DC motor+chopper: armature current falls to zero, speed not! why? 2

[krunom]

Im using chopper with 50Hz frequency. But my DC-motor has time constant L/R=1ms and thats much smaller then 20ms (chopper period). Because of that, armature motor current isnt constant, and with every "chopp" armature current falls to zero and goes up again (exponential-like)... and so all the time.
But, when chopper works, my motor has smaller speed (proportional to average motor armature voltage) but this speed has CONSTANT VALUE (motor doesnt move with "steps", he moves with CONSTANT speed).

i thought: motor will be moving with (smaller) CONSTANT speed ONLY IF:
(L/R)>(chopper period)=(1/chopper frequency)

I want to know: if armature current falls to zero, why my motor doesnt stop in this moment? what is the condition for stopping the motor? why is motor working with CONSTANT speed with chopper with TOO SMALL frequency?


am i wrong? when not, whats wrong?

thanks


P.S. when i say constant, i think aprox. constant (i cannot see any speed-change "with my own eye&quot

 

[RAMConsult]

You did not say what type of load the motor is driving but even the rotor has mass; therefore rotational inertia will keep it spinning until bearing friction and/or some form of braking torque slows it down.

 

[RadarRay]

When the chopper voltage is high, you putting current into the motor. When it is low, neglecting your relatively fast time constant, you are putting in zero current. This creates an average current. Your motor spins at a rate where the torque created by the average current = the load torque. Load torque is friction at steady state, friction + intertia during acceleration. And as Ramconsult stated, you have rotor momentum trying to keep the motor turning, once it is turning.

The speed is probably varies a little but is smoothed by the inertia of the motor. Your eye is not exact enough.

If your comparing motor speed to armature voltage average, which I assume you measure with a voltmeter, remember that the voltage you measure is CEMF+IR drop of the motor. So your speed with be slower by the factor of the IR drop.

The motor is not an instantaneous device. Neither the current, because of the various motor time constants, nor the speed because of the inertia.