# 48v Series Wound motor behavior voltage division between field and armature?



## major (Apr 4, 2008)

rmay635703 said:


> This is something I have been wondering, I need a close approximation of what the field voltage will be over the working range of a normal 5hp 48 volt series wound EV motor.


Hey Ryan,

It depends on the load. For a 48 volt series wound motor, I'd say the field voltage would be in the 0.25 to 1.5 volt range.

Reading over your plan, I am confused as to your intent.

Regards,

major


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## rmay635703 (Oct 23, 2008)

major said:


> Hey Ryan,
> 
> It depends on the load. For a 48 volt series wound motor, I'd say the field voltage would be in the 0.25 to 1.5 volt range.


That would be PERFECT! At 1.5 volts I could continously shunt up to 60 amps, nice! Now are you sure it doesn't get higher than that momentarily at higher speeds?



major said:


> Reading over your plan, I am confused as to your intent.
> 
> Regards,
> 
> major


The project is very simple, it is too cost prohibitive to modify this vehicle to a higher voltage at the moment and the gearing is wrong anyway, so I am going to make a field weakening circuit that I can control more closely (my others haven't worked well, got hot and little benefit), 

We have long stretches of flat slow road around here and I would like to get this vehicle to maintain 40mph on flat sections.

The vehicle as it is now maintains 31mph on flat ground with fully charged batteries, originally it would only go 24mph, now I would like to finish my initial work with a field weakening circuit, I have a nice pair of low resistance high rating carbon pile variable resistors. Their ohm values match my field nicely both above and below making them a suitable way of slowly dialing up the shunt current as my speed increases to maintain some level of efficiency during field weakening. They have very low ohm values adjustable from .6-.0264 ohms which should definately be sufficient.

Originally I thought about making field taps but my motor doesn't have any internal ones to hook up

Thank You
Ryan


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## major (Apr 4, 2008)

rmay635703 said:


> That would be PERFECT! At 1.5 volts I could continously shunt up to 60 amps, nice! Now are you sure it doesn't get higher than that momentarily at higher speeds?


Hi Ryan,

Of course it does. For your calculations use the field resistance. I guessed it would be about .008 ohms. Then at very low currents, 30 or 40 amps, the resistance times the current (Ohm's Law) gives you about 0.25 volts. At 200 amps, I*R = 1.6 volts. So, at 400 amps, it'd be 3.2 volts.

Now I don't know the actual field resistance of your motor. So I could be off. But it'd probably be in the neighborhood. Not anywhere near 14 volts you mentioned before. Unless at stall, 1500 plus amps.

And remember, the field resistance will increase with temperature. If the motor gets to rated temperature, field resistance may increase 30 to 40 percent.




> Their ohm values match my field nicely both above and below making them a suitable way of slowly dialing up the shunt current as my speed increases to maintain some level of efficiency during field weakening. They have very low ohm values adjustable from .6-.0264 ohms which should definately be sufficient.


Like I said, I don't know the resistance of your motor. But the values for your resistors sound higher than the field would likely be. So, I think it would be good to start there.

Field weakening is not always good. Best done at light loads. You'll draw more current for a given load. And the diverted field will affect commutation. I have recently seen a report of a fellow who did this. Went faster, for three days. Then needs a motor replacement because his brushes dusted away and motor fried. So, if you can, and you should, observe commutation when you weaken that field. If you get streamers exiting the motor, stop and increase your diverter resistance.

Faster is nice, crispy motors are not.

Regards,

major


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## rmay635703 (Oct 23, 2008)

major said:


> Hi Ryan,
> 
> Field weakening is not always good. Best done at light loads. You'll draw more current for a given load. And the diverted field will affect commutation. I have recently seen a report of a fellow who did this. Went faster, for three days. Then needs a motor replacement because his brushes dusted away and motor fried. So, if you can, and you should, observe commutation when you weaken that field. If you get streamers exiting the motor, stop and increase your diverter resistance.
> 
> ...


I will be adding field weakening somewhere north of 22mph where the motors amp draw is very low (I am only drawing around 40amps near top cruising speed) The motor is rated at 5hp so I would assume field weakening up to an overal amp draw of 75amps through the motor would be safe. 

I don't think streamers will occur when the amp draw is at or near the motors continous rating.

Again this motor is being used in a heavily gear reduced state to restrict speed so its basically at freewheeling RPM, its spinning so fast it has little power, if I could I would gear this vehicle up with a higher ratio, it could easily handle 35-45mph as is if the gearing were correct but the gears are asian and metric, nonstandard on this side of the ocean.

Thanx
Ryan


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## major (Apr 4, 2008)

rmay635703 said:


> I will be adding field weakening somewhere north of 22mph where the motors amp draw is very low (I am only drawing around 40amps near top cruising speed)


Hi Ryan,

What is the motor RPM at 40 amps?

major


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## rmay635703 (Oct 23, 2008)

major said:


> Hi Ryan,
> 
> What is the motor RPM at 40 amps?
> 
> major


5200 RPM thats all the more I can get even with the small hills around here, I have tested the motor up to 6200rpm going down a long steep hill, the spec from china states 6350 continous.

My goal would be to get the motor to maintain 5k RPM instead of drop down to around 3800-4200rpm on the flat.


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## major (Apr 4, 2008)

rmay635703 said:


> 5200 RPM thats all the more I can get even with the small hills around here, I have tested the motor up to 6200rpm going down a long steep hill, the spec from china states 6350 continous.
> 
> My goal would be to get the motor to maintain 5k RPM instead of drop down to around 3800-4200rpm on the flat.


I guess going from 4000 to 5000 with field weakening would be o.k., if commutation doesn't get out of hand. But 6000 down hill is living on the edge, IMO. And with the field weakening system, you could see higher down that hill.

Be careful out there,

major


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## rmay635703 (Oct 23, 2008)

major said:


> I guess going from 4000 to 5000 with field weakening would be o.k., if commutation doesn't get out of hand. But 6000 down hill is living on the edge, IMO. And with the field weakening system, you could see higher down that hill.
> 
> Be careful out there,
> 
> major


I know full well what might happen (I have another cheapo motor source lined up just in case)

When I am on the opposite end of town where there are actually hills and not gentle one degree inclines I coast going up the hill and back down so my speed does exceed the magical 40mph area, I brake at that time as I am only 300-500rpm from what I believe is the danger area and although I have pushed it slightly I dont plan on finding out empiracly what my motor does past that point when I actually meet the true redline.

My redline is marked clearing on the RPM meter and I know the absolute max speed as well and we take care when we get around that spot, its too bad I can't put a clutch or neutral on here 

Better still would be the proper 4 speed transmission that was in the car before it was imported to the US and disabled. 

Ah well live and learn.


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