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How do I connect a direct on line (DOL) starter to a single phase motor?

Published date: 04 May 2018

There are a number of issues:

We do not manufacture a complete starter specifically for single phase motors but our 3 phase starters can be wired for single phase applications.

Relative to their power output, single phase motors draw much greater current than 3 phase for 2 reasons:
1.      The supply voltage will normally be 240 or 220VAC instead of 415 or 380VAC as is the norm with 3 phase supplies. As power is the product of voltage x current then the current will have to be 415/240 = 1.73 times as great to achieve the same theoretical power.
2.     Single phase motors are considerably less efficient than 3 phase so losses are greater and current consumption greater still.

This means that typically, single phase motor current will be at least double that of 3 phase motors of similar power output. As the power ratings of our starters are based on 415VAC 3 phase it becomes necessary to base starter selection on the full load current of the motor and NOT the power rating.

It is possible to use one of our reversing starters with single phase motors but it does require extensive rewiring to do so. This issue is complicated by the fact that there are a number of designs of single phase motors and the method of reversing varies between them. Therefore wiring a reversing starter for single phase operation is possible but can only be advised for those who fully understand the wiring techniques of their specific single phase motors and have the knowledge and experience to work out how to apply that to the reversing starter. Therefore reversing of single phase motors is not covered here.

Single phase motors may have a number of connections in their terminal boxes, for main windings, start windings, capacitors, klixons or PTC devices etc. Regardless of the number of terminals present, they will all feed back to a simple live and neutral feed at some point, and it is in this feed that the starter will be connected so it will not be necessary to interfere with any of these internal connections (as would be necessary if dealing with a reversing starter). An example of the terminal arrangement for a single phase motor is shown below:
Example of a single phase motor and the terminals

The method of connection is similar to that of a 3 phase, except that for the motor overload to function properly all three poles must be populated. This is because the overloads have phase loss protection, so expect to see similar current flowing in all three poles. If only 2 poles are populated (live and neutral), the overload will sense no current in the third pole and trip accordingly (this can occur in anything between a few minutes, even seconds, up to 2 hours, according to conditions and circumstances).

To prevent this the procedure is to run either the live or neutral (it does not matter which) through one pole of the contactor and overload, loop back up to go through a second pole in the contactor and overload and then to the motor. The remaining power feed wire will go straight through the third pole of the contactor/overload and then to the motor. The control wiring will be as that for the 240VAC coil version of the 3 phase system. The diagram below shows the wiring for a single phase motor and the path through the contactor and overload:
Wiring for a Single Phase Motor starter

Please see the attached diagram for guidance. This diagram illustrates possible wiring using a Tesys D (LC1D****) contactor and Tesys LRD overload (LRD**) and stop control is assumed to be by operation of the stop button on the overload. If a starter incorporating a Tesys K series contactor (LC1K****) is used then the connections between 95/14 and 96/A2 are made internally. In this case the start button connection will be to terminal 95 instead of 14

NOTE : These notes and diagrams provide a schematic method of achieving the control but it remains the responsibility of the installer to ensure that any safety requirements, local legislation and directives, circuit and personnel  protection are met and/or provided. Installation of any moving or motorised industrial equipment should only be carried out by qualified persons familiar with electrical and machine safety requirements.

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