What is a variable speed drive ?
A variable speed drive varies the motor’s frequency and voltage to control its speed. The basic schematic of a variable speed drive system is shown in Figure 1. It normally consists of an AC/DC converter and an output DC/AC PWM controlled three phase output.
Figure 1: Block Diagram of a Variable Speed Drive
When using a variable speed drive correctly one can obtain very accurate speed control of the electrical motor leading to:
- better process control,
- less wear in the mechanical equipment,
- less acoustic noise and,
- significant energy savings.
Harmonics in Variable Speed Drive Systems:
Variable speed drives are a non-linear a.c. load and cause input line harmonics to exist. Moreover, its output is made up of a high frequency sinusoidal based PWM three phase output. In practice it is desirable to obtain a sinusoidal output waveform, however this is very difficult to achieve unless a specifically designed filter is used on the output.
Energy Savings:
Variable speed drives can obtain substantial energy efficiency and energy savings in applications where the electrical motor works at different speeds and load conditions such as for example in variable-speed pumping, particularly when concerned with demand-based pressure systems. Even in the case of constant-speed pumping, if properly designed, variable-flow designs can save energy.
Pros and cons of variable speed drives:
- Variable-speed control achieves energy savings.
- Provide a “soft” start to limit the high starting current in motors.
- Systems are less noisy than direct-on-line systems.
- Add cost to the system.
- Add harmonics to the system and can require filters (For more detail please refer to the other article regarding Harmonics in the Blog).
For further information on application of variable speed drives to your specific application you can contact us.