I have a somewhat stronger 12V dc motor (stronger meaning it was a part of a door opening system earlier). I'm using it with a PWM motor controller.

After using it i noticed that if i give it a constant PWM signal then the voltage measured at its wire connection drops down like 8-9 %.
for example: i'm using arduino, i give the motor a PWM= 20 signal (at a range PWM 0 - 255 -> 0 - 12 V), the motor spins, i measure the voltage at the motor's connections (it's 1.93 V), i make the motor's shaft unable to rotate (the motor stalls), i measure the voltage again (it's 1.78 V).

I tried it with different input signals (the stalling was performed at a relatively low voltage and for short period of time (so it won't damage the dc motor too much) and i made the measurements with two different DMMs. The 8-9% voltage drops are fairly consistent in the range of used voltages (0.7 - 3.1 V), so i'm assuming it would be the same in the whole range of applied voltage too (0 - 12V).

What could cause this? Isn't just the current that changes at stalling? I measured the current too, and it changes respectively to the motor's datasheet (not higher not lower). I was thinking about stalling causing inner temperature causing voltage drops, but the voltage drops are too similar to me for this case, and also if i stop the stalling (let the motor run again), then the voltage imediately get backs to its original value.

 

At stall speed, the current draw is maximum and your voltage supply can't supply the demanded motor current. So it drops.

 

If your using MOSFET drivers . if the drive MOSFET is on for a 50% duty cycle, motor voltage is 50% of battery voltage and, because battery current only flows when the MOSFET is on, battery current is only flowing for 50% of the time so the average battery current is only 50% of the motor current!

 

MOSFET switches off, it not only interrupts the motor current but it also interrupts the current flowing from the battery. The wires from the battery have inductance (so does the battery) so when this current is interrupted this inductance causes a voltage spike: in the circuit the main capacitor absorbs (most of) this spike. When the drive MOSFET turns on again, battery current is asked to flow quickly – which it cannot.

 

At stall speed, the current draw is maximum and your voltage supply can't supply the demanded motor current. So it drops.

I am using a 12V power supply, it is capable of supplying much larger currents than the dc motor's maximum stalling current, so there isn't any problem with demanded currents.

 

Did you measure the voltage from the power supply (at the power supply terminals of the PWM circuit) when the motor is running then when it is stalled? I betcha it drops when the motor is stalled.

 

Did you measure the voltage from the power supply (at the power supply terminals of the PWM circuit) when the motor is running then when it is stalled? I betcha it drops when the motor is stalled.

I measured it at the dc motor's wire connections (sorry, if it isn't called that way, i mean at the two closest point to the dc motor (at the dc motor's two cables)).

 

When the motor is spinning it creates a back e.m.f. that opposes the supply voltage. Hence the current is lower at higher speeds.
When the motor is stalled, the motor load is the DC resistance of the motor winding and hence the current load is greatest.

As others have indicated, the drop in voltage measured at the motor is a result of poor voltage regulation at the power supply and the resistance of the power supply and connecting wires.