Principle of dc motor, voltage is proportional with torque and current is proportional with speed. In PWM method, we change duty cycle and it changes voltage. I would like to know how do we control current? I believe that, If I control voltage and current, I can arrange my torque and speed so I can control dc motor more effectively. Probabaly, I am missing somewhere. Thanks for your attention. I’m looking forward to your reply.
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\$\begingroup\$ The trivial answer is you control Vavg with PWM and sense current and speed under load \$\endgroup\$– D.A.S.Commented Jul 8, 2018 at 15:58
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voltage is proportional with torque and current is proportional with speed
No. Torque is proportional to current. The back EMF is proportional to speed.
A workable first-pass model of a motor is a voltage source in series with a resistance. The resistance is the physical resistance of the windings.
The voltage of the voltage source models the back EMF, and is proportional to speed. When the motor is being driven by applied voltage (not externally by mechanical means), the voltage of the voltage source opposes the applied voltage.
Consider a mechanically unloaded motor. Everything start out off, then you apply a fixed voltage to the motor. Initially, the motor is not turning, so the current is simply the applied voltage divided by the resistance. This is the "stall current". That current causes torque, which makes the motor turn. That turning causes the voltage source voltage to go up. The current is the applied voltage minus this back EMF voltage, divided by the resistance. The current therefore goes down as the motor increases in speed. That means the torque is less as the motor goes faster.
Eventually a equillibrium is reached where the motor spins fast enough to leave just enough voltage across the resistance, to cause just enough current, which causes just enough torque to overcome friction and other losses to keep the motor spinning at its current rate.
The rest of your question makes no sense since it is based on false assumptions.
Fast enough PWM of the motor voltage results in the motor "seeing" the average voltage.
answered Jul 8, 2018 at 15:23