Single vs three-phase motors

Electrical Power & Protection

November 2013 Electrical Power & Protection

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 ongoing 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,” explains Edson Cristofolini, product and projects manager at Zest WEG Group.

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,” continues Cristofolini. “This is possible because the VSD will rectify the single-phase voltage from its input and switch the voltage on and off with 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 online starter with contactors and thermal relays, since all the protection is now built in the VSD,” says Cristofolini.

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 analog inputs and outputs as standard and allows communication with a PLC or scada system,” adds Cristofolini.

“Whilst cost comparisons, between a single- and a three-phase motor typically focus on the cost of the 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,” points out Cristofolini.

“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.

For more information contact Jamie Wilson, Zest WEG Group, +27 (0)11 723 6000, [email protected], www.zest.co.za

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Electrical Power & Protection

Single vs three-phase motors

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