Single versus three phase motors with a VSD
By E Cristofolini and F Labuschagne, Zest WEG Group
Electricity+Control, November 2013 (pages 4 – 6)
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Single phase motors are generally used on farms, office buildings and residential applications where low output power is required and no three phase system is available. However, with ongoing improvements in technology and cost reductions on small size variable speed drives (VSDs), three phase motors with a VSD can now be used more widely.
In general, three phase motors present a better efficiency and power factor than single phase motors. They are also more reliable, require less maintenance and are not as heavy as single phase motors, which are only really used where no three phase voltage is available though with the more easily accessible VSD, which can have a single phase supply, this is no longer a problem.
Besides performance and reliability, by using a three phase motor with a VSD, it is no longer necessary to have a DOL starter with contactors and thermal relays. The VSD also provides additional features not available on a normal starter for single phase motors, such as: current reading, speed reading, under voltage protection, phase protection, fault history and speed variation for power saving.
When comparing single phase and three phase motors from a commercial point of view, it is important not only to compare the prices of the two but also to take into account the starters. By doing so, it becomes evident that, based on current market prices, in most cases it is cheaper to use a three phase motor with a VSD than a single phase motor with a starter.
- Three phase motors present a better efficiency and power factor than single phase motors.
- It is cheaper to use a three phase motor with a VSD than a single phase motor with a starter.
- VSDs have become user-friendly – requiring little maintenance.