Motor Over Load & Winding Resistance

Reduce the mechanical load on the motor and see what happens.

If the motor starter uses overload heaters, change them to a step or two higher. If it is equipped with an adjustment (potentiometer) that is not already maxed out, turn it up a little.

Fire pumps are not equipped with overload protection at all, it being deemed that running the pump to failure is better than quitting to spray the fire.

Does the motor actually run hot? For 400V, 1.13A sounds about right for 0.37kW (0.5hp). What does the nameplate say for FLA (full load amps)?

I have to agree with Tornado, increase the size of the over loads.

Your amperage across the windings seem consistent although higher than 0.8-09A on the other motor. Did you check to see if the increase in amperage is due to whatever the motor is driving - example - a binding pump, worn gear box, bad bearing somewhere?

I would rule out any possible defects in with what is being driven before replacing the motor.

My Apologies - Meant say I agree with Tornado AND PWSlack

This motor is attached to a gear box driving a rotary feeder of a Bag Filter (in Cement Plant. Rotary feeder is OK. Problems looks like on Electrical Side

On Electrical side it took command, so it means that control circuit is OK

Regarding the starter it is Semi code (Siemens). There is no problem with in the semi Code as we check it with another Semi-code

There found some Unbalance in phase currents so is all tripping is due to this unbalance Phase current (may be)?

How we will make the resistance of all phases equal? Is there any shortcut for doing this may be inserting a resistor somewhere

I have confusion that how much it effect the cuurent in the winding with the change in winding resistance

Regarding the temperature of the motor it runs only for three or four minutes and its body temperature is quite normal.

Are you taking your measurements at the motor pecker head or at the starter?

How far is the starter from the motor?

As far as adding a resistor to even out balance, sorry, I can't help you out there.

You mentioned, winding resistance: Red-Yellow = 51 Ohm, Yellow-Blue = 46 Ohm, Red- Blue = 51 Ohm. Motor current: 1.12 A, 1.13 A and 1.12 A

Though there is a difference between the winding resistances, it doesn't look so abnormal. The motor current is also balanced.

What you need to do is already suggested.

First of all,

(1) Look into the motor nameplate data and find the motor rated current. If it is less than 1.13 A, increase the overload setting to up to 115% to 125% of the motor nameplate current and monitor the motor body temperature for few hours.

(2) Make sure that the mechanical load including the gearbox is not jammed, which is causing the motor to draw more current.

In case the motor still trips, replace the overload relay. Overload relay may be faulty.

If possible, try exchanging the motors to confirm the problem is in motor or something else (motor load side or the overload relay).

The idea of inserting resistance is not good at all – never do it.

Good luck.

- MS

Check the connection of the motor, some small motors now are 415V star connected and 240V delta connected. Connecting this type of motor in delta causes them to overload for no apparent reason. If a delta connected 415V motor was removed the new motor may have been connected in the same way as the old motor and not as the nameplate on the new motor states. We have had this problem in our plant.

Normally the FLC of this rating motor is 1.2 Amps. But you have to verify the exact rating. Then increase the overload setting to match the FLC and clean the motor fans and the grill behind the fans and operate the same. if the temperature of motor body increases beyond 75deg C, You have to check the Gearbox and bearings .

Good luck.

Sorry!

some of the answers are wrong because they failed to notice that the voltage is 400 and not 220V.

0.37kw , 400V on a 3phase motor will give, with a p.f.of 0.8 and an efficiency of 0.8 a current = 0.84A

to get 1.2A, your p.f.=0.7 and eff. =0.63 ==> ~1.2A

Therefore, it is importatnt to check the nameplate of the motor to accurately say whether it is getting overloaded!

If Overloaded ( if nameplate is ~1A or 0.8A), then definitely check the driven equipment for bad bearings on the motor itself or else...etc.

The difference in resistences is not much to worry about since the resultant impedence of the windings at 50Hz is ~400 Ohms here.

I hope this will help make a decision.

When a Motor overloads its either due to the driven load ,or due to motor bearings,check whether there is abnormal sound and vibration.What type of load does the motor drive is it a fan or a pump,if its a radial fan then thel start up time delay has to adjusted on the motor protections to accomodate the high starting currents while starting.Moreover the resistances of the motor are normal,the voltage is normal,no unbalance in current,check your terminal connections and confirm if connected as per manufacturers recomemdations.Confirm free movement of rotor shaft by rotating it.check motor alignments etc.

Wow, wow, wow. I cannot believe all the responses saying "Turn up the over load relay!"

A thermal overload relay is the primary motor protection device, it is NOT a luxury! Turning it up / changing elements is the quickest way to kill a motor, short of a lightning strike. With motors, heat x time = failure. More heat, earlier failure and increasing the OL setting on a motor that is already proven to be overloaded is risking a fire. The rule of thumb, also known by us old timers as the "horsey rule" (because a motor is like a dumb horse, it doesn't know when it should stop) is that for every increase in heater element size, you decrease the motor life by 20%. You could say the same for every click of the dial on an adjustable OL relays as well.

Aghvel Niazi,

You cannot "control" the motor resistance, that is part of the design of the motor. Besides, as was pointed out earlier, there is nothing wrong with the readings you are seeing. Your motor is very well balanced, you are well within 1% current balance which is more than acceptable.

The Siemens Simocode relay is your OL protection device. Double check all of the settings against what your motor nameplate says. Don't set the OL level to what the other motors are pulling, set it to this motor's nameplate FLA. No two motors are exactly alike, there are minor differences in manufacturing and application that will make them slightly different from one another.

The Simocode relay does have Phase Current Imbalance protection capabilities, make sure you have it properly set. Many people mistakenly set this very low, because they read that motors should be within 1% balance. But that is 1% voltage balance, which equates to about 10% current balance and the Simocode Imbalance protection is based on motor current. What it does is that in the presence of an imbalance, the Simocode biases the OL tripping curve down in order to compensate for the added heating effects of imbalanced current. When you set it at 1%, it begins to bias almost immediately. So either set your current imbalance protection to 10% or higher, or turn it off altogether, you don't appear to need it here and most small motors like this are a little more tolerant.

If the problem persists, or the current imbalance was not enabled, and you have the Simocode set to look for the motor nameplate FLA, then you have either a loading problem or a bearing problem. Spinning the load by hand means nothing; I can spin a 400HP compressor by hand, that doesn't mean it won't over load a 5HP motor!

Don't assume the motor is bad. In the scheme of things, this is one of the lower possibilities.

I should have added more detail in my suggestion.

0.37kw ≈ 0.5hp. If this motor were 460/3/60, its FLA would be about 1.0 (per NEC). Since it is 400/3/50, the FLA is probably higher (say 1.2). The OP didn't give this info, but at least it didn't seem that his motor was really overloaded. I don't think he said anything about it running hot.

Neither he nor I went into any discussion about the ambient temperature of the motor vs. the ambient temperature of the overload heaters/relay. This could allow (or require) adjustment of the OL rating in either direction.

An extreme case is blast freezer fan motors, which are cooled really well in their ambient, and thus can safely draw more than their conventionally rated amps.

Another unmentioned item was the motor service factor.