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Motor Starting Fuse Problem

Guest Anonymous

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I have a 415V 50Hz motor with 5.2kW no load, 22.6kW Full Load power. The motor is always requested to start at No load. This application is an air compressor, so motor starts and loading valve is closed for about 10seconds, before it opens for compressor to go on full load. Power factor for motor is around 0.91. So

5200 / (415*1.73*.91) = 8A

22600 / (415*1.73*.91) = 34.5A



So my question is that I have a DOL starter with a fuse rating of 50A. We operate at ambient temperatures close to 52degC.


Is 50A Fuse enough, or should we go for 63A? Is there any rule of thumb to calculate these things accurately from the No Load / Full load currents??


thanx for help

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Hello Anonymous


(Register on the forum, it's free and you will recieve advise of answers to your questions!!)

The problem with fuse ratings is related to the starting current of the motor, and the cable ratings of the cable supplying the motor.


Strictly speaking, the fuse should be rated to protect the cable, but it must also withstand the start current of the machine.


The starting current of the motor is a function of the motor design and the starting method used.

If you Direct On Line start the motor, it will draw locked rotor current. This is independant of the connected load. The Locked Rotor Current (LRC or LRI) is typically 6 - 8 times the full load current rating of the motor. In this case, the start current under full voltage conditions is probably going to be 200 - 250 Amps.

Fuses have an overload characteristic and an overload time and you need to study this curve to ensure that it will with stand the start current/time required.

Motor rated fuses are available with a longer overload time and these will enable motor starting with smaller fuses.


If you use a reduced voltage starter, then the start current will be reduced. The actual start current reduction will be dependent on the type of starter and how it is set up. - If you give us more information, we may be able to help further.


Best regards,

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