Thermal Magnetic Breaker for Motor

[Elekk]

For magnetic only circuit breakers, a separate overload (thermal) relay is required to protect motor, which makes sense.

But thermal magnetic circuit breakers already have a built-in overload (thermal) relay. Still a separate overload is used in the starter to protect motor.

Does anybody know why we need a separate overload while there is already a built-in overload in the thermal magnetic circuit breakers?

 

[Sidewinder]

To control & protect the motor as the TM OL will have an aux which drops the power contactor(s) out.

The mcb is only to protect the cable not the current consuming equipment!

 

[Elekk]

I think power contactor is still needed, only thing that is dropped out is the overload relay. Is it correct?

 

[ProDave]

When I designed industrial machines, we only used Motor protective circuit breakers which combined thermal and magnetic breakers in one package.

The thermal breaker was set at the motor full load current, and the magnetic breaker was set at the startup current. A decent MPCB has adjustment on both.

Very often the MPCB's were add on units that piggy back onto the side of a contactor of the same make, which made control panel construction neat and simple.

If you have a combined thermal / magnetic MPCB that is all you need.

 

[Elekk]

Canoe, I am not talking MCB, I am talking about GE Record Plus MCCB 160A with thermal magnetic trip unit. Is it MPCB? Does it need a separate overload downstream at the starter?

---------- Post Auto-Merged at 19:43 ---------- Previous post was made at 19:41 ----------

Please see drawing # 1 at the link: http://controlparts.com/schemati.htm.

This does not have a separate overload downstream at the starter. Is it correct arrangement?

 

[Evans Electric]

You are obviously talking about a huge motor there Elk , and being huge it would most certainly have its own seperate protective device , it would never be relying on a MCCB . To get it started there would be an SD or slipring starter anyway.

 

[Sidewinder]

You will find the overload in this drg is a tm overload hung off the op (T terms) of the contactor,, so unless I have misunderstood your comment at post #6 in the automerge bit is wrong?

The aux terms will be in the e-stop chain effectively.

I would argue yes the MCCB you quote in post #6 would require adequate control of o/l current within the motor control to comply with 60204-1 etc.

---------- Post Auto-Merged at 20:29 ---------- Previous post was made at 20:28 ----------

Deke,

Possibly even a soft start or inverter!

Not a HUGE motor though as it is only 400V

 

[Evans Electric]

Soft Start was the phrase that wouldn't come into my head, its my age y'know, in the absence of the "Thanks " button, thanks.