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Discussion Starter #1
Hopefully not too much of a newbie question but I would assume that
permanent magnet motors don't spin freely while NOT energised? Does the
magnetic field create resistance to movement? Can I spin one of these
motors by hand? If two motors were tied together mechanically (belt, chain,
same shaft, etc) and only one motor was energised would the other impede or
create resistance?

Thanks,
Dan
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Discussion Starter #2
Robert Johnston wrote:
> On 11/11/2010 5:56 PM, Dan Baker wrote:
>
>> Hopefully not too much of a newbie question but I would assume that
>> permanent magnet motors don't spin freely while NOT energised? Does the
>> magnetic field create resistance to movement? Can I spin one of these
>> motors by hand? If two motors were tied together mechanically (belt, chain,
>> same shaft, etc) and only one motor was energised would the other impede or
>> create resistance?
>>
> AIUI, if there is no power being taken from, or put into, a PM motor
> (IE, if it is disconnected), it will freewheel with the only resistance
> being that of it's bearings. If the motor is shorted, it will act as a
> brake, bringing the combination to a stop.
>
There is significant hysteresis losses between the iron in the armature
and the field magnets. As much as 10% of running torque (IIRC). Any
fixed field motor with ferrous materials in the rotor is susceptible to
this (nearly all BLDC, PMAC, and PMDC motors). This is why most bicycle
hub motors have a freewheel clutch inside them.

Cory

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Discussion Starter #3
They can also have significant cogging, depending on the design.

Cory Cross <[email protected]> wrote:
> Robert Johnston wrote:
>> On 11/11/2010 5:56 PM, Dan Baker wrote:
>>
>>> Hopefully not too much of a newbie question but I would assume that
>>> permanent magnet motors don't spin freely while NOT energised? Does =
the
>>> magnetic field create resistance to movement? Can I spin one of these
>>> motors by hand? If two motors were tied together mechanically (belt,=
chain,
>>> same shaft, etc) and only one motor was energised would the other imped=
e or
>>> create resistance?
>>>
>> AIUI, if there is no power being taken from, or put into, a PM motor
>> (IE, if it is disconnected), it will freewheel with the only resistance
>> being that of it's bearings. If the motor is shorted, it will act as a
>> brake, bringing the combination to a stop.
>>
> There is significant hysteresis losses between the iron in the armature
> and the field magnets. As much as 10% of running torque (IIRC). Any
> fixed field motor with ferrous materials in the rotor is susceptible to
> this (nearly all BLDC, PMAC, and PMDC motors). This is why most bicycle
> hub motors have a freewheel clutch inside them.
>
> Cory
>
> _______________________________________________
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>



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Discussion Starter #4
Can you explain "cogging"?
So far from the comments it's anywhere from 0-10% loss in power (+ the
cogging). It would be hard to build a mechanical system that could
disconnect the second motor from the other motor and driveline that is less
than 10% loss. Perhaps an electric clutch?

Has anyone considered this? I want 30hp of motor but don't want to pay to
run it at 30 hp all the time. It appears that one big motor is great but
best efficient at full power. The curve goes down as load is reduced. So
the thought would be to use two motors during heavy load (acceleration) and
fall back to one when cruising. The only extra cost would be the weight of
the motor and the effieciency loss of mechanically adding it in. Plus two
smaller motors are often cheaper than one large one. Is anyone using
anything like this? Is my logic flawed?
Thanks,
Dan


On Fri, Nov 12, 2010 at 2:25 PM, Peter Gabrielsson <
[email protected]> wrote:

> They can also have significant cogging, depending on the design.
>
>
Cory Cross <[email protected]> wrote:
> > Robert Johnston wrote:
> >> On 11/11/2010 5:56 PM, Dan Baker wrote:
> >>
> >>> Hopefully not too much of a newbie question but I would assume that
> >>> permanent magnet motors don't spin freely while NOT energised? Does
> the
> >>> magnetic field create resistance to movement? Can I spin one of these
> >>> motors by hand? If two motors were tied together mechanically (belt,
> chain,
> >>> same shaft, etc) and only one motor was energised would the other
> impede or
> >>> create resistance?
> >>>
> >> AIUI, if there is no power being taken from, or put into, a PM motor
> >> (IE, if it is disconnected), it will freewheel with the only resistance
> >> being that of it's bearings. If the motor is shorted, it will act as a
> >> brake, bringing the combination to a stop.
> >>
> > There is significant hysteresis losses between the iron in the armature
> > and the field magnets. As much as 10% of running torque (IIRC). Any
> > fixed field motor with ferrous materials in the rotor is susceptible to
> > this (nearly all BLDC, PMAC, and PMDC motors). This is why most bicycle
> > hub motors have a freewheel clutch inside them.
> >
> > Cory
> >
> > _______________________________________________
> > | REPLYING: address your message to [email protected] only.
> > | Multiple-address or CCed messages may be rejected.
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> > | OTHER HELP: http://evdl.org/help/
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> >
>
>
>
> --
> www.electric-lemon.com
>
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Discussion Starter #5
Cogging, torque ripple or pulsation, means that the rotor prefers to
be at a fixed angle increment relative to the stator, it will resist
you if you try to move it away from this angle, up to a point, then it
will try to pull itself to the next angle increment. Kind of like
compression in an ICE.
It doesn't necessarily mean there's losses associated with this, it's
kind of like going over a hill, you regain on the down side what you
lost on the up side, but you asked about resistance and being able to
spin it by hand, in which case it is relevant.

As for using two motors, sure it's doable, been done even, there may
even be efficiency to gain if you design the system carefully. The
devil hides in the details as always, if you don't have enough data or
knowledge to accurately model the system, you're not likely to gain
anything but complexity.

If you leave the second motor in the drive line when not powered
you'll still have the full iron losses, which tend to dominate above
some speed. You also just cut the available copper in half thereby
doubling copper losses so there's nothing gained by doing it this way.

If you're gonna clutch the second motor in and out of the drive line
then you need two controllers with the all the added complexity that
brings.




Dan Baker <[email protected]> wrote:
> Can you explain "cogging"?
> So far from the comments it's anywhere from 0-10% loss in power (+ the
> cogging). It would be hard to build a mechanical system that could
> disconnect the second motor from the other motor and driveline that is le=
ss
> than 10% loss. Perhaps an electric clutch?
>
> Has anyone considered this? I want 30hp of motor but don't want to pay=
to
> run it at 30 hp all the time. It appears that one big motor is great b=
ut
> best efficient at full power. The curve goes down as load is reduced. =
So
> the thought would be to use two motors during heavy load (acceleration) a=
nd
> fall back to one when cruising. The only extra cost would be the weigh=
t of
> the motor and the effieciency loss of mechanically adding it in. Plus two
> smaller motors are often cheaper than one large one. Is anyone using
> anything like this? Is my logic flawed?
> Thanks,
> Dan
>
>
> On Fri, Nov 12, 2010 at 2:25 PM, Peter Gabrielsson <
> [email protected]> wrote:
>
>> They can also have significant cogging, depending on the design.
>>
>> On Thu, Nov 11, 2010 at 4:22 PM, Cory Cross <[email protected]> wrote:
>> > Robert Johnston wrote:
>> >> On 11/11/2010 5:56 PM, Dan Baker wrote:
>> >>
>> >>> Hopefully not too much of a newbie question but I would assume that
>> >>> permanent magnet motors don't spin freely while NOT energised? Do=
es
>> the
>> >>> magnetic field create resistance to movement? Can I spin one of t=
hese
>> >>> motors by hand? If two motors were tied together mechanically (be=
lt,
>> chain,
>> >>> same shaft, etc) and only one motor was energised would the other
>> impede or
>> >>> create resistance?
>> >>>
>> >> AIUI, if there is no power being taken from, or put into, a PM motor
>> >> (IE, if it is disconnected), it will freewheel with the only resistan=
ce
>> >> being that of it's bearings. If the motor is shorted, it will act as a
>> >> brake, bringing the combination to a stop.
>> >>
>> > There is significant hysteresis losses between the iron in the armature
>> > and the field magnets. As much as 10% of running torque (IIRC). Any
>> > fixed field motor with ferrous materials in the rotor is susceptible to
>> > this (nearly all BLDC, PMAC, and PMDC motors). This is why most bicycle
>> > hub motors have a freewheel clutch inside them.
>> >
>> > Cory
>> >
>> > _______________________________________________
>> > | REPLYING: address your message to [email protected] only.
>> > | Multiple-address or CCed messages may be rejected.
>> > | UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub
>> > | OTHER HELP: http://evdl.org/help/
>> > | OPTIONS: http://lists.sjsu.edu/mailman/listinfo/ev
>> >
>>
>>
>>
>> --
>> www.electric-lemon.com
>>
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>> | Multiple-address or CCed messages may be rejected.
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>>
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