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Can permanent magnet motors spin freely?

34K views 21 replies 10 participants last post by  ken will 
#1 ·
I want to convert a car to a hybrid by adding an electric motor directly inline with the engine. This would be sort of a limited range electic assist, or depending on where the motor is placed the IC engine could be declutched so the car ran on electric power. Will all permanent magnet motors have a drag on them when spun without a load? Pm servo, brushed, BLDC? I'm thinking an AC induction or synchronous motor may work best but from what I've read is a lot more complicated.

Forgive me this is my first post, any input is appreciated
 
#2 ·
I want to convert a car to a hybrid by adding an electric motor directly inline with the engine. This would be sort of a limited range electic assist, or depending on where the motor is placed the IC engine could be declutched so the car ran on electric power. Will all permanent magnet motors have a drag on them when spun without a load? Pm servo, brushed, BLDC? I'm thinking an AC induction or synchronous motor may work best but from what I've read is a lot more complicated.
Forgive me this is my first post, any input is appreciated
Hi,
I see that no-one has answered this, I am not the most qualified to answer this question as my mechanical knowledge is limited.
A free spinning PM is basicly a generator and has therefore electromagnetic resistance so it is not good to be located in line with a ICE for driving purpose but may be usable as generator to load batteries as a sort of range-extender which would then drive another motor for example.

The usual way to let ICE and PM function together in a drive, is to use a sort of planetary gearbox (in simplest form it is a summation gearbox) where you have 3 possible driving modes, only ICE, only PM or both motors together. The most advanced solution I have seen, which also covers range-extension, is the Chevrolet Volt / Opel ampera.
 
#3 ·
Hi, I asked the same question some 18 months ago, but with a DC motor.The drive train configuration worked ok without problems, but my 9" 72v motor in line with the driveshaft rarely exceeded 2500rpm and I eventually overheated the brush holders. It is direct drive and requires a large motor. Major was most helpful with my questions.
 
#5 ·
A spinning permanent magnet motor is only a generator IF the leads are connected. Otherwise it spins freely.

The Scott Drive motor can be easily spun by hand, but becomes rock-solid when the leads are connected.
If a permanent magnet motor is spun with the leads disconnected it makes very high voltage in the open circuit. If it turns fast enough it can short through the insulation. There is still electromagnetic resistance as the magnets pass the field coils or as the field coils pass the magnets - it just depends on which parts is spinning. If the windings are shorted then there is less resistance because the power is disipated as it is built but it will never spin freely.
 
#6 ·
I would look into the AC motors from HPEVS, I have also thought about putting one in between the engine and transmission of a RWD car. An extra 100fltbs of torque at low-rpm would require less ICE throttle so better fuel economy, also the regen could be activated to slow the car and get a little charge into the pack.
 
#10 ·
Hi Major, thank you for your reply. Was initially confused by what PaulS was saying (no offence!) but I think you've managed to clear things up a little.

So with regards to my earlier confusion, does this mean a PM can freewheel without much issues regarding voltage generated as long as it doesn't overspeed? Also, it will not generate significant torque to resist motion?
 
#11 ·
So with regards to my earlier confusion, does this mean a PM can freewheel without much issues regarding voltage generated as long as it doesn't overspeed?
That is correct.

Also, it will not generate significant torque to resist motion?
This is correct. You may experience a minor drag from the rotating magnetic field depending on the type of motor construction (surface magnets or IPM). I would expect this to be on the order of a few percent of rated torque when open circuit.
 
#12 ·
So I'm bringing this thread back cause I'm still left wondering. I built an electric bicycle that uses a brushless hub motor. The permanent magnets spin around the stator that is also the rear axle. With the bike off and the phases completely disconnected its significantly harder to pedal. I'd say it takes about 30% more energy to maintain speed pedaling on flat ground.

From my experience at least, it seems that induction machines are the only motors that can truly freewheel :confused:
 
#13 ·
Wound field motors will freewheel, except for brush friction. But they would have no magnetic resistance to motion when unexcited.

As far as the bike wheelmotor goes; there are some strange designs, IMO. Do you have a reference showing the design?
 
#16 ·
I have converted a few bikes to electric.

There is an increased resistance felt when ridding, but I feel magnetic drag is the least of three.

#2 The extra weight of the motor and batteries adds to rolling resistance.

The motor acts like a flywheel and most people notice the difference in acceleration, which I feel is the #1 reason for the perception of increased "drag".
 
#18 ·
What kind of motor are you using? I'm using a Nine Continents (9C) motor with 28mm wide magnets. My setup is not light by any means, 20lbs of battery, big fat 26 x 2.4 tires with 2.50mm downhill tubes and 15lbs of motor plus my 230lb fat ass. Alot of that weight is rotating weight so acceleration is deff slower with out power assist. Ball bearings are relatively friction less right? In the 90's percent efficient I'd think. So I can't see why the additional weight itself would cause alot of force that would slow the bike down while its simply coasting. (I could be talking out of my ass here though, been a while since college physics)

But I still think there's a ton of magnetic drag. For example, if you flip the bike over onto its handlebars and seat and crank with your hands like when you were a kid, that wheel comes to a dead stop after free wheeling maybe 5 revolutions (estimate, I could try it when I get home). What explains this?
 
#19 ·
If you flip the bike upside down and slowly turn the wheel by hand, not with the pedals, you should feel feel the magnets more then less then more. If the drag is constant it is probably the bearings. Ball bearings are pretty good, not as good as needle bearings, but good. The grease can get thick or dirty.

Rolling resistance is a problem with fat, low pressure, knobby tires.
 
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