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:The current will not reverse, but the induced voltage across the motor will be reversed (Len's Law)
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: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?
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: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. According to Len's Law, the voltage across the motor winding must be reversed to (try to) 'maintain' the current, which will eventually reduce to zero.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?
If you have an external mechanical torque to maintain the kinetic energy and you have field current, then your dc motor will act as a generator.
Well, we need to clarify whether we're discussing about the current flow direction in the motor winding or power supply terminal. Pls advise.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.
Fleming's rules said:Fleming’s rules give the direction of the magnetic field, motion, and current in electrical machines. The left hand is used for motors, and the right hand for generators and dynamos.
I agree with your analysis for now.nicleo said:Well, we need to clarify whether we're discussing about the current flow direction in the motor winding or power supply terminal. Pls advise.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.
I think there are two stages after the driving power is removed:
1) The motor is running, and the current (not reversed) in motor decays to zero.
2) After the current decays to zero, the rotor still rotates due to its moment inertia.
Say in motoring mode, the rotor rotates 'clockwise'.
The current, when the switch is ON, flows from the positive terminal of power supply (PS) to negative terminal of the PS:
Current path: +PS -> S1 -> motor winding -> S4 -> -PS
When the switch is OFF, the current in motor winding will be NOT change direction:
Current path: -PS -> D2 -> motor winding -> D3 -> +PS
The current direction at PS terminals reversed.
When Stage 2 (i.e. swtich is 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)
Fleming's rules said:Fleming?s rules give the direction of the magnetic field, motion, and current in electrical machines. The left hand is used for motors, and the right hand for generators and dynamos.
I think we can formulate equations to model the situation. The key (when State 1 -> State 2) is the current (in motor winding) changes 'direction'. So, in the simulation model, we can implement this by detecting the zero-crossing of the current waveform. Regarding the 'FULL' equation, I think usernam can find the equation (in the reference where the diagram in his post was copied from) that models the 'regenerative' operation mentioned in his post.bittware said:I agree with your analysis for now.
But the key point is boundary between "Stage 1" and "Stage 2". That is to say when the motor exits "Stage 1" and enter "Stage 2"? Is a quantitative calculation applicable? If yes, how to do that?
I was only neglecting its effect while equating Do*Vs = Back EMF. Since usually Back EMF is in the order of a 100 V and Ia*Ra drop is around 4 or 5V it can be neglected.If you don't take winding resistance into account
Yes for an extreme condition you will take D1=0. I was just giving a general case. Perhaps you may not want a high (di/dt) for a sustained period time. In that case you will not resort to the extreme condition.I can't understand why you introduce an extra D1 duty PWM phase. For extreme condition, take D1=0, I can't get significative result from your equation.
To usernam and bittware, what do you think we just focus our discussion on 'hard switching' first so we won't be confused?usernam said:bittware I need to get a few things clear. What do you mean by PWM switches off? Do you mean S4 is off and current is free wheeling from S1 to DC Motor to D3.
Hello nicleo,nicleo said:Model:
Vs = Vemf + L di/dt + Ri ....... (1)
Vs = driving voltage supply
Vemf = back emf voltage
L = inductance of motor winding
R = resistance of motor winding
After entering stage 2, the mathematical model of the system should reduce to
0 = Vemf + L di/dt + Ri ....... (2)
To bittware:
The stator of the DC motor has permanent magnet or field winding?
Hello usernam,usernam said:bittware I need to get a few things clear. What do you mean by PWM switches off? Do you mean S4 is off and current is free wheeling from S1 to DC Motor to D3.
I agree. However, regardless of what switching style is, I think the phenomenas are the same.nicleo said:To usernam and bittware, what do you think we just focus our discussion on 'hard switching' first so we won't be confused?usernam said:bittware I need to get a few things clear. What do you mean by PWM switches off? Do you mean S4 is off and current is free wheeling from S1 to DC Motor to D3.
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