Motor Normally Operated of Rated Current

Check your meter(calibration) and connection(star or delta) in nameplate.

Dear pnaban,

meter is calibrate by last month, and the motor 400vac is for star connection .

What's the service factor?

power factor :0.66

The power factor is no doubt quite low (and hence reactive current would be high) at no load; but even so, 1.58A does seem unusually high.

What is the ambient temperature? Motors are commonly rated for 40°C rise, so 69°C might or might not be excessive.

Dear Tornado,

Ambient temperature:-15°C - + 40 °C

Insulation class:F

, i test at Environment conditions:~30 °C

Motor normally operated at 80-85% of rated current w/o load... For is information is right?? i can't found this information

There isn't really a standard for no load current of motors. Some mfrs do (and others don't) provide this information.

if all motor manufacturers print no-load current in the name plate,it would be very useful to check a motor's no load current after rewinding/replacing bearings etc

Maybe....

Because at no load, the angle between the voltage waveform and the current waveform is quite high, and at full load, it is quite low. Welcome to Power Factor.

None of the figures here give any immediate cause for concern.

Make sure that a motor overload device is wired into the starter circuit and that the unit is set correctly for the full load current of the motor. After that, don't worry, chicken curry.

When the voltage is higher than nameplate, even if within tolerance, the motor windings saturate a little and use more current as heat, that's why you are seeing the high temperature. At 1.58A on a motor rated 1.5A, your current is only 5% high, your overload relay pickup point is likely no lower than 115%, so i would not expect it to trip. Combined with the low power factor when unloaded, you are not seeing anything that unusual, especially with a motor that is only 62% efficient! By the way, that SUCKS, i would look for a better motor if I were you.

You have not told us the motor rated speed. As the number of poles increases and speed falls, the power factor gets worse and difference between full load and no-load currents becomes small.

For example, a modern high efficiency, Brook Crompton Motors type "W", 700 rev/min, 0.37 kW motor has FLC 1.63 amp @400V 3 phase, efficiency 62%, power factor 0.53 at full load, falling to 51%, 0.34 p.f. at half load. Refer http://www.brookcrompton.com/pdf-files/20113E_W_CI_IE2.pdf.

For example, with 0.6 power factor, the inductive current is 0.8 x FLC.

N.B. Right angle triangle with sides of length 1, 0.8, 0.6 represents 0.6 power factor.

Since the losses are high with 0.62 efficiency and magnetising losses are little reduced at no-load, while resistive loss will fall little if current is near FLC; little change no-load to full load is expected.

Note that if the losses are still near FLC value ~ 40%, the resistive side of the current triangle would be 0.4 X 0.6 = 0.24. With inductive side at 0.8, the resultant line current would be Square root (0.8 x 0.8 + 0.24 x 0.24) or about 0.84 rated.

You have raised the the line voltage, increasing no-load current - in any case, the tolerances on no-load magnetising current are high, like 10%.

Also, if you are using an ammeter on 10 amp range, a 2% full scale error is 0.2 amps.

So, in summary, if you have an 0.37 kW low speed motor, your numbers are not alarming. Worry if temperature rise is high at rated load.

67model

Add to post #12,

If you have winding 69 Celsius at 20 ambient, that is 50 degrees rise compared to probably 80 max for class B insulation.

No load at 85% rated current would be 0.72 x rated copper loss. For a small motor with 60% efficiency, expect 10% mechanical, 15% copper and 15% iron losses. Dropping copper loss to 72% rated only reduces loss 5% (in 40%) compared to full load. Temperature rise of about 50/0.72 = 69 degrees would be expected at worst, forgetting that rotor current would fall a lot.

67model

There is no hard & fast rule for the calculation of motor starting current. It depends on construction and manufacture.. we do have some old motor(Made in Romania ) which are 0.65KW rating and taking 4 Amp. of current. It's temperature after running few hours is 80 degree C.

Those are good candidates for early replacement on efficiency and on personal safety grounds.

my only question is.....other than a simple spin test under power to check rotation quickly and to verify that the motor works, why are you running it without a load?? the ratings are really only useful if the motor is performing work your voltage is within range. fire it up!