new single phase 2.2kw motor tripping mcb

[leepandp]

hi i have a single phase 2.2 kw motor with start run capacitors. we have just replaced this motor as the centrifugal switch was u/s on the old one. power is supplied to the motor through 2 contactors which are also new, and the 16A motor rated mcb has also been changed but the mcb still trips occasionaly, further back on the incoming supply there is a 32A mcb that i dont think is motor rated but does not seem to trip as often as the 16A "c" rated one
any ideas?? im pulling my hair out here!!! many thanks in advance!!

 

[ScottyUK]

A 16A Type C breaker is marginal for a 2.2kW 1-ph motor. I'd expect a 20A or 25A Type C, with cable sized to suit. The breaker is there for short-circuit protection of the motor and thermal protection of the cable; its curve is a lousy match to the motor thermal curve and thermal protection should use a motor protection relay or a combined motor protection breaker as offered by many manufacturers.


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If we learn from our mistakes I'm getting a great education!

 

[leepandp]

ah ok thanks scottyuk i will go to site and uprate the mcb today what do you think about the 32A mcb further back in the supply not being motor rated? occasionaly this is tripped aswell currently the motor is wired with 4mm cable which i believe is ok for appx 26amps thanks for the reply!!!

 

[ScottyUK]

4mm² cable is good for 26A, you could certainly consider changing the upstream breaker for a Type C. It is possible to get a Type D for loads with really severe inrush, although personally I tend not to use them much except on awkward loads such as iron-ballasted discharge lighting.

Check your fault clearance times are still ok, although if this is a fixed installation rather than a socket outlet you'll almost certainly be ok.

Are you UK-based?


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If we learn from our mistakes I'm getting a great education!

 

[Paulusgnome]

Clearly, the answer depends on the regulations that apply where this motor is installed.
Here in NZ (and in Australia too) you would use a 16A or 20A D-Curve MCB to protect the feeder cable (16A if you used 1.5mm2 feeder cable, 20A if you used 2.5mm2), but a D-curve would be the norm for a motor circuit.

 

[ScottyUK]

A type C breaker should not trip on instantaneous overcurrent until 10x rated current. A type D trips at 20x rated current. Most motors are accepted to have a starting current of 6x - 8x rated, so a type C is a good match. Some motors do have a higher starting current and a type D would be justified, but not in most cases.



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If we learn from our mistakes I'm getting a great education!