Efficiency of a 3-phase motor.
Efficiency of a 3-phase motor.
(OP)
Hi everyone,
This is my first time dabbling in the energy management field and I already find myself turning to a forum for some help!
I am trying to figure out the cost of running a three phase motor. I have ended up with:
Cost=Volts*Amps*PF*1.73/1000 * hours*cent/kWh
From what I have read this is good for a general indication of the cost. However would I be correct in saying that the efficiency of the motor is not taking into account and if so, could it be used in conjunction with this formula to give a more accurate running cost.
The motor is powering a conveyor and at the minute I am using the nameplate values.
This is my first time dabbling in the energy management field and I already find myself turning to a forum for some help!
I am trying to figure out the cost of running a three phase motor. I have ended up with:
Cost=Volts*Amps*PF*1.73/1000 * hours*cent/kWh
From what I have read this is good for a general indication of the cost. However would I be correct in saying that the efficiency of the motor is not taking into account and if so, could it be used in conjunction with this formula to give a more accurate running cost.
The motor is powering a conveyor and at the minute I am using the nameplate values.





RE: Efficiency of a 3-phase motor.
If you are using motor nameplate amps and power factor, that will give you the power into the motor assuming it is running at full load, which is generally not a good assumption.
Power factor, efficiency and amps will vary as the motor load changes.
RE: Efficiency of a 3-phase motor.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Efficiency of a 3-phase motor.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Efficiency of a 3-phase motor.
If used FLA off nameplate and power factor from SWAG !?!), then no need to account for efficiency.
If used hp off of nameplate, then don't need to worry about voltage or amps (if voltage is assumed at nameplate), and efficiency is the only thing you need to account to determine full load input power = Pout / Efficiency with suitable unit conversions for HP <-> watts
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RE: Efficiency of a 3-phase motor.
That's a bigger error than efficiency.
RE: Efficiency of a 3-phase motor.
If you actually measure the values in your formula you do not want to include efficiency because you want to calculate the power going into the motor since that what you pay for to run the motor. But then, a meter that will measure volts, amps and power factor should also measure power directly and would likely give a kWh number if you let it record for a while.
RE: Efficiency of a 3-phase motor.
RE: Efficiency of a 3-phase motor.
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Efficiency of a 3-phase motor.
The math was an trending example. Larger motors are typically about 95% efficient at full load.
RE: Efficiency of a 3-phase motor.
http
500hp 2-pole 96.7% efficient at 3/4 load
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Efficiency of a 3-phase motor.
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Efficiency of a 3-phase motor.
1) Use larger diameter wire (less IR^2 losses) and increase the slot "Packing Factor." A slight increase in cost.
2) Use thinner and/or better Electrical Steel for the laminations. Hold the Lamiation Suppler's "Feet in the Fire" while negotiating cost.
3) Improve the Windage loss. Zero cost after re-design.
Charge Premium Prices for the Motors. See all Government Rule Changes as an opportunity to increase profit margins.
RE: Efficiency of a 3-phase motor.
The volts * amps * PF * Rt3/1000 gives you electrical input power which is your cost. If you now multiply this by the efficiency, you will have the mechanical output power.
As you are interested in the cost, you can use the electrical input power, measured or calculated using measured or rated values, or, you can use the motor rated power divided by the efficiency.
NB The rated values are at full load. As the shaft load reduces, the input power reduces and the cost goes down. The motor has a high efficiency down to less than half load.
Best regards,
Mark Empson
L M Photonics Ltd
RE: Efficiency of a 3-phase motor.
These power meters could easily pay back when used as a load management tool especially if your system is supplied by utilities which charge demand, penalize for low PF,and or employs "time-of-use" (TOU) schemes in their power rates.
RE: Efficiency of a 3-phase motor.
But you remind me that if they have a relatively steady conveyor loading they could use the plant PM to measure just the conveyor for, say, 30 minutes.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Efficiency of a 3-phase motor.
This is the number of Watt-Hours per revolution of the disk.
The Kh value is printed on the meter face. I usually time 10 revolutions and take an average to get the time for one revolution.
3600 seconds per hour divided by the time in seconds for one revolution times the Kh equals the kilowatt demand on the meter.
Quality data at an affordable price.
Bill
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"Why not the best?"
Jimmy Carter
RE: Efficiency of a 3-phase motor.
RE: Efficiency of a 3-phase motor.
Thanks to everyone who replied (really sorry about the delay).
Some good info here, I should have plenty to work with.
Thanks again!