other types of motors. VFDs are not usually recommended for NEMA design D motors because of the potential for high harmonic current losses. (Design D motors are those that have high locked rotor torque and high slip.)

Additional Benefits of VFDs

In addition to energy savings and better process control, VFDs can provide other benefits:

• A VFD may be used for control of process temperature, pressure or flow without the use of a separate controller. Suitable sensors and electronics are used to interface the driven equipment with the VFD.
• Maintenance costs can be lower, since lower operating speeds result in longer life for bearings and motors.
• Eliminating the throttling valves and dampers also does away with maintaining these devices and all associated controls.
• A soft starter for the motor is no longer required.
• Controlled ramp-up speed in a liquid system can eliminate water hammer problems.
• The ability of a VFD to limit torque to a user-selected level can protect driven equipment that cannot tolerate excessive torque.

Next: How Much Will I Save?