Electronics: Power consumed by a DC motor. How do I calculate?

Question

I want to measure the power consumed by a DC electric motor.

Will the "power = v x i" equation work when dealing with an electric motor since the motor will consume current intermittently?

In other words, will the reading of a basic ammeter be reliable with an electric motor - if not, how can I calculate true power consumption?

Thanks in advance.

Answer 1

There are two sets of reading...NO LOAD AND FULL LOAD.

No load consumes less current but full load consumes much higher current.

Apply the load to the motor as what the motor design for,then measure the current as this load is applied. Calculate the power consumption.

Answer 2

Well, I'm not sure how this could be calculated without a test, perhaps by finding the motors resistance. however, if you want to find the motor's max consumption, then you would want to lock the motor shaft and use an amp meter to record the motor's draw. then, run it at no load to find it's no load consumption. Because the amount a motor draws varies depending on it's load, you can use the results from these two tests to get an idea for where it will average under load.

Answer 3

power = v * i  is the right way to find power consumption for a given current and voltage.

Whether or not that information will be of any use to you depends on why you want to know the power consumption.

Answer 4

"consume current intermittently" ??

But power is always V x I

But it will depend on what mechanical load you have on the motor, which is perhaps what you mean by "intermittently"?  

Actually, I'm assuming you give the motor time to get up to it's final speed each time. If not, you can have a power versus time curve as the motor speeds up.

In other words, with a fixed voltage applied, you can have many different power readings, depending on the mechanical load.

edit: Yes, a true RMS meter will read the accurate RMS current, but true RMS meters are expensive. Most are average responding RMS calibrated, which means they read RMS only for a sine wave.

So the only way is with a true RMS meter, or a scope. It doesn't have to be a good one, as we are only talking about 60Hz here. just get a peak-peak reading and you can convert to RMS later.

.

Answer 5

Yes, but only IF the voltage is constant.

Most ammeters can give you an rms AC reading of current.

Power at a particular instant in time is always V*I.  If the voltage is constant, multiply the voltage by the rms value of current to find the average power.

If the voltage isn't constant, multiplying the Vrms value by the Irms value won't give you the correct result.  You need sum the instantaneous V*I values and divide the time over which the values were summed (integrate and divide by the integration time).

One way to do that is to add a current sense resistor (high power very low value sub Ohm power resistor) in series with the motor and use a 2-channel oscilloscope with math function.  Channel A will be the voltage on the resistor (representing current) and Channel B will be the voltage over the whole circuit (remember channel A and B grounds need to be referenced to the same point).  Most modern scopes have Math functions and you can display Channel A * Channel B (which will be related to power and needs only to be scaled by the resitor value).  Look at the rms measured value of the Math waveform.