Electric Motors

Yes; 5/6.

If I want to bore you with mathematical formulae then I shall say:

rpm=freq.*60/p where p is number of pole pairs.

Torque=K*HP/rpm Torque=K*HP/freq./60*p

If number of poles will be the same and the torque will stay unchanged then:

K*HP1/freq1=K*HP2/freq2 or HP1/HP2=60/50 HP2=HP1*50/60

But I don't want to bore you.

And some de-rating because of additional heat generated as a result of decreased efficiency.

The K in your formula is proportional to current. The assumption is that the current is the same in both cases.

Actually, it is not too far off. I=V/Z where Z is the impedance of the motor. We don't know the winding resistance of the motor, but if the inductance is much higher than the resistance, we can assume Z is proportional to the frequency. Then the impedance is reduced by a factor of about .83 and the voltage is reduced by a factor of .87.

All true if this motor is an induction motor that is driven directly by this grid. If this is a brushed universal motor or a brushed synchronous motor then a few equations will be different. If this motor uses some driver electronics between it and this different power grid (VFD or stepper) then there could be no noticeable difference at the motor at all or the driver could inhibit operation due to this under-voltage condition.

Depending on the mechanical load, a derating of the wrong motor performance parameter (speed or power) could also produce other problems.

In addition to the above, if the motor relies on any type of fan for cooling (most do) then it must be derated even further to account for the change in flow rate. The volume of cooling air drops by (5/6)³ to 58% of the 60 Hz value, a very significant difference above half load.

You may also find that if there is any significant altitude derating under 60 Hz conditions then a further deduction in maximum power is required.

That depends also on if the motor cooling was originally designed for a speed range - no derating for reduced cooling may be OK. Shaft power is still 50/60 .

Not worried about insulation life?

http://www.baldor.com/support/Literature/Load.ashx/BR457?LitNumber=BR457

Page 65.

Baldor/Reliance/ABB

Depending on the frame size they are rated TEFC from 2:1 to 20:1 constant torque range. ie:warranted and designed operating range.

Since voltage is also reduced no dear ting is required.v/f is almost constant

A 3 phase, induction wound motor pretty much develops the same maximum torque at 50 or 60 Hz. Horse power is a function of torque and RPM. Lowering the frequency from 60 to 50 Hz lowers the motor RPM, but the torque capacity pretty much remains the same. Most 3 phase, induction wound motors will run fine at the lower frequency, as long the voltage is correct for the lower frequency. What is your load? The load's demand for horse power may change at a different rate than the motor. Your load may require much less horse power, but at less load the motor will not be as efficient.

Grand, the motor is 125 HP two speed,driving a dough mixing machine. The load's demand for HP is variable and instantaneously fluctuating to higher value.

keep the current constant you will get the derating

If voltage at motor terminal is reduced from 460V to 400 V then excitation current will be less. This combined with lower frequency (50 Hz in place of 60Hz) will result in less no-load loss (hysteresis and eddy current loss). Hence iron loss will be less.

If the motor has to develop the same torque to drive the mechanical load then load current will be more ( T=Φ.I , as flux is less due to less excitation current so load current will be more). Hence copper loss will be more.

As a result, total loss (iron loss+ copper loss) will be more or less same. Hence no de-rating is needed.

No, the frequency was also dropped. The v/hz actually went up slightly, but within typicall operating range. Excitation current will still be within rating.

The power will be derated by 50/60.

Yes, and the best source of advice is the Technical Helpline for the motor manufacturer. Pick up the phone, and make the call.

Dear Mr.georgerin72,

A similar topic raised about 15 days back, and you can search with in this forum. and read.

The answer for your question is YES. But you have to take care of Cooling Effect of Motor, since the speed will be 5/6 of the speed at 60 Hz. For the same load, the current drawn will be more, hence he heating effect in the winding will be more. V/HZ ratio should be same or the difference in both cases should be less.

Dhayanandhan.S