Electric motor help

[JB160]

While doing a run test of an air cooled heat exchanger that uses a 60hp 900 rpm 460V/3PH/60Hz electric motor. The motor was pulling 82 amps as measured on the VFD but the motor at full load is rated for 74.6 amps @ 460 volts. if I calculate the horsepower draw at the 82 amps using the following formula:

Amps X Volts = Watts
Watts /746 = HP

I get a HP draw of 50.5. Is this a correct way to calculate the HP draw at those amps? 74.6 amps would calculate to a HP of 46, could this be due to the service factor of the motor?

 

[waross]

That formula is for DC and single phase.
For three phase you need to add a factor to account for the out of phase contributions of current on three phases. Multiply your results by root 3 (1.73)
Your apparent power should be 74.6 Amps x 460 Volts x 1.73 = 59437 Volt Amps, VA or VA/1000 = 59.437KVA
Watts = 60 HP x 746 = 44760
44760 Watts / 59437 VAs = 0.75 That ratio includes the power factor and the efficiency.
If your motor has a power factor other than 1.0 you should have told us.
A 60 HP motor with a service factor of 1.1 has a usable HP of 66 HP and the some of the calculations I have done will be off by 10%.
Multiply the rated HP by the power factor and you do the calculations this time.
You may be overloading your motor.
You should be reading the display on the VFD. Don't try to measure the motor current on a VFD with a clamp type ammeter.
Try dropping your speed setting about 3% or 4% and see if your current drops to 74.6 Amps.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[JB160]

Thanks for the info.. BTW.. We use a 1.15 service factor for our motors.

 

[jraef]

Your apparent power should be 74.6 Amps x 460 Volts x 1.73 = 59437 Volt Amps, VA or VA/1000 = 59.437KVA
Watts = 60 HP x 746 = 44760
44760 Watts / 59437 VAs = 0.75 That ratio includes the power factor and the efficiency.

Bill,
You made a small leap in that. We do not really know the PF here, because you are comparing his MEASURED kVA (59.437) against a NAMEPLATE maximum kW rating (44.76). We don't have a MEASURED kW, so we cannot determine the PF here.

So JB160, you don't know the actual HP right now. You can take a slightly more educated GUESS at it simply using what waross said indirectly in his post:

Nameplate FLA = 74.6A
Measured Amps = 82A
82/74.6 = 110%
Rated HP = 60HP
Connected HP = 60 x 1.1 = 66HP

This still is not really accurate though because even though your motor has a 1.15SF, one issue with running a motor into the Service Factor is that once you go over 1.0, the amps, torque and expected motor life are no longer guaranteed to be accurate. In other words the performance criteria and accuracy goes out the window with the overloading. SF just says that thermally, the motor can take it.

HOWEVER, I know of NO motors that are rated above 1.0SF if operated from a VFD! The added harmonics on the output PWM waveform going to the motor and the possibility of reduced cooling when running at lower speeds essentially consumes the added thermal capacity in the motor SF design, you cannot use it twice. You need a bigger motor, or you need to lower your expectations on motor life by a large factor, i.e. use months instead of years, or you need to run it without the VFD. Pick one...

"Will work for (the memory of) salami"

 

[waross]

I guess that I wasn't clear. I meant to say that the efficiency and the power factor combined accounted for the difference between the rated kW and the measured KVA. I was under the impression that the nameplate amps was the current at the rated service factor. I don't have time to check that tonight. Maybe tomorrow.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rhatcher]

Jraef,
Waross's calculations are correct. He took the nameplate rated volts and rated amps to determine rated kva and then divided rated kw/rated kva to determine a factor of 0.75 that represents the motor's rated efficiency and power factor. Since efficiency and power factor only vary slightly when operating in the range of +/- 20% of rated power, this can be used to determine a good estimate of power output. Using Waross's calculated factor of 0.75 and the load amps of the motor:

82A * 460V * 1.73 = 62.256KVA
65.256KVA * 0.75 = 48.942KW
48.942KW / 0.746 = 65.6HP

Of course, the method you used achieves similar results and is much easier. But, I want to make sure that Waross gets credit for being right and also does not lose much time trying to figure out where he went wrong. In either case, the OP is overloading his motor and will have a reduced service life.

As a general rule, each 10C increase in operating temperature cuts the insulation life in half. If you allow an increased temperature rise of at least 10C from operating at 1.09 service factor plus the additional heating from the pwm waveform of at least another 20C and then add in a 10C fudge factor, I'd give it about a year, maybe two if he's lucky. This is why, as you stated, that no manufacturer rates motors to operate above 1.0 service factor on a VFD.

Of course, this is assuming that the motor has inverter duty winding insulation (Class F with 1000V p-p insulation). If not, all bets are off, he'll be lucky to get a few months even without running above rated load.

Waross offered a good suggestion of lowering the motor speed by about 3-4%. This should get the load amps in the range of the nameplate value. However, assuming that the motor is presently running at the rated speed that is required for the heat exchanger fan to provide proper cooling, reducing the speed will reduce the cooling and therefore reduce the effectiveness of the heat exchanger.