Speed control - chopper with DC motor 1

[krunom]

i use chopper to control speed of DC motor. i assume: with chopper i decrease armature current (its constant), but armature voltage is "jumping and falling". Well, torque is proportional to armature current and speed is proportional to armature current. It isnt clear to me: difference between speed and torque?
Im decreasing armature current and it has constant value and armature voltage is "jumping and falling" (with chopper-frequency)... so why my DC-motor has now decreased CONSTANT speed and speed doesnt "jump and fall" like armature voltage???why is the speed CONSTANT???

Ia=torque/km
Ua=speed*km

 

[RadarRay]

For starters Torque is proportional to armature current but the speed is proportional to armtature voltage, NOT current.
The current the motor actually sees is (Vsupply - Cemf)/ armature resistance. In your case it is a chopped voltage supply (I assume).
The motor has losses associated with it and then a load. If you supply more current the motor speeds up, if you supply less current the motor slows down. the motor maintains it speed when the current or really the torque created by the current balances the losses. By varying the average voltage through your chopper you vary the current and thus the speed.

Speed does not jump and fall because the motor is averaging your chopped supply out.

Your equation at the end is incorrect. There are two different motor constants for the equations. Kt is the torque produced per amp (Torque/Amp). Kv is the Cemf created per motor revolution. Kv may be in rad/sec or rpm.
So Va is Speed * Kv.

 

[krunom]

you may noticed: i made "stupid" mistake but then i corrected it: speed is proportional to armature voltage, torque to armature current...

ok, all that stuff about constants..but that isnt so important to me, i know: that are DIFFERENT CONSTANTS... so thank you... that was another my "stupid" mistake...

my question is: armature voltage is "jumping and falling" and speed is proportional to arm. voltage!!! why i have CONSTANT speed??? why the speed is NOT jumping and falling??
ok, i have CONSTANT (decreased) arm. current, but arm. current doesnt have any relationship with speed!? or does it?

simply: i have problems to understand relationship torque-speed...

torque=inert.(d_speed/d_time)

 

[RadarRay]

The voltage is changing much more rapidly than the motor can follow it. The motor is averaging the chopped voltage.

The voltage that creates current is the voltage above the CEMF of the motor. This is because the actual current in the motor is as I said above (Vsource-Cemf)/Ra. This creates the motor current. Now the torque the motor is putting out will balance what the inertia load and frictional load of the motor is. That is the speed the motor will stop at.

This may be hard to see because you are using a chopped voltage. First understand how the motor gets to speed with a fixed supply. Then it is just a matter of understanding that the motor cannot respond instantly to a chopped supply, so it performs an average of what is applied to it.
And by average I don't necessarily mean a true arithmetic average but a filterin of sorts.