Motor Start Up Current

[RobFurn]

When a motor starts your going to get high in-rush currents which exceed the normal operating current for a short time. To cope with that you would use a C or D type MCB, fine with that

Questions: If say the operating current of a motor is 20A what do you expect the in-rush current to be 20% more 30% more. Is there a Calculation or Standard? does this differ for 3ph and single ph Motors?

Also what do you base the cable CSA on? the running current or do you have to beef it up to cope with the in-rush current? TBH if you have to beef it up whats the point of the C of D type breaker after all its the cable your protecting.
Ta

 

[THRIPSTER]

Rob,

Typically a motor will draw 7-10 times its normal running current on start up but it does depend upon mechanical load, supply characteristics and motor type. Anyway, using the 7-10 times normal current you can then consult the breaker/fuse characteristics to see whether it will trip/blow within the time it takes for the motor to run up.

Full load current is used to calculate the size of the cable for Volt drop but you have to be watchful that this size cable enables enough voltage to be avaialable at the motor terminals in order to reach operating speed within a specified time - so there is no simple answer to that without studying all circuit characteristics. You should be OK for domestic though unless there is a very long run on low CSA cable.


Regards

 

[RobFurn]

Thanks Thripster thats a help

 

[ricicle]

Typical characteristics for three phase squirrel cage motors

D.O.L = 7 - 10 x FLC
Star Delta = 3 - 5 x FLC

But as said above it depends on other factors well.