How to determine small dc motor voltage/unmarked motors...

Thread Starter


Joined Nov 20, 2013
Is there a way to determine the voltage requirements of two wire dc motors if you don't know what they came out of and there are no markings?

I know you can run it and guess from whether it gets hot etc, but is there some coil resistance test that will give any useful info?

I have some printer motors (not steppers) , two wire ...what voltage would they most likely be, they are out of a small homeowner type Pixma.

I Found a pixma pdf parts list but absolutely no datasheet info on the
actual motors.
They ran at 6 volts at a speed that seemed ok , but they might be way off.

Sometimes I get motors and have no idea what they came out of.


Joined Aug 23, 2012
Could you attach the picture of motor, maybe someone know that?
I will use my finger to touch the motor to sense the temperature and judge the voltage or current is ok or not.
Last edited:


Joined Apr 28, 2012
That's very bad if you dont know the rating.

The bearings, contact springs, brushes and so on may have a limit of some certain rpm and beyond that, they will wear out rapidly.

One criteria tough is the turn-on voltage, and the turn-off voltage, similar to relays.

Turn-on voltage is typically 70% of the lowest operating voltage.

You can also test it under reasonable load.

Max. voltage is normally double of the low operation voltage for many kinds of motors, as long as they dont overheat, and dont produce sparks.

If they make weird noises or run at abnormal rpm, the voltage is too high, same if they turn hot and begin to smell.

Also most motors wear out faster at higher rpm.

small motors might be similar looking but one is only good for 3 or 4 volts, while another only develops torque at 18 volts.


Joined Jul 18, 2013
DC brushed motor are generally limited to 3k rpm tops, so this is one way of roughly determining what the voltage may be.
It can be done by either supplying a gradual increasing voltage or back-feeding the motor at a certain known rpm and measuring the DC generated and extrapolating it for say 3k rpm.


Joined Jul 18, 2013
If you need more there is this.
I put this together when looking in to finding the characteristics of an unknown motor.

I think that a simplified model of a DC motor can be derived assuming the armature inductance to be zero and ignoring the resonance effect.
With these stipulations the equations are:
1. V=Ia R + Ke omega (Ia=armature current, R=armature resistance, Ke=electr. constant, omega=speed)

2. Tg=Kt Ia (Tg=costant, Kt=torque constant)

3. Tg=J d(omega)/dt (J=inertia, d(omega)/dt=accel.)

The DC motor transfer function is:
Gm(s)=(1/Ke)/(1+s(Rj/KtKe)), which can be written Gm(s)=(1/Ke)/(1+sTm)
where Tm=mechanical time constant.

To measure these parameters r, I suggests the following:
A. Measure with an ohm-meter the armature resistance, then apply voltage to the motor without load and measure the current and speed. From equation 1. you can easily derive Ke.
B. Apply nominal current to the motor (with the shaft locked) by means
of a variable voltage source. Measure the torque on the shaft. From this you can derive the torque constant Kt=Torque/Amp.
C. You will find that Kt is approx. equal to Ke
D. For the inertia you can obtain it by calculation from the size and
material of the rotor.

Note 1: inductance can be ignored- the electrical time constant is
very short compared to the mech time constant so that it can usually be

You can measure the mech time constant by running the motor up to
speed at no load, disconnecting the supply and letting it coast down- plot speed vs time and fit to exponential N=No(e^-t/Tm) time to drop to 36.8% of original speed is the time constant.

Note2: If it is a permanent magnet motor, you can determine the internal emf by spinning it at rated speed and measuring the open circuit voltage. The voltage at any other speed will be directly proportional to speed. To measure the winding resistance, lock the rotor so it doesn't turn and measure the current with a small voltage applied (so as not to exceed rated current) Don't bother using a multimeter's ohm range- not worth the effort.
For inductance, you should use a scope- apply a voltage, rotor locked and look at the current trace vs time.
This will be of the form i=K[1-e^Rt/L] where i is the current at time t.
In most cases the inductance can be ignored as its effects are generally swamped by the mechanical inertia in transient cases and is of little importance for steady state.