# Efficiency and Range - single vs twin motor



## brian_ (Feb 7, 2017)

There would likely be negligible difference in efficiency, so no difference in range. The only reasons to use two motors on the same shaft are that

you can't get one motor (or one controller) which is powerful enough, or 
two motors are needed to separately drive the left and right wheels.


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## ishiwgao (May 5, 2011)

In general use case, I agree with what Brian said. An exception will be for super low load use cases, where 2 motors might be better than 1. In this case, you will be able to turn off 1 motor and load the other motor more, thereby bringing the load to the higher efficiency area in the torque-speed map of the motor.

I would think that the idea of using 2 motorsdoesn't really come in efficiency, but more of a possibility for performance advantage if configured properly.

E.g., 
1) possibility of AWD vs just FWD or RWD
2) possibility of electronic torque vectoring which can be tuned on the fly rather than a conventional differential
etc...


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## brian_ (Feb 7, 2017)

ishiwgao said:


> In general use case, I agree with what Brian said. An exception will be for super low load use cases, where 2 motors might be better than 1. In this case, you will be able to turn off 1 motor and load the other motor more, thereby bringing the load to the higher efficiency area in the torque-speed map of the motor.


True, but it's hard to imagine the efficiency gain being worth the complication.

In some applications (such as the boat that this is presumably about), there is some relatively steady operating state requiring a low power level (such as a slow cruise) and an occasional much higher-power state (accelerating, or running the same boat on plane). An optimal two-motor configuration might be to size one motor to be at its most efficient point in the low-power scenario, and size a second motor so the total available power is sufficient for the highest power requirement. Of course this or any other optimized two-motor scheme requires a controller to look at the current requested power and determine suitable different commands to the two motors.


Then there are the mechanical considerations, especially if powering only one motor at some times...

If two motors are used to drive one shaft, the most direct way to mechanically arrange this is for at least one of the motors to have a double-ended shaft (so that the two motors can simply be coupled in a line), or for at least one of the motors to have a hollow shaft (so it can be installed over the shaft which runs through to the second motor).

If the motor shafts are not double-ended or hollow, then there must be some sort of combining gearbox or chain or belt drive. If that isn't need for speed reduction, then it is a source of additional bulk, weight, cost, complication, and inefficiency. If it does also provide the speed reduction, it is still more complexity and the portion of the system which is not being driven (by a motor which is turned off in the low-power scenario) is adding mechanical drag and not just idling but actually back-driving to provide the torque needed to spin the unpowered motor.


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## YumaEV (Nov 21, 2014)

GeorgeMan said:


> Hi all, possibly a silly question but here goes...
> If the overall weight and battery capacity remains consistent, would there be a difference in range if using one single motor of say 100kw, vs two 50kw motors?
> Assuming everything else was constant like vehicle speed, rpm and weight etc


As the saying goes, "it depends on a lot of variables, there are available now several size AC motors on a single shaft. 
Along with dual Motor controllers. The big advantage is the difference in performance acceleration vs speed and distance. Depending on the weight and rolling resistance of the car, it may require a 30 hp AC motor to achieve the desired acceleration but once up to cruising speed 15 hp is sufficient to maintain the speed. So but turning off 1 controller there would be some energy savings due to the loss in the controller. A quick google search found


Electric Dual Motor B Face Single and Dual Shaft-HPEVS


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## remy_martian (Feb 4, 2019)

The losses in the controller output stages would be LOWER (half the current in each of two controllers is a quarter of the power lost in each, netting you half the losses by running two, not counting control overhead) the more controllers you throw at it....the losses to your wallet will be higher.


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## YumaEV (Nov 21, 2014)

I agree with your losses estimates. The question wasn't cost it was distance  I liked the idea of 2 motors so I can have a Hot Rod/Cruiser switch, Also possibly a limp home mode.


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