Reverse

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jlgh
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Reverse

Postby jlgh » Fri Dec 18, 2009 10:53 pm

I'm an EV newbie so this may be a dumb question....

Can you reverse a series DC motor like Netgain's Warp? With a PM electric motor (I assume, unless there are any gotchas), just reversing the polarity does it: but reversing the polarity on a series wound motor reverses both field and armature.

Do DC controllers have facilities to handle this or are you stuck with mechanical reverse :cry:

Tom Thomson
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reverse

Postby Tom Thomson » Sat Dec 19, 2009 2:48 am

jl
Yes, you can. On my first conversion, that's the way I did it. The motor was a D&D ES 15A-6 and the reversing contactor was an Albright SW182. By the way, I am now using a gearbox, and the Albright is no longer needed and is for sale - $50 +ship
Tom Thomson
Indiana USA

jlgh
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Location: Midlands

Re: reverse

Postby jlgh » Sat Dec 19, 2009 8:59 am

Tom Thomson wrote:jl
Yes, you can. On my first conversion, that's the way I did it. The motor was a D&D ES 15A-6 and the reversing contactor was an Albright SW182. By the way, I am now using a gearbox, and the Albright is no longer needed and is for sale - $50 +ship
Tom Thomson
Indiana USA


Many thanks Tom - that's very useful. Did the contactor reverse the field or the armature?

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Night Train
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Postby Night Train » Sat Dec 19, 2009 2:48 pm

You can reverse either the armature or the field as the effect will be the same, the motor will spin the other way.

One thing to be careful of is if the brushes are advanced on the motor. Advancing the brushes will allow it to commutate better at high rpm at high voltages. Clearly this only works in one direction of rotation. If you reverse the motor the brushes are then retarded and the motor won't like that much at all.

Tom Thomson
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Postby Tom Thomson » Sun Dec 20, 2009 12:51 am

jl
Both-since the field and armature are connected in series.
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Postby GregsGarage » Mon Dec 21, 2009 8:43 am

Night Train is correct, you need to switch polarity of either the field or armature. If you switch both then the motor won't reverse. The advanced brush timing issue won't be a problem for the occasional low speed reversing.
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timpootle
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Postby timpootle » Mon Dec 21, 2009 2:01 pm

Your question has been answered - yes you can reverse a series wound motor using the appropriate contactors. What you can't do, to my knowledge, is use a series wound motor to give you regenerative braking. This involves reversing the field current but not the armature, or vice versa. As these windings are in series (hence the name) there is no tap-off in the internal wiring of a series wound motor to achieve this.

If anyone can tell me I am wrong I would very much like to hear how.
Tim Crumpton

jlgh
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Postby jlgh » Mon Dec 21, 2009 6:25 pm

Many thanks for the helpful replies everyone

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EVguru
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Postby EVguru » Thu Dec 24, 2009 10:28 am

timpootle wrote:Your question has been answered - yes you can reverse a series wound motor using the appropriate contactors. What you can't do, to my knowledge, is use a series wound motor to give you regenerative braking. This involves reversing the field current but not the armature, or vice versa. As these windings are in series (hence the name) there is no tap-off in the internal wiring of a series wound motor to achieve this.

If anyone can tell me I am wrong I would very much like to hear how.


Reversing the voltage polarity to a series wound motor reverses the magnetic field of both the armature and stator (field coils), so the motor still runs in the same direction. This means you can even run them from an AC source and that's what they do in power tools where they are known as 'Universal' motors.

For a series motor to be reversable, you must have more than two terminals (ignoring some oddities like motors that reverse by moving the brushes). A series motor suitable for EV use, such as the ADC or Netgain will usually have four terminals, two for the armature and two for the stator.

Reversing the polarity of either the field or the armature will reverse the rotation, but if you want to be able to use regeneration, then you would reverse the armature, so that the residual magnetism in the stator iron will provide the excitation to start the motor generating.

What you would do is use the controller to short circuit the motor. The current would build up at a rate limited by the motor inductance and when the controller detects a preset limit it would unshort the motor. Then as the energy stored in the inductance is released it boosts the voltage above the battery voltage and so charging the battery. The duty cycle of the PWM is used to control the regen curent in much the same way that is done for drive.

The Zapi H2 controller amoungst others is suitable for regerative braking with series motors, but there are some limitations. Ideally the brushes should be set at neutral and that conflicts with forward drive requirements (too much brush arcing at higher voltages). Motors with interpoles that 'force' good commutation are available, but are physically bigger and more expensive. The Nelco motor in my Reliant Kitten that Evan still owns had interpoles, but the only other motor I know of off hand was the Kostov from Eastern Europe.

If you accept that all you are going to get is the equivilent of engine braking from an ICE, then most series motors will work up to about 96 volt, which is in any case the limit for readily available regen controllers.

In any case you have to be quite careful with regen. If someone crawled underneath your car and cut the rear brake lines, you might think they were trying to kill or injure you, but some people have felt happy to use regen on a front wheel drive car equivilent to machanical braking. You've also got the problem of the rate at which the batteries will accept regen current. You wouldn't want the braking to suddenly dissapear because the batteries were cool and at a fairly high state of charge. When GM built the EV1, they tied the regen into the ABS controller. The rear brakes were applied in proportion to regen and it would seamlessly shift between regenerative and friction braking.
Paul

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jlgh
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Postby jlgh » Thu Dec 24, 2009 11:34 am

Paul that's most interesting. I'd just assumed that regen was (in practice if not theory) the preserve of AC jocks 8)


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