Power consumption data acquisition

[GeorgeB]

In my normal electro hydraulic work, I am able to use current to accomplish all that is required; the customer is interested in not overloading the motor "much".

I've been asked to look at 4 motors (50 hp, 480-3-60) simultaneously and record (I have the data acquisition equipment) the power consumption (kW) vs machine cycle (obtained by pressure transducer). He's after something better than a linear approximation between no load and full load current; that has worked for me in the past.

This will be a rather short test (perhaps 2-4 hours), and we MAY want to look at 4 different motors after initial work.

My issues are:

I know of no way to make the potential connection and leave it for the several hours that is compliant with 70E.

I cannot shut the system down to install CTs, but there is access to insulated motor conductors, so they must be either split core or clamp on. I suppose that we could pre-identify the loads and have CTs installed during a scheduled shutdown. 6 or 8 years back, I saw a unit by Hioki which met the needs, but 4 of their their present 3169-21 and the 12 CTs is a little overkill for this one-time need.

Have any of you done something like this before, and do you have any suggestions?

Thanks!

George

 

[gar]

100305-1720 EST

GeorgeB:

I want to suggest that current is probably the correct measurement relative to overloading of a motor. This is most directly related to temperature rise in the motor rather than power input to the motor.

Suppose the motor power factor is 0.8 for full load on the motor, 100%. Now drop the motor load to 10% and the power input drops to not too much over 10%. Wild guess 20%. But the power factor dropped to a lower value and the current did not even drop to 20%, maybe 40 to 50%. I do not have any good figures to work with so these are illustrative guesses.

Here is some data from Bailey and Gault p 213 from a plot of motor performance curves.

Rated output 35 HP, PF 0.94, 88% efficiency, 41 A, 1740 RPM.
At 5 HP. Pf 0.54, 60 % efficiency, 12 A, close to 1800 RPM.

So my guesses are somewhat in the ballpark.

If you were concerned with mechanical overload of the mechanical drive or hydraulic components, then power would be more useful.

How would you convince someone that current was a better measure of motor overload I do not know.

Other possibilities that probably your customer would not buy are these:
Do a single phase power measurement and multiply by three. Is there a control transformer with a 120 output that could provide a good estimate of the motor supply voltage and phase so a lower voltage could be used for the voltage measurement. No reason not to go to 12 V if your signal conditioning equipment can work with it. Use clamp on current sensors.

The advantage of a three phase power monitor is that the response is faster for short transient power loads. On a machine where there was no output torque transducer, the customer was unwilling to buy this option on the machine, I used a three phase power monitor on the motor input to get an estimate of torque for troubleshooting.

The type of torque curve this kind of machine had is illustrated in plot at photo PA1 at my website http://www.beta-aa.com/pa_plot.html . This plot photo is from a machine that included the straingage torque transducer in the machine. The results from power measurement were far less useful, but still sufficiently useful to look for a binding problem.

.

 

[charlietuna]

Seems that you need to monitor all four motors as individual loads during a specific period of time. Anything other than that would be "guess work". Possibly you might consider data logger rentals along with split core CT's?

 

[GeorgeB]

I want to suggest that current is probably the correct measurement relative to overloading of a motor. This is most directly related to temperature rise in the motor rather than power input to the motor.

I agree, and am sorry that I didn't go into details.

This customer has been sold a system which will reduce his energy usage. This is real, not trick; presently they load 4 motors to about 95% for 5% of a cycle, with average load on the motors of about 15%. They have, as was predicted, been able to shut one pair of motors and suspect the average load has gone from about 15% to about 35%. The hydraulic accumulators aren't ideal, darnit<g>. Much of that very low PF load isn't there either, POSSIBLY reducing their utility rate.

We've been asked to quantify the energy savings by comparing an updated system to an older system. Thus power vs VA.

 

[GeorgeB]

Seems that you need to monitor all four motors as individual loads during a specific period of time. Anything other than that would be "guess work". Possibly you might consider data logger rentals along with split core CT's?

EXACTLY! Do you have a data logger that you have found good for this type application? Rather than a logger, we prefer an analog output to have with the hydraulic pressures and flows in our data logger. Correlating from multiple logs is not easy.

I cannot find anything much at all, and nothing to rent.

 

[gar]

100307-2235 EST

GeorgeB:

The power monitor I have was made by Load Controls Inc. It is three phase with three holes to pass the current conductors thru. Requires center tapped transformer or 14 to 20 on each side of the center tap for power. Output signal is +/-10 V I believe. Three voltage terminals. Name tag says 460 V 10 A 0-10 V.

They probably have what you need.
http://www.loadcontrols.com/
for example
http://www.loadcontrols.com/downloads/PPC3datasheet.pdf

.

 

[GeorgeB]

The power monitor I have was made by Load Controls Inc.

PPC looks like exactly what is needed; I've passed it to the office to evaluate. Thanks.