Three phase motor
and VSD to replace
single phase motor
11 September 2013 – Single phase motors are typically used on farms, office buildings and residential applications where low output power is required and no three phase system is available. However, with the on-going improvements in technology and lowered cost of smaller variable speed drives (VSDs), three phase motors can now be used in these applications.

“Single phase motors have a single winding and are fed with a single phase voltage. There are various arrangements that can be used but the most common design is the one that uses a starting capacitor or a starting and a run capacitor, both equipped with a centrifugal and a stationary switch,” Edson Cristofolini, product and projects manager at the Zest WEG group, says.

Three phase motors, on the other hand, have three windings and are fed with a three phase voltage. The interaction between the magnetic field on the stator and the induced magnetic field on the rotor makes the motor run.

“When using a single phase VSD you would still have a single phase feed to the drive allowing the existing installation to be used without any modification, however there would be a three phase voltage feed to the motor. This is possible because the VSD will rectify the single phase voltage from its input and switch the voltage on and off with insulated gate bipolar transistors (IGBTs), from a DC link provided by a capacitor bank, to create the three phase signal on the output to feed the motor.”

In general, three phase motors present a better efficiency and power factor than a single phase motor and they are also more reliable, lighter and require less maintenance, as no capacitor or internal switches are needed.

“Added to the performance and reliability of three phase motors, when they are run on a VSD it eliminates the need for a direct on line (DOL) starter with contactors and thermal relays, since all the protection is now built in the VSD,” Cristofolini says.

WEG manufactures a variety of VSDs which also provide additional features not available on a normal starter for single phase motors. These include current and speed reading, under voltage and phase protection, fault history and speed variation, which contributes to power savings.

“The VSD also incorporates analogue inputs and outputs as standard and allows communication with a PLC or scada system.

“Whilst cost comparisons, between a single phase motor and a three phase motor typically focus on the cost of the single phase motor, one also needs to take the cost of the starters into account. A comparison undertaken by Zest WEG group indicated that in most of the cases, based on current market prices, it is more cost effective to use a three phase motor with a VSD than to use a single phase motor with a starter. In addition, there is a higher off-the-shelf availability of three phase motors and VSDs,” Cristofolini says.

“It is important to remember that the VSD has on board electronics, thus requiring additional care in use. In spite of this fact, VSDs are becoming increasingly more user friendly and require little maintenance. This makes them the perfect cost effective alternative to a single phase motor with starters,” he concludes.