three phase motor - question about kW/HP-measurement

Discussion in 'Electronic Basics' started by Alexander Fischer, Jul 9, 2003.

1. Alexander FischerGuest

Hi

I have a problem of understanding concerning the power a motor should
deliver. In europe, normally the powere is measured in kW, while at
the US-Net, it is given in HP. As example, a supplier speaks of 200kW
at european 460V/50Hz-Net and of 250 HP at the US-Net with 480V/60Hz.
The conversion of 250 HP would be 184 kW.

Does the motor delivers only 184 kW in such case, or is there a
difference between the values of kW in Europe and HP in the US?
Someone mentioned, that it might be that the european value is
measured by the power the motor takes from the net, while the us-value
is measured by the power available at the shaft.

Could someone clear me up about those things?

regards,
Alexander

2. Alan McClureGuest

Assuming that the HP doesn't change because of the difference in
frequency,
I would say that the motor in question has a specified efficiency of 92%
at full
HP delivered should equal power consumed times efficiency.

ARM

3. Mark EmpsonGuest

In both cases, the power rating is the shaft power of the motor, and to get
HP form KW, divide by 0.746, or to get KW from HP multiply by 0.746. The
conversion is voltage and frequency independent.
Note that the KW rating of the motor is not the same as the power drawn by
the motor, that includes the losses and is the shaft power divided by the
efficiency.

Best regards,
Mark Empson
http://www.lmphotonics.com.

4. Jon ElsonGuest

I think there is, but it is infinitesimal.
No. The difference is that on 60 Hz power, the motor delivers full
rated torque at 20% higher speed! Since Hp is torque x speed, it
therefore produces 20% more Hp (or watts) on US power.

Jon

5. BGGuest

Or 20% less torque, at the same HP.

6. ~^Johnny^~Guest

You can't get something for nothing.
More slip = more torque, but at a lower than rated speed.
Since the motor is rated for a certain slip speed at 50 Hz,
slip increases at 60 Hz, to provide the same performance, at a
higher current.

OTOH,
Increasing current in the windings can drive stator cores into
saturation, increase I-squared-R losses (exponentially, mind you),
among other things... and I =love= to wake up to that BBQ smell of
rosin, phenolic, and enamel all burning at the same time ))

I had a buddy, back in the seventies, who called me up with a problem
with a generator he had purchased (Army surplus). He said it was
frying things, like his air compressor motor...
stereo and TV and other electronics seemed to work fine with it, and
it put out the required 120 volts. I plugged the shop fluorescents
into it, and they flickered, then smoked (damned mag ballasts of the
70's). Then I accidentally touched the input RCA jack center
conductor on the still-playing stereo, and heard that telltale
400-cycle hum...

oops!
Frequency does matter. :-O
--
-john

~~~~~~~~
"The first step in intelligent tinkering is to
save all the parts." - Aldo Leopold
~~~~~~~~