hi
i want to know how to calculate overload setting for motor AC

hi
if the AC motor have 55 KW,how we can calculate for overload setting

Hello amiruddin

You set the overload relay setting to the same as the rated current of the motor.
Best regards,

You can also calculate using the this formula P=I.E ,p=power ,I=current ,E=volatge then you can get the approximate motor current rating.Correct me if i am wrong.

best regards

Hello rene4538

You have not taken into account the power factor or the efficiency which can have a major influence.

P = V x I x eff x pf

The power rating of the motor is the shaft power, not the electrical power.
The best way to determine the current rating, is to look at the motor name plate. - always shows the current rating.

Best regards,

Normally, motors are selected 120% greater power than load. So, first, we check how much motor draws the current at the maximum working speed of machine which should be lesser than motor’s maximum current typed on the name plate. Now you can set the overload slightly at higher current under the motor full load current. This will also save your motor and your load.

hi
first U measure current with clampon meter and then increase relay setting 5 amp more than that.

Dear Amirudin,

The more the data you will give in the question the more closer the answer will be.

For example 55 kw, at what what voltage 380 volts, 415 volts, 690 volts..?

Let us say it is a 415 volts rated motor.

In case of a star-delta starter the location of overload relay matters a lot, it can be located where 100% of current is passing through the relay or 58%is passing through.

For sake of calculation, if the motor name plate rating is 100 amps, then follow these steps.

1. Set the relay at 58 amps and start the motor and

2.Check the current at the output of relay with a clampon meter after the motor is running at its full speed and full load.

3.Let us say it is 43 amps, then set relay 3amps above 43 that is 46 amps.

A properly designed motor should have OLr setting at 80% of its full load current ( @ Name plate )

Hope it helps.

Good Luck !!


Chaterpilar

for the overload setting
1
a) 125% of the full load current rating of a motor having a marked service factor of 1.15 or greater
115% of the full load current rating of the motor which does not having a marked service factor or where the service factor is less than 1.15
2
where a motor overload device is so connected that it does not carry the total current designated on the motor name plate , such as for wye-delta starting , the percentage of the motor nameplate crrent applying to the selection or setting of the overload swhall be clearly marked on the motor starter shown in the motor starter manufacturs overload selection table

find the actual running of the motor Check the voltage.depending on the load set the relay to that load if it is less than full load & if the load is more than full load set the relay at full load of motor