# Measuring motor resistance and speed of a geared DC motor

1. Mar 31, 2017

### bbq_build

Hi, I want to measure the motor resistance of a gear DC motor by experimentation and Ohm's Law. Should I measure the current without the gear head, with the gear head, stall current (with gear head) or stall current (without gear head)?

I am trying to obtain values for Equation 4 in:
http://ctms.engin.umich.edu/CTMS/index.php?example=MotorSpeed&section=SystemModeling

Similarly, when measuring the spinning speed, should I have the gear head installed or removed during the measurements?

Thank you

2. Apr 1, 2017

### jim hardy

I assume he meant

Usually in lab we determine
Ke by and measuring armature circuit voltage at open circuit with known RPM and excitation.

we measure Ri by mounting the motor in a dynamometer(torque measuring device) locking the shaft which forces θdot to zero, applying some current and measuring voltage;;;
reading torque at same time lets us measure Kt

do you have access to armature and field terminals , or are they connected internally?

3. Apr 1, 2017

### bbq_build

Not sure what you mean by armature and field terminals. I have no access to dynamometer. We only have access to a function generator, three digital multi-meters, an oscilloscope and power supply.

Basically, we need to measure the resistance, inductance, turning speed, back EMF constant, torque constant and voltage caused by back EMG in order to model the motor as shown in the video:

Is the current in the video just a reading from the multi-meter placed in serial with the motor (without gear) or it is a stall current (with gear head installed and the hand forcing the motor to stop spinning)?

4. Apr 2, 2017

### jim hardy

Both.
If you stall the motor what happens to its back EMF (θdot = 0) ?
So you're left with V=IR. Seems measuring V and I would tell you R.

If you run it completely unloaded(no gears) and measure current,
then you know from R X that I what is voltage drop across R of armature. Rest of V must be due to back EMF ?
Reason to do that measurement unloaded is so armature current doesn't distort the internal flux from your fixed field (described in the exercise you linked) which will give you a wrong number for Ke.
Ideal would be to measure voltage at zero armature current but you'd have to spin the motor with an electric drill or something.
Unloaded may be as close as you can get to zero current.

I didn't see him figure out inductance in that video.
What experiments can you think up to determine inductance of the armature ?
Think of it as just an inductor.

Does the motor have a place to connect wires? How many? How are they labelled?

5. Apr 2, 2017

### David Lewis

The current is whatever your current meter in series with the motor shows. It is simpler to characterize motor behavior without the gear drive. The gear drive adds friction and multiplies torque.

6. Apr 2, 2017

### David Lewis

If the motor's not turning, it's also a good idea to measure resistance with the shaft in different positions. (When a brush bridges two commutator segments, it will affect the resistance.)

7. Apr 2, 2017

### bbq_build

Thanks. What do you mean by "from R X that I what is voltage"? Could you please clarify?

Yes, there are two terminals at the back of the motor. No label.

8. Apr 2, 2017

### bbq_build

Thanks for the excellent suggestions. I shall make several measurements and take the average.

9. Apr 2, 2017

### jim hardy

You determined R in previous step
you measure I (current) when motor is running lightly loaded
ohm's law V = IR tells you how many volts are dropped in R of the armature and brushes,
remainder of applied volts must be "Back EMF" .

knowing back emf and rpm lets you figure Ke

So are you saying we have no idea how this motor establishes its field ? Indeed we don't know if it's series field, shunt field, a combination of the two(compound), or permanent magnet field ?
How do you know it's the same type motor described in the exercise you are trying to perform, linked in first post as
http://ctms.engin.umich.edu/CTMS/index.php?example=MotorSpeed&section=SystemModeling
which specifies constant field ?
Or is that unknown ?

Not being argumentative, just trying to nail down the question. A question well stated is half answered. I often have to do experiments three times because of "Assumptions" that turned out not so.
That's how we learn.

old jim