AC motor control help


Thread Starter

Ben Brown

I have a 8A single phase 240v motor (which has a fan attached and is used as a blower) I need to switch this motor on and off using a timer circuit (which I already have) but i don't know what to use to interface the two together, the timer gives out 12v dc when on and my original idea was to connect this to a relay to switch the motor on and off, but would this work ?, as the motor is an inductive load i'm worried that the contacts might arc or it may cause interfearance or spikes etc... is this an acceptable way to control this motor or is there another (more reliable) solution, if anyone can help with information or circuit diagrams it would be much appriciated. (i don't need any speed control or anything fancy, just basic reliable on/off operation). thanks a million, Ben

Ralph G. McDonald

You will need a relay with the AC contacts rated for the motor HP @ 240VAC. If it is available with a 12VDC coil, check the relay coil for both pull-in and holding amps required. If it is greater then the ouput capability of your 12VDC timer circuit, then you will need an interposing relay. In any case you should install a snubber circuit across the relay coil connected to the 12VDC timer, because you will get an inductive surge from the relay coil itself when the circuit opens. Think of how an ignition coil stores energy and then releases it when the points open. You don't have both primary and seconday windings, but there can sometimes still be enough energy released to damage unprotected solid state components.
What I would suggest would be to have a motor starter contactor that is sized to the FLA and HP of the motor you are using. This will ensure that the arcing will not deteriorate contacts excessivly and shorten life. A contactor can be selected with coil voltages of 12vdc so this is no problem. As far as EMI caused by the motor switching, you can get contactors with surge suppression CKT built in or you can add them as needed. This is a common way for motor control that uses electromechanical swithing. There is also solid state options but this is more expensive and probably not needed in your application.

Meeuwis Bouw

I think a solid state relay is the best option in this case. Use a type capable of handling the inrush current of the motor (six times 8 amp).
Meeuwis Bouw,
Superfos Packaging
Thanks to everyone that has replied to my posting....your help and assistance was very much appriciated.

Ben Brown.

Jeffrey Eggenberger

You will need a motor starter, this is a special contactor (big relay) that includes motor overload protection. take a 12VDC relay with contacts rated at 120VAC and use this to bring in the starter (unless you can find one with a 12VDC coil, which I doubt will be easy.) Motor starters are designed to handle the surge and arcing that comes with large amperage motors.


Johan Bengtsson

Well, it is quite standard to do it that way, provided you size the relay accordingly.

Yes you will get an arc (for a short period) and spikes and interferance and all this, but not more or less than you would get by operating the same motor with a manual on/off switch.

There are solid-state relays that would make this less troublesome but I doubt it actually would be any problem with an ordinary mecanical relay.

/Johan Bengtsson

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I would avoid solid-state relays - quite often there are more troublesome with an inductive load.