What is a variable speed drive and why are they used in the industrial process?

A variable speed drive can control the speed of equipment or a process and by using a variable speed drive we can operate the process or equipment at the required speed. We can see VSD’s in many industrial applications, the assembly lines would require to operate at different speeds and it can be accomplished by using a VSD. In certain process conditions, it would be required to control the speed of a pump, fan, or motor, so all of this can be done with the help of a variable speed drive. We can see a variable speed drive in many industries such as chemical, food, power plant, mining, etc. So all of these industries would need some kind of speed control and this can be achieved by using a VSD.

What are the benefits of using a VSD in process control?

  • Operation is smoother
  • Acceleration can be controlled
  • Different operating speed for each process can be done
  • Process variable change can be compensated
  • The slow operation can be done for setup purpose
  • Production rate can be controlled
  • Accurate positioning can be achieved
  • Torque or tension can be controlled
  • Energy and time can be saved

We can reduce the energy consumption by using a VSD, electric motors consume a lot of energy so by controlling the motor by using a VSD we can reduce the power consumption. By using a VSD we can also increase the production, if we control a motor by using a VSD we can operate the motor at desired speed for our process. So because of this less error and production will increase too, and thus good income. Due to the proper speed control, there won’t be any jerks in the conveyor or in the belts. So because of the VSD equipment life can be extended and also the maintenance will be reduced.

What are the components of a VSD and how does a VSD operates?

The major components of the variable speed drive are a rectifier, DC bus, inverter, and regulator/controller. The rectifier would convert the AC into DC power, the DC bus has a capacitor and it would store the power and smooth output voltage ripples and power distortions. The inverter would convert the DC to AC and the controller would do the control operations. The VSD operation takes place by converting the incoming fixed frequency supply into a variable frequency output. So low frequency can be used for slow speed and high frequency can be used for faster speed. We can vary the output by enabling the motor to generate more or less torque according to the requirement. So we can use a motor with VSD to turn the large load at a slow speed or it can turn a lighter load at a higher speed.

What are the types of variable speed drive?

Mechanical variable speed drives

The mechanical variable speed drive would use mechanical means to convert a fixed speed motor output into a variable speed output. These drives are of two types and they are variable-pitch drives and traction drives. A variable pitch drives are pulleys and belt drives in which the pitch diameter of one or both pulleys can be adjusted. The traction drive type would transmit the power through metal rollers. The input and output can be adjusted by moving the rollers to change the diameters of the contact path. This type of variable speed drive is not efficient when compared to an electronic drive.

Hydraulic variable speed drive

This type of drive would use hydraulic oil as a medium and through which the torque will be transmitted. This device has high mechanical and hydraulic losses when compared to other types of variable speed drives. There are three types of hydraulic variable speed drives and they are hydrostatic drives, hydrodynamic drives, and hydro-viscous drives.

The hydraulic drive is composed of a hydraulic pump and also a hydraulic motor. Speed is regulated by varying the displacement of the pump or motor. The hydrodynamic drives would use oil to transfer torque between an impeller on the constant-speed input shaft and also a rotor on the adjustable speed output shaft. In the hydro viscous type of VSD it would have one or more disc connected to the output shaft. The torque will be transmitted from the input shaft to the output shaft through an oil film between the discs. The torque which is transmitted will be proportional to the pressure exerted by the hydraulic cylinder which presses the disc together.

Electric variable speed drive

In this type of speed drive, electronic devices are used to control the speed and torque of an electric motor. There are three types of electric VSD and they are DC motor drives, AC motor drives, and eddy current drives. Electrical VSD is composed of an electric motor and also a speed control unit or a system.

How to do the pre-commissioning check of the VFD?

  • The power and motor cables should be in the correct size and termination
  • The power cable shields must be properly earthed to the PE terminal
  • There shouldn’t be any faults with the cables
  • The installation and termination of the control cable must be properly done.
  • The control cable shield must be properly earthed at one end only
  • The cooling fan connection must be checked and we must make sure that the proper voltage tap is selected on the auxiliary transformer

How to select a VSD?

  • VSD must be selected according to the full load operating current of the motor
  • Low harmonic
  • It should be immune to vibrations
  • VSD must be selected according to environmental conditions such as temperature, humidity, etc.
  • Supply voltage ratings must be considered
  • Input and output frequency
  • Output overload
  • Efficiency

How to do the VSD commissioning?

The drive manual must be checked before the commissioning

  • Check if the rating of the drive and the motor are proper for the installation
  • The motor cable must be disconnected so that there won’t be any unexpected motor starts, then turn on the power
  • The electrical parameters must be checked – the power supply voltage and DC bus voltage should be measured, we need to check if the microprocessor has completed the test sequence, make sure that the control system is ready, and also check if the cooling fan is running.

How to set parameters of VSD?

A VSD parameters should be set properly before it is connected to a mechanical load.

Base voltage

The voltage must be set according to the motor’s rated voltage, we must make sure that the VSD is set at proper output Voltz/Hz ratio to the motor.


The frequency must be selected according to the motor frequency

Maximum speed

The maximum speed should be set to 50Hz or above

Minimum speed

Mostly the minimum speed is set zero hz for a pump and higher for constant torque applications.

Rated motor current

While setting VSD the motor current should be considered

Motor acceleration and deceleration

The acceleration time and the deceleration time of the motor must be considered

What are the advantages of VSD?

Starting current control

During the start-up of a motor, it would take six or seven times more current than the full load current and due to this motor will become hot and also the life of motor would be decreased too. So when we use a VSD with a motor the motor can be started with zero voltage and frequency and because of this the motor would take 40/60 percent of the load current and thus power consumption can be reduced

Torque limit control

A VSD torque can be set in a way that the motor won’t operate beyond the limit and thus there won’t be any damage to the machine.

Operating speed control

A VSD can control the motor speed with the help of a controller and because of this, a VSD is really suitable for industrial production.

Power line disturbances will be reduced

So during the motor start-up it would use three times that of the full load current and due to this the drain in the power distribution system will be very high. So due to this voltage sag the equipment which utilizes this distribution system would trip due to this. The devices such as sensors, computers and contactors would be affected due to this voltage sag. So by using a VFD this voltage sag can be eliminated.

Controlled stopping

Stopping a motor properly is an important process and VFD would do this with the help of braking resistor.

Energy saving

A VFD can save the energy by adjusting the motor speed and due to this the process can be done very easily.

What are the disadvantages of VSD?

  • Due to the lack of proper cooling, overheating can be caused at low speeds
  • Overheating can be caused because of supply harmonics
  • Bearings could carry current due to the rotor voltage build up
  • Stator insulation will be affected because of the voltage surges


post-graduate in Electronics & communication.

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