# Can permanent magnet motors spin freely?



## SHARKBITEATTACK (Nov 12, 2012)

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


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## gunnarhs (Apr 24, 2012)

SHARKBITEATTACK said:


> 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.


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## poprock (Apr 29, 2010)

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.


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## ruckus (Apr 15, 2009)

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.


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## PaulS (Sep 11, 2012)

ruckus said:


> 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.


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## Bowser330 (Jun 15, 2008)

SHARKBITEATTACK said:


> 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


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.


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## ishiwgao (May 5, 2011)

PaulS said:


> 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.


I would like to clarify this post. If the BLDC / PM motor is connected to a controller wit hthe cables, but the controller is not "turned on" by the ignition, will it still cause a problem with free spinning? or will it spin freely without resistance?


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## PaulS (Sep 11, 2012)

ishiwgao said:


> I would like to clarify this post. If the BLDC / PM motor is connected to a controller wit hthe cables, but the controller is not "turned on" by the ignition, will it still cause a problem with free spinning? or will it spin freely without resistance?


Each time a magnet passes a coil of wire a voltage and current is generated. If the controller is "off" then it is an open circuit and the voltage will rise until it can bridge the gap or is disipated by a protection circuit in the controller. It will never "freewheel" like some AC motors can.
If the voltage generated is high enough it will damage the controller.

If you want it as close as it can get to "freewheeling" then put a normally closed contactor across it that opens when the controller is turned "on".


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## major (Apr 4, 2008)

PaulS said:


> Each time a magnet passes a coil of wire a voltage and current is generated.


A voltage is generated in the coil. A current will only flow if there is a completed circuit for that coil.


> If the controller is "off" then it is an open circuit and the voltage will rise until it can bridge the gap or is disipated by a protection circuit in the controller.


If the coil is open circuit then no current will flow. If the RPM is near the rating, then the generated voltage across the coil leads will be about the rated voltage.



> It will never "freewheel" like some AC motors can.


It will, or very nearly. The magnets passing the steel core will induce eddy currents and cause hysteresis loss in the core which will present a small amount of torque opposed to rotation, perhaps a few percent of rated torque. 



> If the voltage generated is high enough it will damage the controller.


This is possible at very high RPM, but is usually not a problem.



> If you want it as close as it can get to "freewheeling" then put a normally closed contactor across it that opens when the controller is turned "on".


This will work the opposite as you describe. If you short the coil you allow the coil current to rise in proportion to the voltage and RPM. That current will interact with the flux and produce a torque opposing rotation. It will essentially lock the rotor. Try it sometime.


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## ishiwgao (May 5, 2011)

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?


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## major (Apr 4, 2008)

ishiwgao said:


> 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.


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## SHARKBITEATTACK (Nov 12, 2012)

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


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## major (Apr 4, 2008)

SHARKBITEATTACK said:


> 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


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?


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## SHARKBITEATTACK (Nov 12, 2012)

major said:


> 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?


Sure.










All kinds of good info here http://www.electricbike.com/9c/


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## major (Apr 4, 2008)

SHARKBITEATTACK said:


> Sure.
> 
> All kinds of good info here http://www.electricbike.com/9c/


I don't see any reason in there for it. If the phase leads are open circuit it should spin with minor drag. Do other similar bike hub motors drag? I'd think that would be a real determent. A real bitch to pedal home if you run out of battery  What do the ES forum guys say?


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## ken will (Dec 19, 2009)

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".


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## SHARKBITEATTACK (Nov 12, 2012)

major said:


> I don't see any reason in there for it. If the phase leads are open circuit it should spin with minor drag. Do other similar bike hub motors drag? I'd think that would be a real determent. A real bitch to pedal home if you run out of battery  What do the ES forum guys say?


Well its sort of been brought up... there's 2 main types of hub motors that we use on the ebikes. The more popular style (type I have) is a direct drive motor but there are also geared motors as well. The direct drive motor like I have actually has the capability of regen braking which is nice. I've tested it but haven't devised a way to activate it on the fly. The popular way is to use a microswitch on the brake cable but my bike has hydraulic brakes. 

The geared motors spin faster and are said to be more efficient because the motors spend more time in their efficiency range and the built in reduction also gives them better hill climbing ability. They are more expensive, generally aren't as robust, powerful or compact for obvious reasons. This style of motor actually has built in sprag clutches which allow it freewheel without any drag while pedaling or coasting downhill motor. The downside to this is you can't get any regen.










So I guess its just kind of a well known fact among the ebike community that you don't get true freewheeling unless you go with a geared motor. On the ebike, having the drag isn't a huge issue since its designed to be a "pedal assist bike" but it would totally would suck, if there was an electrical failure and you had to pedal 15 mi back home though!! Been fortunate to not have that happen yet. On an extended range hybrid vehicle conversion it would be an issue where you would be using precious fuel just to overcome the drag of the motor. (I'm just trying to understand the theory though, and rotating magnetic fields make my brain hurt I think I'll have to check out some books from the library about electric machines. I hadn't thought of a series wound motor (forklift motor?) Do these motors produce instant torque like a normal PM brushed DC motor?

EDIT: I should have looked on the website where I bought the bike first. A lot a good info that I could of paraphrased instead of trying to explain it myself.



Ebikes.ca said:


> *I just got my motor but can barely turn the axle there is so much resistance, is this a defective unit?*
> 
> No, with any direct-drive motor there is significant drag force required to turn the hub due to the strong interaction between the magnets and the stator. With the Crystalyte 400 series motors, this is about 0.4 N-m, while the torque to turn the axle of the 500 series is closer to 0.7 N-m. That is more torque than most people are able to apply just with their fingers turning the axle. However, if you mount the wheel on the bike and spin it, you will see that the wheel can still turn more or less freely. If you give it a whirl it should turn 3 or 4 revolutions before coming to a stop. The drag force caused by these hubs is comparable in magnitude to the rolling resistance of the tires on a regular mountain bike.


http://ebikes.ca/store/store_motors.php


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## SHARKBITEATTACK (Nov 12, 2012)

ken will said:


> 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.
> 
> ...


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?


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## ken will (Dec 19, 2009)

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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## major (Apr 4, 2008)

SHARKBITEATTACK said:


> 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?


What is the running torque for your motor? Like 50 Nm? Then you're taking about the few percent freewheeling torque I mentioned. It is due to the magnets continuing to produce flux as it rotates. There is no torque produced from the coils because there is no current in the coils. But the magnet flux flows through the stator teeth and back iron. This has a slight "magnetic pull" in the static condition. But when the wheel rotates, the magnetic flux changes direction in the stator iron each time a magnet passes by a tooth. Every time you change the direction of flux in the steel it uses energy. Also the changing flux induces tiny loops of current in the stator steel called eddy. These eddy currents establish a magnetic field of their own and that will make a force in the direction opposing change. These 2 magnetic forces are what constitute core loss and are present in the PM motor whenever it rotates, energized or not. They account for approximately half of the running losses; resistive or copper losses being the other half. Of course the copper losses are not present when unenergized. It's just the price you pay for the PM field. 



> I hadn't thought of a series wound motor (forklift motor?) Do these motors produce instant torque like a normal PM brushed DC motor?


Oh yeah. The series wound DC motor will produce the highest instant torque for its size. That's what the electric drag racers use. Take a look at what John does with series motors. http://www.diyelectriccar.com/forums/showthread.php?p=369047#post369047


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## jehan12413 (Feb 4, 2010)

SHARKBITEATTACK said:


> 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


 Here is a bit of info which may or may not be helpful to you. I have spent quite a bit of time with permanent mag alternators on windmills. These are basically GM alternators where the wound rotor has been replaced with a permanent magnet unit. I have swapped out the stator coils many times on these units to find the most efficient combo for the rpm achieved. One thing I have noticed is that number of windings in the staor has a huge effect on the freewheeling resistance. For example with a 12v stator coil which only has a few windings you can spin the shaft by hand and it will continue to freewheel unless the output leads are shorted. Now put in a 60v stator coil and there is significant cogging from the magnets and it is very difficult to turn by hand (definately will not freewheel). Short the leads and it is basically locked and impossible to turn.


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## ken will (Dec 19, 2009)

SHARKBITEATTACK said:


> I built an electric bicycle that uses a brushless hub motor. I'd say it takes about 30% more energy to maintain speed pedaling on flat ground.


Some bicycle hub motors were designed for 250 watts, then the manufacturers filled them with oil to keep them cool and sold them as 450 watt motors. 
On a cold day they can be harder to turn. If you give them full power it will heat up and thin out the oil, which will let them turn a little easier.


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