Variable Frequency Drives

A Variable Frequency Drive is a motor control device that protects and controls the speed of AC induction motors. From the VFD basics theory, we know a VFD can control the speed of the motor during the start and stop cycle, as well as throughout the running cycle by outputting adjustable frequency. It also refers to as Variable Speed Drives (VSD), Adjustable Speed drive (ASD) and frequency inverter.

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Variable Frequency Drive

Rated Current (amps)
Single phase 220V - 240V 50Hz/60Hz input, three phase output.
Three phase 380V - 440V, 50 / 60Hz input, three phase output

How does VFD work?

Both three phase and single phase VFDs can convert input power to adjustable frequency and voltage source for controlling speed of AC induction motors. The frequency of the power applied to an AC motor determines the motor speed, based on the equation:
N = 120 x ƒ / p
whereHow does VFD work
N = speed (rpm).
f = frequency (Hz).
p = number of motor poles.

For example, a four-pole motor is operating at 60 Hz. These values can be inserted into the formula to calculate the speed:
N = 120 x 60 / 4
N = 1800 (rpm)
AC supply comes from the facility power network (typically 480 V, 60 Hz AC) while the rectifier converts network AC power to DC power. The filter and DC bus work together to smooth the rectified DC power and to provide clean, low-ripple DC power to the inverter (see Fig. 1: The function of a standard VFD, now there is new technology in vector VFD).
pulse width modulation waveform
The latter uses DC power from the DC bus and filter to invert an output that resembles sine wave AC power using a pulse width modulation (PWM) technique which switches the inverter semiconductors in varying widths and times that, when averaged, create a sine waveform (see Fig. 2: A pulse-width modulated waveform).

VFD Energy savings

The benefits of VFDs are numerous and they offer the greatest energy savings for varies applications, the most common is VFD for pumps and fans. The adjustable flow method changes the flow curve and drastically reduces power requirements.

Centrifugal equipment such as fans, pumps, and compressors follow a general set of speed affinity laws. The affinity laws define the relationship between speed and the variables flow, pressure and power.

Based on the affinity laws, flow changes linearly with speed while pressure is proportional to the square of speed. The power required is proportional to the cube of the speed. The latter is most important, because if the motor speed drops, the power drops by the cube (see Figs. 3 to 5).
VFD flow and speed relationship VFDs pressure and speed relationship VFD power and speed relation

In this example, the motor is operated at 80% of the rated speed. This value can be inserted into the affinity laws formula to calculate the power at this new speed:


Therefore, the power required to operate the fan at 80% speed is half the rated power (see Figs. 6 and 7).
vfd system flow and pressure relationshipVFDs system flow and power relationship

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Find valuable articles, white paters and tutorials below:
Circuit breakerIf the variable frequency drive (motor and supply cabling) is protected by fuses, we normally assume that the fuses will have sufficient interruption capacity to act as a protection device in their own right, without any additional circuit breaker.

If the VFD is protected through a circuit breaker (mechanical contacts, controlled by some bi-metallic device, or an over-current relay), the contacts of this circuit breaker must be rated to withstand breaking of the highest current that can occur under operation of the VFD. This means, that it must typically be rated to interrupt the full short-circuit current that can occur on the busbars on the SUPPLY side of the breaker. The contact rating of this device may have to be many times the maximum possible current for the VFD itself. This value is not dependent on any rating of the VFD itself, except in the very few situations where it may have been installed at the motor end of the cable supplying the VFD. In this case, there must be some additional protection for the cable, at the supply point.
There are two ways in tension control, one way is to control the output torque of the motor, the other way is to control the motor speed. Variable frequency drive (VFD) open-loop control mode is complied with the first way, which doesn't need tension feedback. The term, "open-loop", means there is no tension feedback signal to the VFD, the VFD control the output frequency or torque to achieve the control purpose, and with no relation whether there is an encoder or not. Torque control mode means the VFD controls the motor torque instead of the frequency, the output frequency is automatically changed according to the speed.
VFD open loop tension control
Placing a proper size variable frequency drive near motor instead of in central control panels allows for much faster servicing. It also accelerates initial installations, so the system is productive sooner. Decentralized intelligence, the practice of putting motion control into VFDs instead of in central PLCs, results in simpler PLC programs that interface with smart VFDs rather than complicated programs that are large and difficult to handle.

Be sure to check application torque performance requirements (both steady state and dynamic) and select the appropriate VFD - either V/F, sensorless vector, full vector, servo, and so on. Don't forget to consider the application's environmental requirements. If a higher enclosure integrity is required (such as NEMA 12/IP55 or NEMA 4X/ IP66), some specified VFDs can provide this out of the box while others will have to be "enclosed" in a separate host enclosure to obtain these levels.
The variable frequency drive cable doesn't do much for motor insulation, it is for motor bearing protection (I won't speak to the effectiveness of such cable). Many years of experience says that if you are going to spend the money on the VFD cable, have it installed properly. Regardless, always attempt to keep the cable distance from the VFD to motor short as possible (30m or less). If it gets longer, discuss with your motor and VFD manufacturer. There are many differences between both VFD and motor designs, let alone the system voltage used.
The variable frequency drive is good at maintaining the rotor magnetism while reducing the frequency, since the VFD is designed for that. But most VFDs are not going to pass that energy back to the AC supply unless it has special circuitry to do so. If one busses many VFD drives together using the DC bus connection, the energy will flow back naturally and feed other loads. Otherwise the DC bus will overvoltage and often trip the variable frequency drive. A braking resistor is sometimes used to burn off the excess energy too.
1. You can even run more than 2 motors on a single variable frequency drive; it is absolutely OK, I am using it for quite a long time and the VFD is just working very fine.
2. In one application I had used 8 Motors (screw conveyors) on a single VFD (Gozuk make) we had no problem in that configuration.
3. In another application I used ABB 22KW VFD, to run 2x11 KW induction motors this was used on large EOT cranes and the motors were used for the long travel of the EOT cranes.
Manufacturers of medium and large VFDs typically provide information on the requirements for power cables connecting the inverter to the AC motor and from the VFD isolation transformer to the converter. Cable and panel grounding recommendations are also included. Cable supplier catalogs and Web sites provide detailed information, but not necessarily everything needed to determine if the VFD manufacturer's recommendations are being met. This paper provides typical requirements, reasons for those requirements, and some useful insights to help the reader bridge the gap between VFD supplier requirements and cable supplier published data.
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