Motor speed control using Variable frequency drive


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What is a Variable frequency drive

A Variable Frequency Drive (VFD) is used as a motor controller that controls an electric motor speed by varying the frequency and voltage supplied to the electric motor. They are also known as variable speed drive, adjustable speed drive, adjustable frequency drive, AC drive, microdrive, and inverter.

Frequency is directly related to the motor’s speed (RPMs). Faster the frequency, the faster the RPMs. Depending on the motor load, the VFD can be used to ramp down the frequency and voltage of the motor input. As the application’s motor speed requirements change, the VFD can simply turn up or down the motor speed to meet the speed requirement.

Working of a Variable Frequency Drive

The first stage of a Variable Frequency AC Drive, or VFD, is the Converter. The converter has six diodes. They allow current to flow in only one direction in the direction shown by the arrow in the diode symbol. For example, whenever A-phase voltage is more positive than B or C phase voltages, then that diode will open and allow current to flow. When B-phase becomes more positive than A-phase, then the B-phase diode will open and the A-phase diode will close. The working is similar for the 3 diodes on the negative side of the bus. Thus, we get six current “pulses” as each diode opens and closes. This is called a “six-pulse VFD”, a standard configuration for current Variable Frequency Drives.

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The voltage at the DC bus is the RMS value of the input voltage.

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Adding a capacitor gets rid of the AC ripple. A capacitor absorbs the ac ripple and delivers a smooth dc voltage such that the AC ripple on the DC bus is typically less than 3 Volts. Thus, the voltage on the DC bus becomes “approximately” 650VDC. The actual voltage will depend on the voltage level of the AC line feeding the drive, the level of voltage unbalance on the power system, the motor load, the impedance of the power system, and any reactors or harmonic filters on the drive.

AC to DC conversion of the system is the converter and the DC to AC conversion is the invertor.

Frequency generation in VFD

The invertor has two switches ( transistors) for each phase of the input, depending on the switch tht is closed to complete the circuit, the voltage phase to the motor can be changed. While the upper row switches give a positive terminal to the motor input, closing the lower switches will connect the negative part of the DC bus to the voltge. Thus changing the phase and the direction of the voltage from supply, frequency can be changed

The output of the VFD is a rectangular wave form. VFD’s do not produce a sinusoidal output. This rectangular waveform would not be a good choice for a general purpose distribution system, but is perfectly adequate for a motor.

To reduce frequency, inverter switching speed is controlled. But the frequency can be in the range satisfying the Volt/Hz ratio for given voltage. So to change the voltage Pulse width modulation is used.

This is called Pulse Width Modulation or PWM.Notice that during the first half cycle, the voltage is ON half the time and OFF half the time. Thus, the average voltage is half of 480V or 240V. By pulsing the output, we can achieve any average voltage on the output of the VFD.

Why should I use a VFD?

Applications of a Variable frequency drive are

1 - Reduce Energy Consumption and Energy Costs

In electric vehicles where there is not need to run the motor at full speed, energy costs can be cut down by controlling the motor with a variable frequency drive. VFDs allow you to match the speed of the motor-driven equipment to the load requirement which is specific characteristic for VFD when using AC motor

2 - Increase Production by Tighter Process Control

By operating motors at the most efficient speed for your application, fewer mistakes occur, and thus, production is optimized. On conveyors and belts to eliminate jerks on start-up high through put is needed.

3 - Extend Equipment Life and Reduce Maintenance

Optimal motor speed ensures the long life of the equipment with less maintenance time.

Due to the optimal control of the motor’s frequency and voltage, the VFD offers good protection for motor from electro thermal overloads, phase protection, under voltage, overvoltage, etc… Starting a load with a VFD ensures that the motor is not subjected to the “instant shock” for a smooth start thereby eliminating belt, gear and bearing wear.