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#23281 03/14/03 12:15 PM
Joined: Dec 2001
Posts: 50
I have a 3phase 50kw A.C. motor on a new air compressor that is being installed. The manufacturer of the Air compressor says it is a 75hp motor. How can that be? It seems to me a 75 hp motor would be more like a 90kw motor. Or am I all wet?
This 50 kw motor is on VFD.
My question is, Should I take the mfg's word for it and treat it like a 75 hp or should I treat it like a 50kw motor. I plan to run it on 480volts.
It is a Europeon motor and it is set up for wye/ delta starting.
The nameplate has very little info. it says "400 v 50kw."
Any help would be appreciated.

#23282 03/14/03 12:52 PM
Joined: Apr 2002
Posts: 2,527
"50kW", like horsepower, is the mechanical output of the motor. To convert motor kW to Hp, divide by 0.746 {or about three-quarters}, giving about 67Hp. The claim of 50KW = 75Hp is not accurate.

Adding the wye-delta starting and 400V [50Hz?] rating, it looks like the compressor was not intended for use on the North-American continent. Also, a 50Hz motor powered at 60Hz will try to run 20% faster, but may heat up from overload in the process. I would be really careful to be sure the vendor doesn't blame everything else if the equipment doesn't work satisfactorily. Watch out for jacking up the starter-overload setting, or switching sheave sizes on the belt drive to get around the motor not being capable of continuously operating with a true 75Hp load.

Working in reverse, multiplying 75Hp x 0.746 = 56kW [not 50] motor nameplate is needed to meet the 75Hp claim.

[This message has been edited by Bjarney (edited 03-14-2003).]

#23283 03/14/03 01:15 PM
Joined: Sep 2002
Posts: 1,498
Likes: 1
C-H Offline
Is there some kind of duty cycle on this motor? The 75Hp could be for a short period of time. The air compressor in our office/workshop just runs for a few minutes at a time. (And it makes me jump each time it starts) The marketing people will of course cite the maximum power. [Linked Image]

Another thought: If both voltage and frequncy are increased 20%, wouldn't that mean that the power too increases 20%?

[This message has been edited by C-H (edited 03-14-2003).]

#23284 03/14/03 07:25 PM
Joined: May 2001
Posts: 30
Out of curiosity, what exactly is the formula for determining the FLA on a 3 phase motor when only the KW is known?
For instance, If they say this motor is a 75hp motor, I would look at the typical FLA for a 75 hp motor and it should be around 96 amps at 480volts.
I always thought it would be:
96 X 480 = 46080
46080 X 1.73 = 79718.4
Divide by 1000 = 79.72kw

Comes out to about 80kw not 50
What am I doing wrong?

#23285 03/14/03 07:58 PM
Joined: Jan 2003
Posts: 33

You are finding KVA not KW. You would need to multiply the KVA by your powerfactor to get KW.

#23286 03/14/03 09:40 PM
Joined: Apr 2002
Posts: 2,527

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#23287 03/14/03 10:32 PM
Joined: Oct 2000
Posts: 2,722
Broom Pusher and
As stated, ExI=VA, not W ("E"=Volts / Electromotive Force, "I"=Amperes / Intensity, "VA"=Volt-Amperes / Apparent Power, "W"=Watts / True power).

To get the amount of True Power ("W") which is flowing within the Apparent Power ("VA"), you would need to know the Motor's Efficiency, in Percentage, or the Power Factor, in Percentage.

Here's some formulas (more can be found in the Technical Reference Area):

Find "I", when KW is known - for 3Ø systems of all voltages:

I = HP × 746 ÷ 1.73 × E × PF

To find KW:

KW = I × E × 1.73 × PF ÷ 1000


* I = Amperes,
* HP = Horse Power,
* E = System Voltage,
* KW = Kilo Watts,
* PF = Power Factor (in Decimal equivalent of known percentage),
* 746 = Equivalent Watts to 1 Horse Power,
* 3Ø = 3 Phase.

Good Luck.

Scott35 S.E.T.

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
#23288 03/17/03 11:42 AM
Joined: Sep 2002
Posts: 1,498
Likes: 1
C-H Offline
I just realized that I overlooked something: I think the motor is a red herring. It's the VFD that will determine performance. The frequency and the voltage the motor operates on will be determined by the VFD. If the air compressor, VFD and motor come as a set, the manufacturer could have designed it to operate at a different performance than the nameplate on the motor indicates.

Some motors as small as 4Hp are designed for both 400V and 690V operation. It wouldn't suprise me if this huge 75Hp motor is too. If I'm not mistaken, this in combination with a VFD would allow use with higher frequency and voltage and hence higher power than the nameplate indicates.

I'm no expert in motors, and I hope someone will correct me if I've written something stupid.

[This message has been edited by C-H (edited 03-17-2003).]

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