# Switching from series to shunt on the fly?



## ElvishWarrior (Apr 10, 2008)

I've been trying to find ways to come up with the voltage I need with a battery pack that is minimal in size and cost.

Basically I need to be able to run a D&D ES-31B or similar motor at 144V but don't need more than 2.5kWh (though more would be fine if it didn't come at the expense of size and money.)

Having not been able to find deep cycle batteries of such a small size, I was wondering about making the voltage go twice as far. I'm not too familiar with the limitations of shunt motors, but my impression is that they provide higher efficiency than series motors but poor low end torque.

When run in series, the voltage accross the polar coils and the voltage accross the commutator coils add up to the pack voltage. When run in parallel, each coil would receive the total pack voltage. It would seem that by wiring the same motor in parallel you could get the same performance using half the voltage at double the current.

This would assume that the voltage drop accross the series coils and polar coils is the same or at least very close. If not, one of the coils would limit the allowable voltage such that less than half the series voltage could be used.

Am I on the right track with any of this? My original plan was to use a 144V battery pack, but if I can get away with 72V in series for starting out and then switch to parallel when I need more speed, that would open up many more options. I could probably both fit and afford a deep cycle battery pack at that voltage, and it would open up new options for controllers too.

Speaking of controllers, I've seen some listed as being for "separately excited" systems. Does this relate to the same sort of thing I'm thinking of? Has anyone else proven the idea of switching between series and parallel on the fly to be a good or bad idea?


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## 3dplane (Feb 27, 2008)

Hey Elvish !
I can't wait for one of the gurus to chime in but till then all I can tell you is like you assumed the field coil in a series wound motor is made of very few turns of large diameter(or cross section) wire.(very low resistance)The controller would try to pulse voltage into almost a dead short.In a shunt wound(separately excited) motor the field coil is made of more turns of smaller gauge wire (to keep current low I guess) and requires the proper controller.I feel that even if the controller survived(it will not) you would not gain the benefit you are looking for due to high field strenght limiting the rpm.
( have you heard of field weakening?) Anyway (disclamer) thats how I see it.
Barna.


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## ElvishWarrior (Apr 10, 2008)

That makes perfect sense the way you've explained it. I thought however that I read somewhere about some motors that could be switched between series and parallel. Would it be the shunt motors then that could be switched to series but not the other way around? Or a special design that kept the armature coils and field coils at the same resistance?

For my application, I'm thinking perhaps just using two lower voltage motors would solve my voltage limitations. I'm just worried about running out of space under the hood.


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## Dennis (Feb 25, 2008)

> That makes perfect sense the way you've explained it. I thought however that I read somewhere about some motors that could be switched between series and parallel. Would it be the shunt motors then that could be switched to series but not the other way around? Or a special design that kept the armature coils and field coils at the same resistance?
> 
> For my application, I'm thinking perhaps just using two lower voltage motors would solve my voltage limitations. I'm just worried about running out of space under the hood.


What you are referring to is wiring two motors in a parallel circuit or to series circuit on the fly, also called series-parallel shifting. It does not mean the windings, but how the motors are wired to each other.


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## ElvishWarrior (Apr 10, 2008)

I was actually talking about both scenarios. It appears from what I've read in this thread that it may not be feasible to have an efficient motor designed such that it can be switched from series to parallel. Switching to double motors is something I'm looking into to solve my need for a small size low energy but still deep cycle battery pack.


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## major (Apr 4, 2008)

ElvishWarrior said:


> "When run in series, the voltage accross the polar coils and the voltage accross the commutator coils add up to the pack voltage."
> 
> Hi ElvishWarrior,
> 
> ...


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## ElvishWarrior (Apr 10, 2008)

Thanks Major for the very informative post. I was aware of Eg but did not know that it affected only the armature coil.

Thanks also for the overview of terminology. A huge part of having a productive discussion is speaking the same language.

So now I know what would happen if a series motor were wired in parallel. (And knowing is half the battle... GI Joe!) What if a shunt motor were wired in series? And could there possibly be such a thing as a motor that could be wired either way for different tradeoffs?


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## major (Apr 4, 2008)

Hi ElvishWarrior,

You ask " What if a shunt motor were wired in series? "

You would have a very bad performing motor. Shunt motor field coils are designed for motor voltage, so when put in series with the armature, there would be way, way too much voltage drop and nothing left for Eg. The thing might rotate slowly, but would basically be incapable of doing any real work.

Then you ask "And could there possibly be such a thing as a motor that could be wired either way for different tradeoffs?"

With either series or shunt wound motors, one can alter the field strength by external means to accomplish a degree of speed control. Usually referred to as field weakening. But this does not switch the fields from series connected WRT the armature to shunt, or visa versa. Just won't work for an efficient application of a reasonable sized DC motor. If you want to use field control, I suggest you look at separately excited, or SepEx motors and controllers. But, however you implement field control, it does not give you complete control over the range. Just above base speed. And field control gives you no current limiting. So you still need an armature controller, or chopper. Once you have the chopper, it can do the speed control over the entire range. That is why the basic set-ups use a series motor and chopper. The SepEx system can offer some advantages such as contactorless reversing and regeneration which are very difficult to come by with the series motor. SepEx systems work well for low to medium voltage and power, but get above 50 V or 10 kW and the motor may need extra measures (size and cost) to commutate acceptably.

Regards,

major


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## ElvishWarrior (Apr 10, 2008)

Low voltage is fine with me. 10kW is just a tad bit low for my power needs, but I'd be open to using twin motors if they would fit. Maybe on a subsequent conversion using a lighter vehicle.

Anyone know where to get sepex motors? Series is all I've been able to find.


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## ElvishWarrior (Apr 10, 2008)

If I read correctly, it looks like a Warp 13 motor can be switched from series to parallel. Is this correct, or is it really that it is offered in both series and parallel?


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## MrCrabs (Mar 7, 2008)

I think its offered 1 way or the other.... You need to call and talk to them to get a quote and order one, so it sounds like its a custom job.


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## major (Apr 4, 2008)

Hi ElvishWarrior,

I looked at the specs on the WarP13. It has four field terminals, S1, S2, S3, & S4. This allows the series field coils to be configured in either "4 coils in series" or "in series-parallel, 2 in series and the 2 pairs in parallel". This is just a method of field weakening which avoids the use of a diverter resistor. A little more efficient, but requires an eatra contactor when done on the fly. The performance curves confirm that both the "series" connection and the "parallel" connection are series motor characteristics. Not a series and shunt motor.

Regards,

major


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## Wirecutter (Jul 26, 2007)

ElvishWarrior said:


> So now I know what would happen if a series motor were wired in parallel. (And knowing is half the battle... GI Joe!) What if a shunt motor were wired in series?


 Ah, well I can speak to this. The answer in my case was _nothing_. Ok, well I got some sparks as I connected the battery terminals. The motor didn't turn. What probably happened was a combination of things. The high voltage drop through the field windings kept the current down, so the armature couldn't develop enough of a field to turn.

I subsequently caved in and purchased a proper "Sepex" controller, and in the process gained regen braking and contactor-less reversing. Sepex = Separately excited = shunt wound.

One technique to speed up series wound motors' top end is field weakening. (Dennis can speak about this...) At higher RPMs, the motor can sacrifice some torque for speed by causing some of the motor's current to bypass the field. Basically, a very low resistance is switched in across the field, which puts a higher proportion of the current through the armature. Sepex motors are the logical extension of this technique. You gain efficiency by carefully controlling the field current, using only as much energy as is needed for the desired motor torque.

-Mark


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## dataman19 (Oct 7, 2009)

Great info,,
Now I know what "Senior member" really means..
thanks
dataman19


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