The Variable Speed Drive, also commonly known as an Inverter, allows for speed control of an AC motor through varying the frequency of the current being supplied to it. Whilst they are available for single phase motors, they are most commonly found controlling 3-phase motors.
The most basic inverter allows for the manual control of the motor’s speed and direction, along with a start/stop function, and a jog function. In addition it allows for the setting of pre-set speeds. This can be particularly desirable when using the inverter to reduce energy bills. By setting a pre-set “night-time” speed on equipment that is running 24 hours, but doesn’t need to be at full power when there are fewer people around (i.e. cooling fans) it is possible to reduce energy consumption by up to 30%.
The more advanced inverters start allowing the addition of various options to expand upon their versatility, as well as allowing the control of multiple motors. The most common option is an Encoder Feedback Unit, allowing the inverter to receive information back from an encoder fitted to the motor, and to react according to pre-programmed instructions to the information being received. This could be changing the motor speed, direction, or even bringing it to a stop. Remote keypads are available, for when the inverter is being installed a distance from an operating station – and for even more flexibility the inverter can be connected to a computer by Ethernet, USB, or RS232 serial interface, and can be programmed directly from the PC using a variety of industry standard protocols, or by using the propriety software that is freely downloadable.
The inverter is also available in a single-phase version, using single-phase 230/240V input, and converting it to a 230/240V 3-phase output, allowing smaller 3-phase motors (up to 2.2Kw) to be controlled and run from a household supply.
An example of how to save energy costs with an Inverter:Let’s take an example of a cooling fan that runs 24 hours a day, 7 days a week. It is powered with a 5.5Kw motor, and for this example we will take an energy cost of £0.12kW/h day-time rate, and £0.10kW/h night-time rate.
Without an inverter, the motor is running continuously at full power, the annual energy cost is £5199. If an inverter is fitted, and the motor is set to run at full power for 14 hours a day (e.g. between 6am and 8pm), and at night it is set to run at 70% power, the annual energy cost now falls to £4,406 per year – resulting in annual savings of £793, on just one motor.