Towing a directly powered EV conversion

Electric cars, trucks, ATVs, NEVs - things bigger than a motorcycle.
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skykooler   1 µW

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Towing a directly powered EV conversion

Post by skykooler » Mar 22 2021 9:28am

I've been looking at towing an EV conversion I've done behind an RV. Two of the ways of doing this (flat towing or a tow dolly) keep the rear wheels on the ground, and therefore require a transmission that is able to operate while the engine isn't running.
In my case, the motor (series-wound, not permanent magnet) is coupled directly to the driveshaft - the transmission has been removed. Is there any concern with turning a motor when the motor is not powered? Is this doable or should I look at removing the driveshaft before towing?

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amberwolf   100 GW

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Re: Towing a directly powered EV conversion

Post by amberwolf » Mar 22 2021 9:57pm

Can the motor be forced to spin faster than it's rated speed during any part of the trip, by the wheels forcing the shaft to turn the transmission in reverse? If so, you could end up with "commutator grenading", when the rotational forces exceed the strength of the mounting of the commutator bars.

Other than that, there's nothing dangerous about it (it can't generate electricity, for instance), though it will wear the bearings, brushes, etc, unnecessarily.

( If the speed the EV will be towed at is lower than or equal to what the EV would be able to achieve on it's own, then the motor won't be spun any faster than it would in operation. If the speed it will be towed at is higher than it could achieve on it's own, then you need to look into the rated speed of the motor)

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Re: Towing a directly powered EV conversion

Post by skykooler » Mar 24 2021 11:45am

Thank you! The towing vehicle has a 60mph governor which is slower than the EV can reach, so that shouldn't be an issue. I'll keep an eye on wear, then.

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Re: Towing a directly powered EV conversion

Post by Hillhater » Mar 24 2021 6:51pm

How far are you going to tow ?
Spinning that motor via the road wheels/ diff, etc willboth needa significantly higher tow force than a simple car with the Gearbox in nutral.
At the least your tow vehicle fuel usage will be increased.
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Re: Towing a directly powered EV conversion

Post by amberwolf » Mar 24 2021 9:01pm

Hillhater wrote:
Mar 24 2021 6:51pm
Spinning that motor via the road wheels/ diff, etc willboth needa significantly higher tow force than a simple car with the Gearbox in nutral.
The motor doesn't have magnets, so without power to the field coil it shouldn't be able to induce a current to create drag. The only extra drag would then be from the bearings. If those have that much drag, it would also heat the motor significantly (possibly unacceptably) in normal operation....

What source of drag am i missing that would cost taht much extra energy? (other than the differential, which is always there anyway, and the driveshaft's U-joints, and any bearings for the shaft)

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Re: Towing a directly powered EV conversion

Post by Hillhater » Mar 24 2021 11:34pm

No, not electrical drag, just mechanical.
Having to spinup that (xx lbs) of motor armature to 4k+ rpm every time you accellerate , and even to keep it spinning is going to give significant drag.
..ever ridden one of those flywheel spin bikes in a gym ?.
If it were me, i would put the rear whells onthe dolly and tow it backwards on the front wheels.
Or just remove the tailshaft at the diff and flat tow.
Avoids any possible issues and reduces the tow load.
But i am not doing it !
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Re: Towing a directly powered EV conversion

Post by amberwolf » Mar 24 2021 11:41pm

Hillhater wrote:
Mar 24 2021 11:34pm
No, not electrical drag, just mechanical.
Having to spinup that (xx lbs) of motor armature to 4k+ rpm every time you accellerate , and even to keep it spinning is going to give significant drag.
Doh! I didn't even think of that. :lol: :oops:

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