calculator for motor power

[panthripu]

Hi guys
I am looking for some type of calculator where i can get power drawn by a induction motor if I know the current being drawn by it. Calculating by formula is also a way but I need manufacturer curves ( siemens and ABB) to get curves , current Vs power factor. Where can i find it ?

 

[Besoeker]

Hi guys
I am looking for some type of calculator where i can get power drawn by a induction motor if I know the current being drawn by it. Calculating by formula is also a way but I need manufacturer curves ( siemens and ABB) to get curves , current Vs power factor. Where can i find it ?

You need power factor as well and that varies quite a lot with load.

 

[Jraef]

You need power factor as well and that varies quite a lot with load.

Yep, sorry. You can't calculate power accurately without knowing the power factor. You can ESTIMATE it based on educated guesses of the PF, but the best way to KNOW the power consumption is to measure it.

If you know current, you must be measuring it, right? Just get a better meter or transducer, one that also looks at voltage so that PF can be determined by comparing the angular displacement of the two values.

 

[Besoeker]

Yep, sorry. You can't calculate power accurately without knowing the power factor. You can ESTIMATE it based on educated guesses of the PF, but the best way to KNOW the power consumption is to measure it.

Totally agree.
But sometimes it's a requirement, at least for me, to predict that the bid stage of a project. I still need some basic data on the motor parameters.

 

[gar]

130207-1154 EST

panthripu:

Get a wattmeter. Then to determine output power you need to estimate motor efficiency. To measure output power directly you use torque and RPM from appropriate transducers.

If the motor is 3-phase Y a measurement on one phase is probably satisfactory with a single phase wattmeter. If the motor is delta, and you can get the current measurement of one coil and its voltage, then again a single phase meter may be sufficient.

If the current is under 200 A, then you can get a TED 1000 system and continuously monitor your motor. This is a single phase power and energy meter. If your voltage is not 120, then simply use a small 10 W transformer to change your voltage to 120 and do appropriate scaling.


The TED 1000 is no longer made, but some have appeared on Ebay. The 5000 model is the replacement, and available new, but in some ways the 1000 is better. The new price for a 5000 is about $200 US.

In what country are you located?

.

 

[Besoeker]

130207-1154 EST

panthripu:

Get a wattmeter. Then to determine output power you need to estimate motor efficiency. To measure output power directly you use torque and RPM from appropriate transducers.

The question was what power is drawn. That sounds like the power drawn from the supply rather than the shaft power delivered by the motor.

 

[Jraef]

Totally agree.
But sometimes it's a requirement, at least for me, to predict that the bid stage of a project. I still need some basic data on the motor parameters.

Well, you and I can probably estimate better than some when we need to for bid porpoises, but it's still an estimate...

 

[panthripu]

calculator for motor power

thanks for all your comments.
I agree with all of you that accurate power can be measured with instruments only. But at this moment I need only approximate values. I have siemens simeas P but for that I need some CT to connect them to motor cables to get current signal. If I may get clamp on type CT ,then it will solve my problme. But the problem is how to find suitable CT to measure up 300A of current.Any well known manufacturer ?

 

[Besoeker]

Well, you and I can probably estimate better than some when we need to for bid porpoises, but it's still an estimate...

For me, it's calculation based on available data and modelling the motor. And at different speeds and loads. VFD applications is business for us, you see.
And there are significant financial consequences for getting it wrong.
As I've said before, it puts you between a rock and a hard place. Be conservative with offered efficiency and the competition can win the business with a better offer. Whether or not they actually deliver is another matter. Be optimistic and fail to deliver, and it's tested for that, the financial penalties will destroy or negate the margin. It's a tough call but we've done OK at it. We've delivered the performance in accordance with the agreed contract terms.

Estimate it might be, but with very little margin for error.
I an old person, quite some way beyond normal retirement age. My missus would like me to give up my day job. That's why I've been trying to make some of the spreadsheets I use for such calculations user friendly for others - see the thread on that. I will probably reduce my involvement. I now work from home about three days a week. But it can still be frenetic at times.

 

[Phil Corso]

Panthripu...

Following are three methods to use if a clamp-on power meter is unavailable!

1) Load-Factor Estimate by Thermometer.
Where available, remove lifting-eye from motor carcass, fill hole with oil, insert old-fashioned thermometer. If a lift-eye hole is unavailable build a temporary reservoir with putty, or other suitable material, add oil, then insert thermometer. Load-factor, LF, is the ratio of measured temp-rise, Tm, to rated temp-rise, Tr, Then compare Temp-rise above ambient to the nameplate rating!

2) Load-Factor Estimate by Resistance.
Determine stator-winding resistance for "hot" and "cold" conditions. Load-factor, LF, is the ratio of measured Th to Tr. Then, for copper and aluminum windings:

Th = (Rh / Rc) x (Km + Ta) - Km. Since,
Tm = Th - Ta, then solving for LF,
LF = Tm / Tr = (Th - Ta) / Tr, yielding,
= (Kr - 1) x (Km + Ta) / Tr, where,
Ta = ambient temp when motor is 'cold'.
Th = ambient temp when motor is 'hot'.
Tr = motor's rated temperature-rise.
Kr = Hot to cold resistance ratio = Rh / Ra.
Km = Material constant, Cu=234.5 & Al=228.
Rh = Stator ?hot? resistance, Ohms.
Ra = Stator ?cold?resistance, Ohms.

3) Load-Factor Estimate by Slip.
If an rpm meter is unavailable (and the location is unclassified, or determined to be non-hazardous) use a Strobe-light. If a strobe is unavailable, use a fluorescent-lamp connected to the same supply as the motor's. It is now a "strobe" light.
The resultant shaft image will appear to slowly rotate backwards. Count the number of
rotations for one minute, and divide it by the number of poles to obtain actual slip. Then, calculate the ratio of actual to design slip. Multiply design Hp or kW by this ratio to determine actual output.
Note: The count is divided by the number of poles because the number of lamp flashes is usually (but, not always) 2 x line frequency!

Caveat: none of the above metods will be as adequate as a power measurement, but you will be able to calculate kW or Hp, kVA, and PF, more accuratly than with an ammeter!

Regards, Phil Corso

 

[Besoeker]

thanks for all your comments.
I agree with all of you that accurate power can be measured with instruments only. But at this moment I need only approximate values. I have siemens simeas P but for that I need some CT to connect them to motor cables to get current signal. If I may get clamp on type CT ,then it will solve my problme. But the problem is how to find suitable CT to measure up 300A of current.Any well known manufacturer ?

I have a Fluke 1000:1 clip on CT that can plug into a multimeter. I generally use it with a 10ohm burden resistor and look across that with a 'scope to get peak, rms and a lot of other data.

 

[gar]

130207-1310 EST

Besoeker:

Yes, you are correct that that as he presented the question he wants motor input power. I just went beyond that because he may really want to also know what power gets to the load.

The question seems to imply that that he has an actual motor system because he is talking about measuring current. If it was just a quote he probably would not have the motor to measure.

On some machines where we had equipment one of the functions of the machine was to drive a nut. Most of the customers of this type machine would not spend the money to add nut torque monitoring to this shaft. In troubleshooting the part or the machine knowledge of nut torque vs time in the cycle could be useful. A machine I was called in to troubleshoot did not have the nut torque transducer. To get approximate information I used a three phase solid state (Hall device) power meter connected to a scope, and strip chart recorder.

This motor had on-off mechanical loading via two clutches and a two speed motor. The clutches changed loading because of two different gear ratios, and the two speed motor effectively did the same thing. Thus, there were really four calibration constants. Calibration really did not matter other than you needed to know which was active. The goal was to look for an abnormality in the build curve. This was effective in finding the problem. It was a clutch with broken springs.

The cost of tear-down was about three people for a half day and production could not run. So it was important to try to predict the problem cause before tearing into the machine. The production day shift was sent home slightly early, and then teardown started.

A curve of the nut torque from a machine with a nut torque transducer can be seen at http://beta-aa.com/pa_plot.html . The curve data was collected and plotted in 1982. The photo date is when the picture was taken. The nut torque is not controlled by the machine in this application, but will be whatever is determined by the specific parts and lubrication in the assembly.

.

 

[mike_kilroy]

current vs pf curves

current vs pf curves

Hi guys
I am looking for some type of calculator where i can get power drawn by a induction motor if I know the current being drawn by it. Calculating by formula is also a way but I need manufacturer curves ( siemens and ABB) to get curves , current Vs power factor. Where can i find it ?

I agree with you that you should be able to calculate reasonable estimate of power drawn by using each motor's current vs power factor curve as you suggest. Where to get them? Both mfgrs you mentioned should be willing to send you these -just ask them for them.