What is a Variable Speed Drive?

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Variable Speed Drives (VSDs) Explained

Variable Speed Drives (VSDs) are also known as Adjustable Speed Drives, Variable Frequency Drives (VFDs), Adjustable Frequency Drives, Inverter Drives, Microdrives or AC Drives. These drives are an excellent method for controlling motor speed in response to varying process demands.

Typically a VSD system consists of a three-phase AC induction motor and a variable frequency power supply. The variable frequency power supply uses solid state components to produce a pulse-width modulated current that varies the power and frequency supplied to the motor. The drive thus takes the fixed-frequency AC supply and converts this to a variable-frequency AC supply. This controls the power use and mechanical power output so that the motor can run at the most efficient speed for the motor and the process.

Control of the motor speed can be based on feedback from the process, e.g., flow rate, temperature or pressure, so that process control is improved. This enables accurate control of the motor speed over a broad range.

As a result, small decreases in the speed of pumps and fans or the pressure of pumps can lead to large decreases in energy use, e.g.,

• Reducing the speed of an AC motor by 20% can reduce the energy consumption by 50%.

• A 20% reduction in the pressure in a pump can reduce the energy consumption by around 28%.

Other benefits include:

• Programmable soft starting, soft stopping and dynamic braking.

• Built-in energy optimisation features.

• Fail-safe bypass of the VSD package in the event of failure.

• The switching method means that the PF of the motor is effectively 1.0 and PFC equipment needs are reduced.

For more information regarding VSDs, please refer to the following related blog posts:

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