Types and categories of variable speed drives (VSDs)
There are several types of variable speed drives (VSDs):
1. eddy current couplings
2. motor-generator sets
3. variable ratio gear boxes
4. fluid/hydraulic couplings
5. pole-changing motors
6. Direct Current (DC) drives
7. electronic frequency converters.
And electrical variable speed drives (VSDs) can be broadly classified as:
1. DC drives - mains AC power converted to DC and used to supply a DC motor
2. drives using AC squirrel cage motors. Output to motor is either:
inverter output (variable frequency)
cycloconverter (variable frequency)
3. primary voltage control (limited speed)
4. drives using AC wound rotor motors (slip recovery drives or Scherbius drives)
5. drives using eddy-current couplings driven by AC squirrel cage motors.
The two basic types of AC inverters generally used in the industry are:
1. Current Source Inverter (CSI) - either load commutated or self commutated
2. Voltage Source Inverter (VSI) - either pulse amplitude modulated (PAM) or pulse width modulated (PWM)
The typical application of a variable speed drive (VSD) unit would be a heating and air conditioning installation, where energy is saved by eliminating the more traditional and less efficient damper control arrangement.
For example, a variable speed drive unit within a pumping installation where the pumping rate is adjusted by variation of the pump motor speed rather than the more traditional throttle and bypass valve systems.
