60Hz motor to run at 50Hz

[op9]

I have been asked to look into a problem of a belt driven fan from the US running here in Australia on 50Hz, drawing far too much current. I am attempting to obtain more info from the site which is across country, but at this stage my question is fairly general, and I am assuming no other abnormal conditions.<br><br>The nameplate specifies operation at either frequency as followes:<br><br>60Hz 1780RPM 75HP 230/460V 184/92A <br>Service factor 1.25 230/115A<br>Eff 94.1/93<br><br>50Hz 1475RPM 75HP 208/415V 210/105A<br>Service factor 1.0 210/105A<br>Eff 93/91.7<br><br>There are 6 individual field coils which were connected by the local electrician as series delta. (ie 2 coils in series for each side of the delta). It has a star-delta starter but only overloads as the fan outlet damper is opened.<br><br>It is being supplied at 415V 3 phase 50Hz and is supposedly drawing max amps at only about 45% full load.<br><br>Would you expect a standard 60Hz wound motor to be capable of the same 75HP at 50Hz given there is a voltage reduction from 460 to 415VAC?<br><br>I would appreciate anyones comments.<br><br> <p>Rod Nissen<br><a href=mailto:nissen@powerup.com.au>nissen@powerup.com.au</a><br><a href= > </a><br>

 

When you lowered the frequency to 50 Hz and the voltage to 415 Vac, you changed the V/f ratio. If the fan were operating at 460 Vac, 60 Hz, the V/f ratio would be 7.67 Vac/Hz. At your settings, the V/f ratio is 8.3 Vac/Hz. This increase in V/f ratio can cause more current to flow to maintain the same torque. I don't know, however, if this is what is causing max rated current at 45% load. I'm not sure how to calculate the effects of changing V/f on current. Maybe the motor is too small for the fan you're trying to drive, or there are excessive losses of a mechanical nature that require more HP than the motor is rated for or you're getting shorts in the motor windings that are reducing winding resistance and causing current to increase. It's difficult to tell. I wish you luck determining the problem and a solution.<br>

 

[jimmy56]

You check the series / delta connections. If the series connections made reverse than current can also increase.

 

[motormouth]

If you really mean a motor wound for 60Hz then, no, it will not produce the same power at 50Hz if the same voltage is applied. Output power reduces by 5/6ths as shaft power is the product of torque (similar at both frequencies) and speed. Clearly, if the speed reduces by 5/6ths so does the shaft power.<br>
However, your motor has multiple windings that are connected to suit the voltage and frequency of the supply. As can be seen by the rating plate details, the current (therefore torque) increases when connected at lower voltage and frequency. Note that the service factor reduces from 1.25 to 1.0 at these lower values so the motor will trip sooner if overloaded.

 

[Comer]

The effective impedence of a large AC motor is dominated by the inductance of the windings. If you apply the same voltage at a lower frequency, it will draw too much current. However, if you apply a proportionly lower voltage at a lower frequency, it will run okay (within limits). The biggest problem is heat dissipation since the cooling fan (assuming it has one) is running slower and moving less air, hence the reduced service factor. <br>
<br>
As regards horsepower, it is useful to look at actual electrical input for the two conditions (per the nameplate): 73.3 KVA @ 60hz and 75.5 KVA @ 50hz. Bottom line here is that you can get 75 HP in either case, but with less reserve at 50hz. <br>
<br>
As far as power output versus current, I agree with xnuc that you should look at the load you are trying to drive. You might also check voltage and current per phase and insulation resistance to ground for each winding (isolated from the other windings) to be sure that imbalance is not a contributor.