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# Kw rating of a motor

## Kw rating of a motor

(OP)
Hi, is the Kw rating on a motor mechanical output? I’m trying to get my head round if I can use a 7.5kw isolator for a 7.5kw motor?

### RE: Kw rating of a motor

Jk1996: Strictly speaking, the mechanical output of a rotating machine is torque - not power. For what I think you're trying to accomplish, you need to know the nameplate current as well as the kilowatt rating. This is because the amount of current required to produce 7.5 kW at 120 V is different than how much you need at 460 V (or any other voltage). In general, the higher the voltage the lower the current. Usually the isolator rating includes BOTH power (kilowatt) and current (ampere).

Converting energy to motion for more than half a century

### RE: Kw rating of a motor

(OP)
Thank you for your reply. I think I was getting confused as from what I believe a motor nameplate displays Kw ratings that are the mechanical output of the motor. The confusion then comes as the isolator is rated for the electrical power. Am I making sense?

### RE: Kw rating of a motor

Well, since induction motors are essentially constant speed, torque is equal to power so, yes the rating is mechanical hp. One hp is 746 watts.

### RE: Kw rating of a motor

Isolator? Disconnect switch or something like that?

Yes, they are typically rated in HP or kW at each line voltage even though the motor rating is shaft power. It's not that difficult to figure out input power for a certain shaft power.

### RE: Kw rating of a motor

Dear Mr Jk1996 (Electrical)(OP)26 Jan 23 13:51
" #1. is the Kw rating on a motor mechanical output? ..."

Yes. The kW rating on a motor is the mechanical output in kW. Note: the SI unit is kW , not Kw .

"#2. .... I can use a 7.5kw isolator for a 7.5kw motor? ..."
Take note that the Power (kW) is dependent on [(V x A x pf)/ (efficiency) ]
Same kW will have the current (A) reduced by half, with voltage (V) increased by two.....
In Europe the switching device is rated in Voltage and Current. See IEC documents.
In the US the switching device is rated in hp, voltage and current. See NEC for detail.
Note: the basic rating values are the voltage, current and the application , etc....
Attention: A say 400 V 20 A switching device may NOT be suitable for 100 V 80 A application. A switching device usually can be used at a lower voltage, but usually NOT suitable to take higher current, although the ( V x A) is the same !
Che Kuan Yau (Singapore)

### RE: Kw rating of a motor

Compositepro: Umm ... no. A squirrel cage induction motor will have the speed vary with mechanical load. More load means slower speed, until you get to a point where it stalls out. This means the torque profile is not a true constant (although it varies very little for small changes in load). A wound rotor induction motor behaves similarly, but since it has another power source for the rotor, it can limit the torque variation by varying the power applied to the rotor. Synchronous and direct current machines have even better torque regulation capability since they tend to have even better rotor power control.

Converting energy to motion for more than half a century

### RE: Kw rating of a motor

(OP)
So when sizing the isolator we can’t just use the Kw rating on the isolator and compare it to the motor nameplate?

### RE: Kw rating of a motor

Dear Mr Jk1996 (Electrical)(OP)1 Feb 23 19:03
"....So when sizing the isolator we can’t just use the Kw rating on the isolator and compare it to the motor nameplate?... "
1. Yes. We can’t just use ......
Take note that kW (NOT Kw) is dependent on the voltage and the current. A switching device has to take the voltage and the current ratings into consideration. For any application, the voltage and the current rating shall NOT be exceeded; even the kW is within the range.
2. In Europe, refer the IEC standards. In the US refer the NEC.

Che Kuan Yau (Singapore)

### RE: Kw rating of a motor

#### Quote:

So when sizing the isolator we can’t just use the Kw rating on the isolator and compare it to the motor nameplate?

Usually, yes. But, there are some motors that have a much higher FLA than normal so you can't do that for those.

A 100hp@480V motor is expected to have a FLA = 124A. So, the protection and control devices rated for 100hp@480V are designed expecting 124A. Slow speed 100hp motors might have the FLA in the 150-200A range so you couldn't just match the isolator to the motor HP rating for these motors.

### RE: Kw rating of a motor

kW is the motor output in SI units, equivalent to HP in Imperial units.
The HP or kW rating of an isolator switch is the maximum size of motor that may be disconnected with the isolator.
That is code.
There may be exceptions for very slow speed motors.
That is engineering experience.
And I know of an expensive failure on a 400 HP system.
Control gear completely destroyed.
400 HP motor shaft bent.
Coupling exploded.
Fan shaft bent.
That was ENGINEERING WITHOUT EXPERIENCE.
And the responsible engineer was warned, and chose to stand on his degrees and dismiss and rudely disrespect the man with less degrees but more experience.

#### Quote (Lionel Hutz)

Usually, yes. But, there are some motors that have a much higher FLA than normal so you can't do that for those.
Well said, Lionel.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

### RE: Kw rating of a motor

I too have an "expensive" story relating to ratings confusion.

Got called into a client to investigate the repeated failure of some 250HP VFDs. Making a long story short, the client ordered six "250HP" drives, because they had "250HP" dewatering pumps. MOST of the right questions were asked and answered, except the crucial one. The motors were 12 pole slow speed pumps. A 4 pole 250HP pump shows as 302A in the NEC chart, the drives were rated 302A, and USUALLY a standard 4 pole motor comes in at slightly less than the FLC charts in the NEC. But these being 12 pole, were 348A FLA. Ooops... X 6!

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

### RE: Kw rating of a motor

Mechanical power output of the motor could be calculated by using the following formula:

Pmech = Pout = τ * ω

where

Pout: output power, measured in watts (W);
τ: torque, measured in Newton-meters (N•m) or foot-pounds (ft/lbs);

### RE: Kw rating of a motor

@ Dear Mr bidrohini (Electrical)5 Feb 23 11:44
[color #204A87]" ...... Mechanical power output of the motor could be calculated by using the following formula: Pmech = Pout = τ * ω......where Pout: output power, measured in watts (W); τ: torque, measured in Newton-meters (N•m) or foot-pounds (ft/lbs);..."[/color
1. Please clarify " .....τ: torque, measured in Newton-meters (N•m) or foot-pounds (ft/lbs);...".
2. The statement uses the word " or "
Do you mean that SI unit 1 Newton-meter (N•m) = 1 foot-pound ?
Che Kuan Yau (Singapore)

### RE: Kw rating of a motor

Sorry,

P=τ * ω where, τ: torque, measured in Newton-meters (N•m)
1N•m= 0.738 ft-lbs

I want to edit my previous answer but can't find the edit option. Thank you for pointing it out.

### RE: Kw rating of a motor

(OP)
Thanks guys this has been interesting. So can I just clarify the kW rating on an isolator is equivalent to the motor name plate kW?

### RE: Kw rating of a motor

#### Quote (OP)

So can I just clarify the kW rating on an isolator is equivalent to the motor name plate kW?
Yes.
To avoid further confusion, the Ampere rating on a switch is intended for resistive loads.
The HP or kW raring on a switch considers the motor starting surge.
The HP or kW raring on a switch will be less that the product of the rated Amps and rated Volts.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

### RE: Kw rating of a motor

#### Quote:

So can I just clarify the kW rating on an isolator is equivalent to the motor name plate kW?

If you check the motor is a typical motor that has a FLA around what's listed in the NEMA standard FLA table.

### RE: Kw rating of a motor

Dear Mr Jk1996 (Electrical)(OP)7 Feb 23 18:40
"..... So can I just clarify the kW rating on an isolator is equivalent to the motor name plate kW? ...."
1. The over simplified answer is NO.
2. In the US , the NEC is the law.
(a) There are numerous conditions and limitations when using a motor-circuit switch as an "isolator"
(b) the motor-circuit switch is rated in hp, NOT in kW.
Note: Motor 7.5kW = 10hp approx. Per NEC typical 3-phase 10 hp motor current [30.8 A at 208 V], [14 A at 460 V] etc....
Attention: A low speed <1200rpm or a high torque motor may requires higher running current. In these cases the nameplate current rating shall be used.
3. Summary: The isolator rated Voltage shall > the system operational voltage and the rated current shall > say 1.2 times the motor nameplate rating.
Caution: use the hp rating with care, as it is dependent on the voltage and the type of motor, see above 2 Note.
Che Kuan Yau (Singapore)

### RE: Kw rating of a motor

To expand on Mr Che Kuan Yau's post:
30.8 Amps at 208 Volts = 11.088 electrical kW.
A 10 HP motor is 7.5 mechanical kW (motor nameplate kW)

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

### RE: Kw rating of a motor

@ Mr waross (Electrical)9 Feb 23 02:41
"...To expand on Mr Che Kuan Yau's post:
30.8 Amps at 208 Volts = 11.088 electrical kW. "

No.
Per NEC Table 430-150 , the typical 10 hp full-load current 30.8 A at 208 V. Expanding " 30.8 Amps at 208 Volts = 11.088 electrical kW " is NOT called for by the NEC. It is incorrect and is in conflict with the intention of the Tabulated value in the NEC, which is the Law in the US.
Che Kuan Yau (Singapore)

### RE: Kw rating of a motor

When it comes to motor control stuff, the NEC is not the law. I work with motor controls every day and generally don't give a shit what the NEC or the CEC says. Hardly need to look at either.

### RE: Kw rating of a motor

I am confused?
What has "Per NEC Table 430-150 ,......" to do with an explanation of the difference between electrical kW and mechanical kW as applied to electric motors?
Using mechanical kW to select a standby generator capacity may result in an unhappy customer.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

### RE: Kw rating of a motor

@ Mr waross (Electrical)9 Feb 23 16:52
"...I am confused? What has "Per NEC Table 430-150 ,......" to do with an explanation of the difference between electrical kW and mechanical kW as applied to electric motors?..."
Refer to the question raised by Mr Jk1996 (Electrical)(OP)26 Jan 23 13:51
Hi, #1. is the Kw rating on a motor mechanical output? #2. I’m trying to get my head round if I can use a 7.5kw isolator for a 7.5kw motor?, .... comprising of two different questions.
Question #1. had been fully answered. Now we are on question #2. , to avoid any confusion.
Che Kuan Yau (Singapore)

### RE: Kw rating of a motor

The kW rating on a motor is its electrical power rating, which is a measure of the amount of electrical energy it consumes. The mechanical output of a motor is typically lower than its electrical power input due to losses from the efficiency of the motor.

You can use a 7.5 kW isolator for a 7.5 kW motor, but it is important to make sure that the isolator has the correct current rating for the motor and that it is suitable for the voltage and frequency of the power supply. Additionally, you should check that the isolator complies with relevant electrical safety standards and regulations.

### RE: Kw rating of a motor

#### Quote (Maximillianrichards)

The kW rating on a motor is its electrical power rating, which is a measure of the amount of electrical energy it consumes.

No, wrong.

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