Questions on Sepex motor theory

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Ampy
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Questions on Sepex motor theory

Couple of technical questions on a Sepex motor (48v Aixam van) here if anyone can help.

On a Sepex motor van, after removing the blown Curtis controller I connected the field to a 12v battery and Armature to 24v to see if it would go.

1) Even though I connected the field and Arm black wires to negative, and Reds to positive ... The van drove in reverse (about 7 mph).
The simple solution was to put the field wires 'the wrong way round' and then it drove forwards, but I just can't understand why the obvious set-up made it go backwards??

2) Without the field battery connected, I didn't expect the van to move at all when I connected the Armature briefly, but it did rock backwards each time that was connected. Can anyone think why that would happen given zero volts through the field?

MEroller
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Re: Questions on Sepex motor theory

Not knowing the specifics of that motor I can only guess at an explanation for your second question: As switching on a current through a coil (in your case the armature) creates an increasingly strong magnetic field, said change in magnetic field strength will induce a voltage and thus also a small current in the field coil too, even though no power source is connected to the field coils. Like this the field coil also shortly creates a countering magnetic field , which causes the short jerk of the motor.

My rides:
QvR vR one, erider Thunder 5kW

Ampy
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Re: Questions on Sepex motor theory

Thanks, that makes sense.
I think the motor is installed backwards, hence the reason the motor drives backwards (or even rocks backwards as the default direction) and its just used with the leads inverted to go forward.
Am I right in thinking Sepex motors don't have any brush advancement, i.e. they are not made to run slightly better in one direction than another?

MEroller
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Re: Questions on Sepex motor theory

Am I right in thinking Sepex motors don't have any brush advancement, i.e. they are not made to run slightly better in one direction than another?

Sepex motors have basically the same setup as Permanent Magnet Synchronous Motors, which have three phases in the stator (thus no brushed commutation, that must be done elctronically). The only difference is in the rotor which does not have permanent magnets in the Sepex motor, but electro-magnets. Those need (more or less constant and only in one polarity) electrical power to be transfered via brush rings (one +, one -) to the rotor. So you can't advance anything with the brushes, they are always in continuous contact with the positive and the negative ring.
Any advancement is only possible with the electronic commutation, or better with sine-based vector control of the stator phases. Both options would the job of the controller.

My rides:
QvR vR one, erider Thunder 5kW

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