# Need help to calculate the resistance and current across a motor

Thread Starter

#### akeiw12

Joined Apr 13, 2007
4
I have a small DC motor that is being used to rotate a centrifugal pump. The motor can supply a variable voltage from 0-30V (controlled on the power supply). I also have a function generator hooked up to the motor, so that I can create a pulsatile waveform from the pump output. In order to operate the motor with the function generator, I had to create an emitter follower, used as a current amplifier (see picture). What I'm trying to do is calculate the power usage of the motor when it is running in both continuous and pulsatile modes. I have a dual-probe BNC cable connected to the motor, and have it going to an A/D board to output my data on my computer. So, essentially, I am measuring the input voltage to the motor, but I also need the input currrent, in order to calculate the power (P=V*I).

So, my question is, how do I find the current?! I was thinking that if I knew the resistance of the circuit, I could just calculate it by I=V/R, but I'm not sure if I'm finding the resistance correctly. I hooked up a digital multimeter to the two motor inputs, and measured the resistance there while zero volts were applied. I was thinking that the resistance won't change no matter how much voltage is applied to the motor, but I'm starting to re-think that because of the emitter follower circuit...though, I really don't know much about circuits, and so I'm not sure if that even matters. When I apply enough voltage to the motor to get it to start spinning, the resistance on the multimeter maxes out...I still don't understand that either. I would really appreciate any help possible! Thanks a lot!!!

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#### beenthere

Joined Apr 20, 2004
15,819
If the power supply can keep up with the pump, why worry about the exact current? By the way, the motor uses the power supply output, not the other way around.

Do not try to measure resistance in an energized circuit - it is very likely to eat your meter. In resistance measurement, the meter supplies a small current to the device under test.

If you want to have some idea of max current, measure the motor draw under load at the full 30 volts.

Your pump control circuit will be happier running the FET as grounded source, with the motor between the power supply and the drain. The FET would be even happier with a reverse-biased diode (like a 1N4004) from drain to source. Use about 100 ohms for the gate resistor, and keep the gate voltage under 20.

Thread Starter

#### akeiw12

Joined Apr 13, 2007
4
Thanks for the reply! I don't know a whole lot about circuits, so forgive me if I ask stupid questions, but I'm a little confused about the last part you wrote. When you say to hook the motor between the power supply and drain, does that mean connect the drain to both the motor and the power supply? And then what gets connected to the source?! (see the picture...this is how I'm interperating what you're saying). I'm not quite understanding it.

Also, I'm conducting an efficiency study of the pump at various flow rates; so I need the input power of the motor at various voltages levels for that calculation.

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#### beenthere

Joined Apr 20, 2004
15,819
The motor has one lead to the power supply and the other to the FET drain. That way, the FET controls current through the motor.

If the motor current is under 10 amps (the limit for most meters), then the meter may be placed in circuit so the same current flows through it as does the motor. Place it in series with the motor and the power supply - or the FET, it makes no difference. Read the current draw at various speeds.

If you're going to pulse the motor, then the current gets a bit harder to read. You will need some components like a small value resistor, a diode and capacitor, and an op amp with some other stuff to get that reading.

Thread Starter

#### akeiw12

Joined Apr 13, 2007
4
Thanks for the help! I got the motor hooked in between the drain and the power supply now, but I'm having troubles obtaining my current readings. I placed the meter across the motor, but anytime I power the motor on and then switch my meter to take current readings, the motor will stop running and the voltage level will drop to a lesser value (i.e. no matter what voltage I'm at, when ever I connect the meter to read current, the voltage drops to 6.0V and gives a current of 2.94). Do you know why this is happening, and how I can fix it so I can get current readings at multiple voltage levels?

Also, is it wrong to assume that the resistance across the circuit while the motor is running, is the same as when the power is off, and therefore a constant resistance independent of the voltage level?

I appreciate all the help! Thanks!!!

#### beenthere

Joined Apr 20, 2004
15,819
To get a proper reading of the circuit current, you could tale the motor lead to the drain loose. Connect the positive meter lead to the loose motor lead, and the negative meter lead to the drain. Now all circuit current will pass through the meter. Notice that you can only read current connected like this.

There will be a certain resistance in the motot due to the length of the coil windings, etc., but this has no real significance to motor operation. Passing current through the coils builds a magnetic field. Current through the armature (through the commutator brushes) also builds a magnetic field, one that varies over time due to the rotation of the armature and the periodic field reversals coming from the current path through the commutator.

The result is refered to as "back EMF". The action of the magnetic fields limit current through the motor. If the motor is held so it can't rotate - stalled - then current goes way up and may burn up the coils. The motor has an impedance, rather than a resistance, that limits current. You can't read it with an ohmmeter. Measure the current and use Ohm's Law in the form R = E/I to get the value of the impedance.

Thread Starter

#### akeiw12

Joined Apr 13, 2007
4
Thank you SO much for all your help!!! I got the meter hooked up and everything is working great! Thanks again, I really appreciate it!!!!!