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Re: (ET) volts
At 01:12 AM 4/15/06 -0400, Christopher Zach wrote:
Um, applying a higher voltage to a PM motor doesn't weaken the field.
The motor speed will be approximately proportional to the applied
voltage. No overspeed. As you said the field strength does not change
(IE it's not weakened)
What I meant was that by increasing armature voltage you're changing the
relationship of armature strength (now 125%) without changing field
strength (because it's a magnet). Thus the motor will behave in the way
that the shunt motors work when you reduce the field strength (ie: They
spin faster, less torque, heat up more, and pull a lot more current under
load). Not good things.
Spin faster - yes
heat up more, pull more current under load - possibly, it will depend on
the load. It some cases it can be less because the extra torque on startup
means you spend less time a peak torques. Unless you have a controller on
it though peak currents will almost certainly be higher. It may not be
enough to matter though.
less torque - No, torque will be the same or higher. Torque in a PM motor
is essentially the product of the field (magnet) strength and the armature
current. With higher voltage the current will go up, so will the
torque. This assumes the armature is not saturating but that's not very
likely.
You could also run into brush issues with the higher current and voltage.
Unless the motors are close to the edge I doubt increasing them to 48 V
will give enough current to demagnetize them but the extra 30% tip speed
might be dangerous. Also the extra load from the higher speed may cause
the motors to warm up more quickly.
Both could be a problem; I would not like to see the blades rip apart.
Worse yet when you hit a rock the blades would be spinning faster. Bad
idea IMHO; get a 48 volt curtis motor controller and set the output for
75% or so.
I would do the same. A 1/3 increase in tip speed is nearly a doubling in
energy. Not something to be cavalier about in a cutting instrument.
I'de be a little worried about the field in a shunt wound motor. You are
going to get a lot more heating there (close to double).
Interesting. I wasn't sure that would cause a problem. Since they are
technically compound motors this would be offset by the increase in field
strength in forward speed, but would do really weird stuff in reverse.
Heat in the shunt in a shunt wound motor is V*V/R. Since the field
resistance is a constant and you are increasing the voltage by a third the
power loss to heat doubles. That's regardless of the compound
windings. The same effect will be true on any field weakening resistors,
they will now need to dissipate twice the power.
The other item that could well have an effect here is that you may well
push the field into saturation in which case the field will not strengthen
linearly with the applied voltage and you may end up with a permanent field
weakening situation. The resulting speed/torque is curve may not be a
simple relationship.
Hm. Eh, I'd just get a 48-36 volt motor controller and be done with it.
Why go 48 volts anyway?
Greater resistance to battery droop if you use a controller. More commonly
available chargers maybe? With proper setup it should be possible to
increase both the peak speed and peak torque. More complex control and
slightly less total energy available though.
Robert
" 'Freedom' has no meaning of itself. There are always restrictions, be
they legal, genetic, or physical. If you don't believe me, try to chew a
radio signal. " -- Kelvin Throop, III
http://www.aeolusdevelopment.com/