Ways to reverse Series DC Motor -100% FWD 30% RVRS
Posted: Fri, 15 Oct 2010, 02:24
I'v been getting that.
I'll try to be more constructive. A while ago, I found a paper on exactly this:
A Four Quadrant Adjustable Speed Drive For Series Wound DC Motors
The author uses a reversing contactor, but has a good description of motor operation and control.
I see the options for no-reverse contactors as follows:
1) A really high current power supply that can reverse the field while the motor is being driven by the PWM. I would guess that the field is in the order of 5-10 volts, mostly the resitance of the copper straps in the pole windings. This is a tough one because it's exactly in the dread zone for semiconductors. It also needs an impressive down-converter for the high current (500-1000A), kind of like a DC spot welder, except that spot welders are all AC.
2) Sepex. The field windings need to be re-done so that the copper is appropriate to lowish currents that are easy to control with a small (50 amp?) H-bridge controller.
Sepex should be no less efficient than Series, as the power needed for the field is the same regardless. If the power in the windings is a constant, the voltage and current are determined by the copper geometry.
Thin wire = lower current, but more turns and higher voltage.
A field winding and current that is compatible with the power supply (160V?) is needed to make the driving electronics practical.
An analog is an automotive alternator. A few amps of field current can produce a lot of amps of output current. I think that typically, this is a factor of 30 or more.
In a series motor, the field windings are carefully chosen to provide
sufficient field when fed with the same current as the armature.
Sepex probably gets a bad name because the field current on the controllers is too little to fully realise the motor capabilities.
Sepex is like a shunt confiruration, except that the field winding current is controlled by a separate controller channel. The field current is proportional to the armature current, so the field PWM must vary according to the current in the armature, whereas the armature PWM is more related to the speed of the motor.
For braking, the field current can increased so the motor's back emf is greater than the battery voltage, causing current to flow through the transistor intrinsic diodes towards the battery. A Sepex DC motor will be considerably less effective at braking energy recovery than an indiction motor will be.
For reverse, a H-Bridge is needed to allow the field to be reversed and motor turn the other way. Because the field current is relatively low, this is likely to be only 4 big transistors with non isolated gate drivers like the IRL2113 ($2 last time I bought any) or its kin.
Overall, the Sepex can provide some operational improvements, but because it involves rewinding the field, it's benefit is marginal, if at all.
I'll try to be more constructive. A while ago, I found a paper on exactly this:
A Four Quadrant Adjustable Speed Drive For Series Wound DC Motors
The author uses a reversing contactor, but has a good description of motor operation and control.
I see the options for no-reverse contactors as follows:
1) A really high current power supply that can reverse the field while the motor is being driven by the PWM. I would guess that the field is in the order of 5-10 volts, mostly the resitance of the copper straps in the pole windings. This is a tough one because it's exactly in the dread zone for semiconductors. It also needs an impressive down-converter for the high current (500-1000A), kind of like a DC spot welder, except that spot welders are all AC.
2) Sepex. The field windings need to be re-done so that the copper is appropriate to lowish currents that are easy to control with a small (50 amp?) H-bridge controller.
Sepex should be no less efficient than Series, as the power needed for the field is the same regardless. If the power in the windings is a constant, the voltage and current are determined by the copper geometry.
Thin wire = lower current, but more turns and higher voltage.
A field winding and current that is compatible with the power supply (160V?) is needed to make the driving electronics practical.
An analog is an automotive alternator. A few amps of field current can produce a lot of amps of output current. I think that typically, this is a factor of 30 or more.
In a series motor, the field windings are carefully chosen to provide
sufficient field when fed with the same current as the armature.
Sepex probably gets a bad name because the field current on the controllers is too little to fully realise the motor capabilities.
Sepex is like a shunt confiruration, except that the field winding current is controlled by a separate controller channel. The field current is proportional to the armature current, so the field PWM must vary according to the current in the armature, whereas the armature PWM is more related to the speed of the motor.
For braking, the field current can increased so the motor's back emf is greater than the battery voltage, causing current to flow through the transistor intrinsic diodes towards the battery. A Sepex DC motor will be considerably less effective at braking energy recovery than an indiction motor will be.
For reverse, a H-Bridge is needed to allow the field to be reversed and motor turn the other way. Because the field current is relatively low, this is likely to be only 4 big transistors with non isolated gate drivers like the IRL2113 ($2 last time I bought any) or its kin.
Overall, the Sepex can provide some operational improvements, but because it involves rewinding the field, it's benefit is marginal, if at all.