AC Drives FAQ (part 3): Variable-Frequency Drives and Safety-Interlock Circuits

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What are the general rules for safeguarding machinery incorporating motors driven by VFDs (variable frequency drives)? For example, should the contactors be on the input or the output?

A contactor on the input of the VFD rather than the output is preferred, although on can make the case for one on the output.

Putting a contactor on the output of the VFD will assure immediate removal of voltage from the motor which is what you want. On the other hand, some drives are easily damaged by switching on their output and that it's possible that the motor could be reconnected to the output of a drive that was operating above zero frequency and this could also damage the drive. (Effectively a direct (non-soft) or full-voltage start on the output of a VFD)

Placing the contactor on the input of the drive would shut off all power to the motor, but there could be a small delay due to the energy stored in the capacitors in the VFD power supply. This would be safe, would not damage the drive and would prevent direct-online switching on the output of the VFD. The drive would always start from zero when the contactor was re-closed. You may have to reset an under-voltage trip on the drive, but that may be automated if required. You may also use soft starts between the VFD and the motor.

(cont...)


Electric Motors + Motor-Control Systems
(Nested-content Series)

Electric Motors

Alternating Current Motor Drives

AC drives connect to standard AC induction motors, and have capabilities of adjustable speed, torque, and horsepower control similar to those of DC drives. Adjustable-speed drives have made AC squirrel-cage induction motors as controllable and efficient as their DC counterparts. AC induction motor speed depends on the number of motor poles and the frequency of the applied power. The number of poles on the stator of the motor could be increased or decreased, but this has limited usefulness. Although the AC frequency of the power source in the United States is fixed at 60 Hz, advances in power electronics make it practical to vary the frequency and resulting speed of an induction motor.

Ill. 54 AC motor drive. Rockwell Automation, rockwellautomation.com.

Ill. 55 Variable-frequency drive controller. Computer Controls Corporation, www.versadrives.com. Three-phase power input; Converter DC filter Inverter;

Ill. 56 The ratio of volts per hertz is regulated to a constant value. Motor output waveforms;

Variable-Frequency Drive

A variable-frequency drive (VFD) system, also known as a variable-speed drive system, generally consists of an AC motor, a controller, and an operator interface.

Three-phase motors are usually preferred, but some types of single-phase motors can be used. Motors that are designed for fixed-speed main voltage operation are often used, but certain enhancements to the standard motor designs offer higher reliability and better VFD performance. A simplified diagram of a VFD controller is shown in Ill. 55. The three major sections of the controller are as follows: Converter -Rectifies the incoming three-phase AC power and converts it to DC.

DC filter (also known as the DC link or DC bus)- Provides a smooth, rectified DC voltage.

Inverter -Switches the DC on and off so rapidly that the motor receives a pulsating voltage that is similar to AC. The switching rate is controlled to vary the frequency of the simulated AC that is applied to the motor.

AC motor characteristics require the applied voltage to be proportionally adjusted by the drive whenever the frequency is changed. E.g., if a motor is designed to operate at 460 Volts at 60 Hz, the applied voltage must be reduced to 230 Volts when the frequency is reduced to 30 Hz. Thus the ratio of volts per hertz must be regulated to a constant value (460/60 = 7.67 in this case). The most common method used for adjusting the motor voltage is called pulse width modulation (PWM). With PWM voltage control, the inverter switches are used to divide the simulated sine-wave output waveform into a series of narrow voltage pulses and modulate the width of the pulses.

With a standard AC across-the-line motor starter, line voltage and frequency are applied to the motor and the speed is solely dependent on the number of motor stator poles. In comparison, an AC motor drive delivers a varying voltage and frequency to the motor, which determines its speed. The higher the frequency sup plied to the motor, the faster it will run. Power applied to the motor through the drive can lower the speed of a motor below the nameplate base speed, or increase the speed to synchronous speed and higher. Motor manufacturers list the maximum speed at which their motors can safely be operated.

Ill. 57 AC motor starter and drive control. Fixed line voltage and frequency; Fixed line voltage and frequency; Line voltage and frequency; Varying voltage and frequency; Base speed; Speed setting; Motor; RPM; Two poles-3,600 rpm Four poles-1,800 rpm

Ill. 58 Inverter-duty AC induction motor. Baldor Electric Company. baldor.com.

Inverter Duty Motor

Inverter duty and vector duty describe a class of AC induction motors that are specifically designed for use with variable-frequency drives . The high switching frequencies and fast voltage rise times of AC motor drives can produce high voltage peaks in the windings of standard AC motors that exceed their insulation break down voltage. Also, operating motors for an extended time at low motor rpm reduces the flow of cooling air, which results in an increase in temperature. NEMA-rated inverter- or vector-duty motors use high-temperature insulating materials that can withstand higher voltage spikes and operating temperatures. This reduces the stress on the insulation system.

AC motors frequently drive variable loads such as pumps, hydraulic systems, and fans. In these applications, motor efficiency is often poor due to operation at low loads and can be improved by using a VFD in place of speed controllers such as belts and pulleys, throttle valves, fan dampers, and magnetic clutches. E.g., a pump or fan, controlled by a variable speed drive, running at half-speed consumes only one-eighth of the energy com pared to one running at full speed, resulting in consider able energy savings.

QUIZ:

1. List the three basic sections of an AC variable frequency drive controller and state the function of each section.

2. An induction motor rated for 230 Volts at 60 Hz is to be operated by a VFD. When the frequency is reduced to 20 Hz, to what value must the voltage be reduced in order maintain the ratio of volts per hertz?

3. How does an AC drive vary the speed of an induction motor?

4. Inverter or vector duty AC induction motors are the types most often specified for use with variable frequency drives. Why?

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There are other limits of installing a contactor on the input side, including possible damage to the DC bus capacitor pre-charge circuit. The pre-charge circuit will be designed for a certain amount of operations/hour. Is it acceptable contactor(s) to use late-make/early-break auxiliary contact on the output, wired back to the drive?

The early-break contact should break around 500ms before the main legs to be effective. Typically-available 10ms early-break aux-contact isolators are ineffective as the residual motor field will not have decayed, resulting in an inductive kick 10ms later when the 3 main legs open. Pffff! goes the VFD output! It is unclear in this situation what a suitable isolator could be. The only practical solution is a lockable isolator, the expectation being that the person with the key understands the need for care.

If one is expecting a lot of switching on the input, then this could be a problem. If an early-break contact on the contactor is used to panic-stop the drive first, then that may be okay. It depends on the timing and also how the drive responds to this sort of treatment. Some are more tolerant than others. Consult the manufacturers of the drive.

In the case of an emergency, nothing can stop the motor faster than the drive itself (most drives having an E-Stop input anyway). How the VFD handles the emergency condition is usually a programmable function: ramp down at max or coast. This assumes, of course, that the drive is undamaged, one would be wise to tie in a line or load contactor to the NO (normally open) output of the drive-status relay. If it's an emergency, break it on the load side (drive output). Nobody at the inquiry is going to sympathize with your concerns to pamper the thyristors when weighed against the potential risk to human life.

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