What are Variable Speed Drives? The Ultimate Guide
Variable Speed Drives, also known as adjustable-speed drives or variable frequency drives, are a piece of factory automation equipment that regulates the speed and torque output of an electric motor. Using an integrated rectifier to convert the fixed-frequency and fixed-voltage AC power supply of an electric motor into a DC supply, a fixed sinewave is converted into a variable sinewave out, driving the motor. With the constant changing load of an induction motor, variable speed drives can quickly change the speed/torque of a motor to keep it as a constant.
According to ABB: "In many cases, motors are controlled by means of a valve that regulates the flow of fuel or a vane that controls the airflow while the speed of the motor itself remains unchanged. These and other methods, such as using two-speed motors or controlling them by switching on or off, are inefficient from an energy point of view."
Variable speed drives are designed to reduce the amount of electricity and energy consumed by a company, ensuring factories are more environmentally friendly. More than 65% of industrial electrical energy is consumed by motors, so by accurately maintaining the speed of a driven machine, VSDs can save energy, and are proven to be a more efficient solution than motors controlled by valves that regulate the flow of fuel, without changing motor speed. VSD producer ABB has estimated figures showing more than 115 million megawatt hours of electricity are saved each year by their drives in operation worldwide, also highlighting the ongoing issue that less than 10% of motors in use globally are actually equipped with VSDs.
Types of Variable Speed Drive
There are various types of variable speed drive in production, each controlled differently and offering varied functions.
- Variable pitch drives: This is a type of mechanical adjustable speed drive which adjusts the pitch diameter of a motor via pulleys.
- Traction drives: Another type of mechanical adjustable drive that transmits power through metal rollers. The said rollers are moved to change the diameters of the contact path to change the input/output speed.
- Hydrostatic drives: This type of drive features a hydraulic pump and a hydraulic motor, and is a type of hydraulic adjustable speed drive. This type of drive controls motor speed by regulating the fluid flow of the machine with a valve.
- Hydrodynamic drives: Another type of hydraulic adjustable drive, the hydrodynamic drive uses oil to transmit torque from an impeller to a rotor.
- Hydroviscous drives: This hydraulic adjustable drive uses multiple discs connected to the input shaft, pressed together to transmit torque to the output shaft through an oil film.
- DC Motor Drives: These drives control the speed of DC motor systems, using either armature voltage or field current.
- Eddy current drives: This type of drive features a fixed speed motor and an eddy current clutch, which contains a fixed speed rotor and adjustable speed rotor. This drive gives a closed loop speed regulation with varying clutch current, allowing the clutch to only transmit enough torque for desired speed operation.
- Slip-controlled wound-rotor induction motor drives: This is a type of AC drive, which controls the speed of an AC motor system. It controls speed by varying motor slip via rotor slip rings.
- Continuously Variable Transmission: Also known as CVT, continuously variable transmission refers to mechanical and hydraulic speed drives used in vehicles and farm equipment.