one circuit breaker for 2 motors

[Yaya]

I am used to seeing a circuit breaker, or fuses, used with a contactor and overload relay in motor control circuits.
However, we've taken delivery of a feeder conveyor which uses a large circuit breaker to supply the two motors on the conveyor, with a contactor and overload relay for each motor.
The exact details of the circuitry are as follows:
Main incomer circuit breaker rating on the conveyor - 630A
Contactor rating 185A - 2 off connected in parallel for each motor.
Electronic overload relay - will be set to 160A
Cables to motor - looks like 16 or 25mmsq - in parallel from the contactors ie. 2 cables are connected to each motor.
The motor is only about 2m away from the switchgear.
Motor ratings - each is 90kW, 161A, 380V.

I'm trying to arrange a supply for this conveyor, but I'm limited to installing either a 250A or possibly 300A breaker at the point of supply. This will be OK, since the conveyor will not likely be running at full load.

Not being in possession of the Buff Book, is there any standard I can look up online to check whether the above arrangement is OK. I would actually like to recommend that the panel be re-wired, with the cable to each motor protected by it's own circuit breaker, but I will need to substantiate this. Presently the cable to each motor does not have any short circuit protection, but if the cable run is so short, will this then be acceptable?
Any comments on this arrangement will be appreciated.

Regards,

 

[waross]

Check the NEC or CEC.
The ampacity of the cable must be the sum of the full load rating of each motor (From the table in the NEC, not nameplate) plus 25% of the current of the largest motor. This gives a minimum cable ampacity of 362.25 amps for the cable. The breaker may be more than twice this rating.
I realize that you are probably not bound by the NEC, but motor characteristics and cable loading are similar everywhere. If you are limited to a 300 amp breaker, you may consider two breakers, and a seperate cable and local breaker for each motor. A 300 amp breaker is marginal for starting one motor rated at 161 amps.
respectfully

 

[Yaya]

Thanks for the reply, waross
You are right - considering the starting current,a 300A breaker is marginal. I remember using a 300A breaker on a 110kW motor once, and it used to trip immediately because of the 1st cycle inrush current.

Unless I install a very large breaker, I may have to go with a fused isolator at the point of supply.

Any further comments regarding the fact that on the conveyor switchboard, there isn't separate breakers for each motor?

 

[jraef]

You are obviously not in the US or Canada so I cannot speak to codes in your location, but what they have done in the design might be perfectly legitimate here in the US. The way it works here is, the 630A breaker is providing only the instantaneous short circuit protection, the overload relays are providing the thermal protection not only for the windings but for the motor leads as well. The only caveat is that the conductors going from the breaker to the line side of the contactors must be rated for the breaker size because that is the only thing protecting those wires. So as long as the instantaneous trip setting of that breaker falls within the allowable range for protecting the windings and motor leads, i.e. up to 1300% of motor FLA (assuming the cables are properly sized), this is considered acceptable, in fact it is done all the time on machines and systems where 2 motors are linked to a common shaft. In your case, with the motor FLA being 161A the breaker magnetic trip setting must be 2000A or less. If it is a non-adjustable mag trip, make sure the trip value is not over 2000A. The cables from the breaker to the line side of the contactors must also be rated for 630A.

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376