resistance controller

I have recently completed a battery loco base on the series of articles beginning in M.E. Vol 162 no 3845.The motor is a converted 12v. car dynamo and the power is supplied by a 12v car battery. Everything works fine until I wire in the speed controller. This is basically a length of resistance wire soldered to studs with a moving arm contacting the studs.This is where the problem starts.When the speed controller arm is positioned at the end where no current has to pass through the resistance wire the motor runs slower than when it has to pass through the whole length of resistance wire at the opposite setting.Please can someone explain this.My knowledge of electricity extends to O level physics and Ohms law only and this was a few years ago now.

Many thanks.

That sounds like the Rex Nicholls tram. I think there is a problem to do with magnetic saturation and/or needing to switch the field coil from series to shunt. the brush position may also not be optimum. have a look at later issues of ME to see if any comments come up in the letters.

One of our club members, now passed on, got a medal for his at the ME show.

There are better motors available now. The converted dynamo was mostly replaced by Sinclair C5 motors that came on the market about that time.

I'm not familiar with the article you make reference too. Was there a circuit diagram, if so could you send a copy here, then we might be able to sort you out.

Car dynamos were shunt wound generators, one side of the field and armature were connect to the frame. No doubt you have lifted those connections and made two new terminals. You will need four terminals in total, two small ones for the field and two larger ones for the armature The field winding should be separate and be connected straight across the battery, full 12 volts. The fire element should be in SERIES with the armature bit by bit via the studs, one end to positive the other to negative. As you notch up the resistance should get less and motor speeds up until eventually, the arm armature its self is straight across the battery full 12 volts, that will be maximum speed. There are better speed control circuits around using electronics, I think Neil the editor might be able to assist in that area, better than me.Let me know how you get on.Ted

Ted's description matches my understanding of how you control these shunt wound motors.

Can you describe your cool circuit in full. I can't see how adding a resistance wire that's (presumably) unconnected at one end could slow the motor. I can't see saturation as Bazyle describes being the cause if it runs ok without the resistance wire.

Neil

It sounds to me as if you have the variable resistance in series with the field winding. As the field current is reduced so the armature has to spin faster to generate the required back emf.

Russell.

I think Russell has the answer that the resistance is connected to the field instead of the armature. I think we still need to see the circuit diagram in case we have all made the same wrong assumption.

Les.

If you are controlling a shunt wound generator (like a car dynamo), the output current will increase with the field (wound rotor) current. The battery is pretty much a constant voltage in this case. Similarly, if you are driving a shunt wound / wound field motor (which is essentially the same electrical machine as the dynamo above), the maximum speed will occur at minimum field current and the maximum torque will occur at maximum field current and low speed.

I think I've got that right - and I think it's what Russell is saying. Intuitively, if the field current is cranked right up, the back emf generated in the armature reaches battery voltage at a low speed and you can't then drive any current into the armature (current = torque). Backing the field current off reduces the back emf and allows current to be driven into the armature at a higher speed.

Murray

Edited By Muzzer on 13/01/2015 10:57:01

I agree with Ted and others analysis. But I worry about using a bit of electric fire element. When I last tried using a car dynamo as a motor it drew about 20 Amps into the armature which seems a bit much for a fire bar wire which gets red hot at about 5 Amps. Interested to hear how it works out.

Back again, initially forget about varying field current (weak fielding) that come after you have completely notched up if at all. Follow my instructions and get the thing running, then play if you feel like doing. In days gone by when DC drives were around Face plate starter were used. On stud one, a current was applied to the field (full 220 volts) then there was about 10 brass studs mounted on some thick black insulating material in a circular fashion. Behind each stud was a resistance wire, such that as the arm was moved across the studs resistance was lowered, until at last stud the armature itself was connected directly across the 220 volts. Then and only then, was weak fielding applied and it was electrically inter locked so that you couldn't attempt to start the motor on a weak field. Weak fielding give speeds above normal and not all motors were balanced well enough to weak field with safety. As some one has already said modern permanent motors are much better, and more efficient than the old dynamo, no field current. Unfortunately the resistors are wasting energy ( heat) and that is not you really want, its cheap, cheerful and energy wasteful. Also, if my memory serves me well, the size of the resistance wire (CSA) varies, thicker at the beginning. I don't think soldered connections will last long . Can you make good mechanical connections as an alternative? Ted

Many thanks to all who have replied to my query. Certainly food for thought. I have made one attempt to forward the

wiring diagram to this post but it was rejected for some reason which I will investigate.it was the Rex Nicholls tram as someone rightly suggested. I have made it to look like Toby from Thomas the Tank engine but the face is that of Thomas not Toby as that was the only mould available. I don't think my grandsons will worry about that though. The loco is powerful enough to move me along the track at a fair speed just on the on/off switch without the resistance controller in the circuit but it would be ideal to have some sort of speed control.

I forgot to mention in my last post that the resistance controller is wired in series to the field winding but I will try to re-send the wiring diag. from the article.

David,

Having re-read your first and last posts, I believe that your motor is behaving correctly.

One way (maybe not the best) of controlling shunt-wound motor speed is to add extra resistance to the field circuit, which has the effect of increasing the speed of the motor.

Can't remember all the theory at the moment and probably Ted's and other's posts above will explain it better than I can if I took it all in.

Shunt wound motors have separate connections to the armature and field so changing the field current (by adding resistance) will not directly, by the same resistor, change the armature current. This has to rely on the effects of induced EMF etc to balance things out.

Someone else will probably explain this better than me but I hope this gives you the general idea.

Cheers, Stephen S.

I think we are (almost) all furiously agreeing here - you need to reduce the field (rotor) current to speed the motor up once it is moving, otherwise it will reach a (slow) speed and stay there. Whether you put one or more resistors in series with the field winding to achieve that or control the field current with a PWM controller (or even a linear one), the effect should be the same.

Murray