Relay Contacts and Arc Protection

Because this may be a well pump motor with a long conductor between the relay contacts and the motor...you wish to put a MOV across the motor conductor, it is preferable to have the MOV as close to the motor as possible to protect the motor, the alternative thinking is at the motor starter to protect the motor and conductor.

At 277VAC at the motor starter should be OK.

Dear CCM,

This is asingle phase capacitor start blower motor. The motor is protected by a fuse and we are not talking about motor protection. The service life (Number of ON-OFF cycles) of the relay is what we are discussing. If arcing take place across the contacts of the relay (I am not using a contactor) they will burn out fast and I may not be get 100,000 cycles of endurance.

So the questions are simple:

1) Will there be arcing across the contacts? Some participants from the Electrical forum think that there will not be since it is an AC motor.

2) If there will be arcing, how will you protect the relay from premature failure?

Thanks,

thankan; where do you find a 277 volt motor? is it 3 phase or 1 phase? pc

Hi Perry,

You can buy them from A.O.Smith Electrical products in USA. Single phase 277 V, Capacitor start motors.

Any motor requires fusing and overload protection, some motors have current/thermal overloads built in but fusing is still required. If you are using a standard motor starter (as oppossed to a logic relay or such) then the unit will proform as required. A MOV can suppress Back EMF and allow the starter contacts to last longer just be sure it is rated for the substantial load it will be applied to it and is AC rated for 277 V. (if a DC unit, rate at least for 400 V)

BTW I have run across Failed MOV's that had motors replaced for no reason!

Conductors need fusing--Motor needs O/L prtection.

Yes conductors need fusing and motors need overload protection. Since it is difficult to run a motor without conductors, motors need both overload and fuses. Also in most electrical codes.

Sarcasm?

The circuit/conductors are sized for the fuse or breaker, period(and code)--Motors, are protected by overload relays sized for them. I have plenty of tools that are protected by thermal overloads that draw less than 10 amps plugged into 20 amp circuits with 20 amp breakers. The overload protection of one motor has nothing to do with the circuit protection, there can be and are more than one on it.

If you are talking about industrial settings--The same holds true within code. Motor protection is soley on the shoulders of overload relays--I have changed out plenty of smoldering motors that have been incorrectly protected by heater size in the overload relay. The breaker didn't care until the amps exeeded the rating for the conductors.

Have otherthings to worry about and I doubt I will respond.

Regards, KT

Dear KT,

We are talking about only 1.7 Amps (Assuming 80% effiency) at 277 VAC. When the supply to the motor is interrupted will there be an inductive kick-back (Back EMF usually several folds the supply voltage)? If so there will be arcing across the relay contacts for an unpredictable duration and the contacts will burn out sooner than its expected life. So in order to QUENCH the arc, it is a general practice to instal a RC network across the contacts.

Regards,

Thankan.

I don't know Thankan.

I deal with motor starters and all kinds of SCRs and Varible frequency drives and almost exclusively 480V 3 phase.

We have around 500 motors ranging from 600hp down to .25hp--I am just curious about your question and look forward to hearing a good answer from someone here.

The arc--I suspect, would be caused by resistance and amps increasing as the contacts open-But the resistance would cause the voltage to drop across the contacts until they opened. If you are looking at problems with the enertia of the turning motor generating a voltage after it is cut from the line, I don't know.

Hope you get an answer--KT

Hello mike,

Please pay attention to my question! Conductors and contacts are two different things. Ofcourse we need conductors (Wires) but we do not need contacts of a relay to run the motor. Just a switch will do to energize and de-energize the motor. Thanks for your answer any way!!

For Your Information - A switch has contacts.

Your question asked "do relay contacts require arc protection".

If the contacts are of the right materials and big enough to dissipate the heat, no the contact does not need arc protection. For the dinky size of your motor this is feasible.