Re: (ET) A Thread for discussion.

["David Roden (Akron OH USA)" <roden ald net>]

On 18 Aug 2000, at 8:16, ThompsonG DFO-MPO GC CA wrote:

> I noticed the obvious that,
> the leads and the connections to the windings in the outer shell were
> extremely heavy and come directly form the batteries. While  the leads to
> external brushes and the connector are very fine.

No, it is exactly the opposite.  The field connections (the field is on 
the outside) are small, and the armature (rotating part, fed electricity 
by the brushes) connections are quite large.

I'm not an engineer, but I'll try to explain this as I understand it.  
There are others on the list who I'm sure will augment my discussion.  
For the sake of simplicity, I'll ignore the issue of speed control and 
treat the motor as if it were constant speed.

The field generates magnetism.  So does the armature.  To describe in a 
simple way what is going on, the brushes change where the magnetism is 
generated in the armature as the motor rotates, so that the armature is 
sort of "pulled around the motor."

Large armature connections and small field connections are typical of a 
shunt or separately excited motor.  

Power is volts * amps.  Thus, 1000 watts can be 10 volts at 100 amps, or 
100 volts at 10 amps, or 1 volt at 1000 amps.

The magnetic power of the field is of course generated by electric power 
(volts times amps).  In the ET, this comes from relatively high voltage 
(36 volts) and relatively low current (an amp or two).  This works 
because there are many turns of small wire in the field -- it has high 
resistance, so the voltage drop across the field is high, but it uses 
relatively little current.

The armature has fewer turns of much heavier wire.  Therefore, it needs  
much more current.

To generate just one horsepower, the motor -- field and armature combined 
-- needs about 25 amps at 36 volts (taking losses into account).  And of 
course its peak horsepower is much higher.

(On the other hand, series motors have fields that are connected not 
across the battery, but rather between the battery and the armature.  
These are wound with just a few turns of heavy wire.  Because of the 
wire's low resistance, it has a low voltage drop, but consumes lots of 
current.  But, as I understand it, for a given motor output, the field's 
share of the electrical power is more or less the same as in a shunt or 
sep-ex motor.)

This is all by way of trying to make it clear that GE hasn't performed 
any kind of magic here.  The laws of physics are immutable -- it takes 
746 watts of electricity to make one horsepower (ignoring losses to 
electrical heating and friction).  The rule of thumb is that most brushed 
DC motors (be they series, shunt, sep-ex or compound) lose about 20% of 
the electric power to inefficiency, so about a kilowatt (1000 watts) in 
produces about a horsepower out.

The field still uses its share of that power.  All GE has done is make 
the field use less current and more voltage than in some motors.  That's 
why the field wires can be fairly small.


David Roden - Akron, Ohio, USA
1991 Solectria Force 144vac
1991 Ford Escort Green/EV 128vdc
1979 General Engines ElectroPed 24vdc
1974 Honda Civic EV 96vdc
1970 GE Elec-trak E15 36vdc
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