Motor energy consumption calculation

[Ted99]

Performing an energy balance on a mixing process. Need to calculate energy consumed of a mixer through course of the mix cycle. What do I need to measure & how do I calculate total ennergy from mixer?
Thanx!!!

 

[jraef]

Amps, Volts and Power Factor, integrated into time. Or you could just go buy a watt-hour meter, that's what they do.

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[ccfowler]

Ted99,

How accurately do you need to have this data?

What power range is involved? Fractional hp, 5 to 50 hp? Much larger?

Since you are concerned with only work done by the mixer to the process, you need to eliminate motor losses, gear drive losses, bearing drag, etc.

 

[Ted99]

Only looking for a rough approximation (+/- 5-10%). Working with a 100 Hp motor, 480V, 3 phase.

 

[ccfowler]

Ted99,

Sorry for the slow response. (Computer issues among other things.)

Assuming that you have a motor that has not been rebuilt and the nameplate data is valid, here is a scheme that I have used to approximate the actual load on a motor. Although seemingly crude, I've often found it to be more practical than using amperage readings where the motor may be running at less than rated power. It is based on using the slip of the induction motor to indicate running torque. All you need is a tachometer and reasonable confidence that voltage is within the motors rated needs.

Assume that running torque varies linearly with slip (0 torque at synchronous speed, and torque consistent with rated horsepower at rated full load speed). For a typical 4-pole motor and 60 hz, 0 torque at 1800 rpm, and full load torque at 1750 rpm. Obviously, use the nameplate data for your actual motor. The calculations are simple, and all the "monkey business" with power factor issues can be pleasantly ignored.

This isn't perfect, but it should satisfy your accuracy needs fairly well.

 

[Ted99]

Thanx all for the input, very useful/educational. Have discovered a few new twists to my problem.
a) The 100 Hp drive I've been working with is providing its power to a 125Hp motor. The mixer has been working without issue, but the reason(s)for such a paring, escapes me.
b) The drive/motor is variable frequency/inverter duty.
*********
1) Under what conditions might such a Hp match up be desired?
2) How should I expect this to impact the energy calcs?
3) Does the variable frequency/inverter drive/motor change how I need to approach this problem.
Regards,
Ted99

 

[ccfowler]

Ted99,

First, since your motor is not just a simple induction motor but is powered through an adjustable speed drive, the relationship that I suggested doesn't hold well. That relationship is only practical when the motor is powered by reasonably "clean" sine-wave voltage.

Second, using a 125 hp motor with a 100 hp makes sense. The "electrical noise" (harmonics, nasty spikes, etc.) impose a greater thermal load on the motor, and the greater heat dissipation capabilities of the motor are needed to avoid overheating.

Third, relying on metering the power input to the drive to estimate the work input to the process by the mixer is not a simplistically accurate method. The "real power" input to the drive includes resistance heating losses within the drive and the motor, and these losses become increasingly significant at reduced power levels. Most likely, bearing and seal losses will be reasonly trivial, but you will have to verify the actual situation for your application.

Still, since your accuracy needs are not overly burdensome, you may be able to use input power measurement along with some realistic efficiency allowances to estimate the work input to the process.

I would do some research into the nominal performance characteristics of the motor and the drive to determine some reasonable basis for making these allowances. Since the motor is probably operating well below its "normal" power rating, its actual efficiency is probably significantly diminished from its nominal, full-load efficiency.

In most cases, the real energy savings provided by adjustable speed drives comes from operating their load at reduced speed. The diminished efficiency of the combined drive and motor at part-load normally pales in comparison to the "work not done" by not driving the load at full speed.

 

[peebee]

Careful on metering either side of your vfd (input or output side)... Make sure you have a true RMS-sensing power meter. Otherwise, the choppy waveforms that you see on either side could give you some very inaccurate power readings.