# Combining Motors



## dualdj1 (Dec 15, 2011)

Hey folks, I haven't been able to find a good answer online anywhere, so figured I'd ask the electric motor experts... (this isn't exactly car related though, sorry).

I have several used 1/20th hp motors that I got out of an old machine. My question is, can I run them in tandem (via a belt, chain, etc) for increased power output? I think I have 6 of them. Would it be a linear increase in power? As in, 6 would be 6/20=3/10hp, 8 would be 2/5hp, etc? Or are there diminishing returns?

Thanks for helping!

Jason


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## frodus (Apr 12, 2008)

well, what kind of motors are they? AC? Single or 3 phase? DC?


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## dualdj1 (Dec 15, 2011)

frodus said:


> well, what kind of motors are they? AC? Single or 3 phase? DC?


single phase 115v AC, 60hz 1amp.


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## frodus (Apr 12, 2008)

Is it for a vehicle? or are they running off AC-Mains?

They should add together, but you also add inefficiency every time you convert energy mechanically (belt, chain, gears).


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## dualdj1 (Dec 15, 2011)

frodus said:


> Is it for a vehicle? or are they running off AC-Mains?


It'll be runnin on AC Mains. I was thinking of (attempting) using it for an air compressor setup.


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## Rational (Nov 26, 2011)

I have learned that a differential is a torque-divider and it's necessary because the wheel speeds are different when going around curves. 

So, each of two motors would feed a differential and the driveshaft would yield the sum of torques at some speed. 

If there's an electrical way to do this I haven't thought of it yet.  On the upside, motors and engines are more like 'torque sources' than 'RPM sources' but if you coupled all the shafts together the back EMF generated for each motor almost certainly would not be what the motor wanted.


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## major (Apr 4, 2008)

Rational said:


> I have learned that a differential is a torque-divider and it's necessary because the wheel speeds are different when going around curves.
> 
> So, each of two motors would feed a differential and the driveshaft would yield the sum of torques at some speed.


Hi Rational,

What does this have to do with the OP's question?



Rational said:


> If there's an electrical way to do this I haven't thought of it yet.  On the upside, motors and engines are more like 'torque sources' than 'RPM sources' but if you coupled all the shafts together the back EMF generated for each motor almost certainly would not be what the motor wanted.


 is an appropriate symbol for you to use. What you typed following it makes no sense. And the use of electric motors in combination to mimic the mechanical differential is well known. You can find it discussed here in this forum if you bothered to search. But again it is off topic for this thread.



dualdj1 said:


> My question is, can I run them in tandem (via a belt, chain, etc) for increased power output? ..... Would it be a linear increase in power? As in, 6 would be 6/20=3/10hp, 8 would be 2/5hp, etc?


Hi dual,

Yes, yes, and yes. Power adds. So if the shafts are coupled, the speed is the same so the torque adds. Of course the torque depends on the load. But if they are identical motors connected to the same power source each would deliver the same torque and the total torque on the load would be the sum.

While this may be an interesting exercise it will likely be considerably less efficient than a larger single motor and less reliable.

Regards,

major


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## dualdj1 (Dec 15, 2011)

major said:


> Yes, yes, and yes. Power adds. So if the shafts are coupled, the speed is the same so the torque adds. Of course the torque depends on the load. But if they are identical motors connected to the same power source each would deliver the same torque and the total torque on the load would be the sum.
> 
> While this may be an interesting exercise it will likely be considerably less efficient than a larger single motor and less reliable.
> 
> ...


Thanks Major, that's what I was looking for. I realize it's not the most efficient, etc, but it's free motors vs paying for a new one. If it doesn't work, oh well. But I just wanted to know if it was at least worth trying, or if it wasn't worth my time.

Appreciate the response!


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## Rational (Nov 26, 2011)

major said:


> Hi Rational,
> 
> What does this have to do with the OP's question?
> 
> ...


I left some details out of my post - sorry.

The OP can feed a differential with two motors and get the torque summed at the driveshaft but for several motors the downside is that he'd need several differentials.

A torque source, like the N'gator coil spring, tries to output constant torque and if it's unloaded it tends to speed up indefinitely. 
An RPM source tries to maintain constant speed and if you lock the rotor it delivers very high torque, so something breaks.
Real motors are somewhere in between.

Wall outlets and batteries are voltage sources; an arrangement of motors coupled like this may work better when each motor is instead fed by a current source, but this is electrically inefficient. LEDs and fluorescent tubes are best fed by current sources, e.g., the ballast does the conversion between the wall outlet and what the tube wants.
A voltage source is probably the electrical analogy of an RPM source, and a torque source goes with a current source.

Coupling the motor shafts together may result in 'hunting' or other undesirable behavior, depending on inertias, damping due to friction or harmonic balancer types of devices, and shaft elasticity. 
For things like motors, loudspeakers and phonograph cartridges which are in both the mechanical and electrical domains you pretty much have to write the messy equations that govern these to predict performance.

BTW, the Economy of Scale effect would agree with you about the efficiency but I don't know how you decided on the reliability aspect.


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## dualdj1 (Dec 15, 2011)

I think I follow what you're getting at. I think for what i'm doing just using a belt will provide enough slippage for any inconsistencies, but i could see where coupling using differentials would be most beneficial.


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## DavidDymaxion (Dec 1, 2008)

I think it is a case of "it depends" for the reliability thing. Does it mean "more reliable" as in more likely to let you limp home in case of a breakdown, or does it mean "more reliable" in terms of not have anything break in the first place?


Smaller motors are less likely to fly apart at high rpm => more reliable
If your system is sized to just enough power, then if one motor breaks you don't have enough power (or the remaining motors overheat) => less reliable
If you use chains and one chain breaks, the remaining motors drive the car => more reliable
If you have the motors on a common shaft and one motor seizes => less reliable
If you exploit multiple motors to have multiple controllers and battery packs => more reliable (although the chance of something breaking gets higher)



Rational said:


> ... BTW, the Economy of Scale effect would agree with you about the efficiency but I don't know how you decided on the reliability aspect.


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