protect relay contacts when switching a DC reversible motor

[chip123]

I am trouble shooting an old DC reversible pump that is controlled by relays. The motor is connected to 32 volts. the motor is 1/6 HP. The system has two relays that control the aemature to reverse the voltage to change direction of the motor. When arrived to repair the installation one of the relys had its contacts welded closed. Someone else had worked on the motor before I arrived. I believe the back EMF is what is arking across the contacts when the relays releases. I would like to add protection to each side of the armature to prevent te arking. Do I just add two condenser to ground?

 

[fsmyth]

Add a 0.1 uf in series with a 10-20 ohm resistor
across the contacts. A pretty generic low-voltage
snubber. There are more involved solutions if the
risetime is really fast, but that should work.
<als>

 

[cbarn24050]

Hi arcing does not usually cause contact welding, the culprit is either starting current, jammed rotor or reversing the motor while its spinning the wrong way.

 

[ScottyUK]

True, but arcing gives rise to poor surface condition, which leads to high contact resistance and results in contact heating or contact welding when the large starting current flows.


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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[chip123]

What size cap (voltage)and what wattage resistor should I use? The motor runs on 32 V 6 amps. Should there be any thing put on each side of the armature?

 

[fsmyth]

Use a poly cap rated for at least twice the working voltage
(i.e., anything over 60v. 200v caps are easy to find.)
Anything 1W or over on the resistor. Bigger is better.
Note that this is generic; to give any closer values,
one would need to know the specs on the motor and the
actual conditions under which it is used.
TRW (and others) makes such a thing in a single
package that looks like a square poly cap.
I am looking at one right now -- (a 0.47uf/100 ohm
package used for relay and small motor applications).

<als>

 

[ScottyUK]

Resistor - bigger is better, but stick with metal film if you can. Once you get over a couple of watts they tend to be wirewound: wire-wound types are inductive, unless they are expensive bifilar types, and make for a less effective snubber circuit.

Capacitor - use a metallised polypropylene film type. They have low self-inductance and are usually self-healing which makes them great for snubbers and other similar duty.


----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!