is a motor an inductor
bittware said:
I am using PWM to control DMOS H-bridge to drive a brush DC motor. When PWM switches off, the current flowing through inductor can't be cut simultaneously. It will find a path through freewheeling diode. But the motor rotor is still spinning, the Back-EMF has a trend to force the current to inverse.
My question is:
Whether the current flowing through motor inductor would reverse or wouldn't reverse at all in a limited switch off time?
nicleo said:
When switch is OFF, the current flowing through inductor could not reduce to ZERO instantaneously. The direction of the current flow will not reverse.
bittware said:
Assuming a extreme condition, if a rotor has been accelerated to a relative high speed and the driving power withdraws, the rotor will act as a generator. The kinetic energy will be converted into electric energy, meanwhile the current is reversed. How to explain this case?
nicleo said:
No. The current will still in the same direction, i.e. not reversed. I assume that in your case, there is no 'external' torque to drive the motor (rotor) after the driving power is removed. The current flows through the motor winding, after the driving power is removed, will not change or reverse direction.
bittware said:
Althogh after driving power is removed no external torque drives the motor, the residual kinetic energy instored in rotor itself will also be converted into electric energy. Right? So I believe a part of kinetic energy will cause revesed current eventually. What I am concerning is when the reversed current appears after the PWM switches off.
nicleo said:
When Stage 2 (i.e. swtiches (S1, S2, S3, and S4) are OFF, just after motoring current decays to ZERO, and rotor still rotates), and if the stator has permanent magnet or the residual magnetic field on stator is strong enough:
Current path: -PS -> D4 -> motor winding -> D1 -> +PS
In this case, both the current at PS terminals and in motor winding are reversed (based on Fleming's rules) because the rotor is still in 'clockwise' direction.
username said:
bittware are you refering to some sort of regeneration. If you are then yes eventually the current in the inductor will reverse. But not instantaneously. It will first decay to zero and if you are still in regenerative mode the current will go negative. Ofcourse you have to provide a path for the reverse current so you must be using a Hbridge or a Class C chopper.
username said:
Now suppose inductor current has finally come to zero while motor is still rotating in the same direction. We turn S2 on and S1 and S4 off. Voltage across the inductor becomes negative (using previous refernce directions) so current through the inductor falls i.e it becomes negative. Current flow is D4 Dc motor and then S2 then back to D4. Now regenerative process has begun. Now we are using the Hbridge in boost converter mode. Now turn S2 off and D1 begins conducting. Voltage across the inductor is positive so the inductor current rises i.e becomes less negative. Current flow is D4 then DC motor then D1 then source and back to D4. So now the average inductor current is negative. Average power flow is from motor (now a generator) to the source.
bittware said:
What I care about is the current variation in inductor when PWM swithes off. Based on common sense, the motor resistor will consume up the energy stored in indutor during recirculation phase. And as for PWM off time is enough long, the current will alternate its direction finally. Bigger the motor resister is, quicker the current changes its flowing direction.
It looks to me that, at least "bittware" and "nicleo", agree that the current in motor winding, if the rotor is still running during PMW OFF state, will reverse direction eventually if the OFF state is long enough. In my opinion, this comment is correct only at certain operating points, or proper control strategy is employed.
Case 1
Power Converter Topology: H-bridge (pls refer to nicleo's diagram).
Control Strategy: PWM, Hard-Switching (i.e. ON-State -> S1 & S4 are ON; OFF-state -> ALL switches are OFF)
Motor Operating Point: Below No-Load speed
Description:
During OFF state (before current decays to zero), the current path is -PS -> D2 -> Motor -> D3 -> +PS.
After the current decays to ZERO, D2 and D3 will be OFF. The voltage appears across the winding will be the Vemf (as motor is still rotating).
As the Vemf is LOWER than Vs (power supply), the current in motor winding will stay at ZERO and will NOT reverse.
Case 2
Power Converter Topology: xxx
Control Strategy: xxx
Motor Operating Point: Above No-Load speed
Description:
Say the motor is driven above No-Load speed (by external force, for example, an Electric Vehicle is travelling downhill).
The generated Vemf is HIGHER than Vs, the current in motor winding will reverse and flow back to power supply through the following path: -PS -> D4 -> Motor -> D1 -> +PS
Case 3
Power Converter Topology: H-bridge.
Control Strategy: PWM +
usernam's regenerative switching sequence
Motor Operating Point: Below No-Load speed
Description:
Say it's in PWM OFF state (all switches are OFF) and the motor current has finally comes to zero while motor is still rotating in the same direction. Then S2 is switched on. Voltage across the motor armature winding reverses (because Ri = 0, Ldi/dt = 0, and Vemf does not change because motor still rotates in the same direction) so current through the armature winding falls i.e it becomes (more) negative. Current flow is D4 -> Motor -> S2 -> D4. As the result, energy is built up again in armature winding. Then, S2 is turned OFF, and the inductance of the armature winding forces D1 to conduct. The current flow is -P2 -> D4 -> motor -> D1 -> +PS, and the power flow is from motor (now a generator) to the source. In this example, the H-bridge, at latter stage, works as
boost converter to enable the regenerative process.
Conclusion
If the Vemf is higher than Vs after current decays to zero during PWM OFF state, the current in motor winding will reverse. Higher Vemf than Vs can be achieved:
1) If motor is driven (by external mechanical force) above No-Load speed.
2) If the H-bridge is controlled as 'boost converter' as described by 'usernam'.
3) If variable Vs is used so the Vs can be reduced to be less than Vemf.