Rewinding PM motor to change voltage
Rewinding PM motor to change voltage
(OP)
I have a 12/24vdc PM motor which turns 5500rpm,no load, at 12v and 11000 at 24v. My power supply is 24vdc. Can I rewind the armature for 24v and reduce the speed to about 5000rpm. If so, can anyone tell me how or where I might find that info?





RE: Rewinding PM motor to change voltage
*Even inside a hard-boiled egg, there is a golden heart!!!*
RE: Rewinding PM motor to change voltage
RE: Rewinding PM motor to change voltage
*Even inside a hard-boiled egg, there is a golden heart!!!*
RE: Rewinding PM motor to change voltage
It may be cheaper and safer (and even wiser ???) to get a 12 V supply to run at 5500 RPM.
*Even inside a hard-boiled egg, there is a golden heart!!!*
RE: Rewinding PM motor to change voltage
RE: Rewinding PM motor to change voltage
RE: Rewinding PM motor to change voltage
If you double the number of turns, your resistance will go up by a factor of four. You will not halve the wire diameter. You will halve the cross sectional area. So the diameter would go up by about the square root of two. For instance, if the wire diameter is 1mm it would be 1.414mm for double the turns. The voltage constant (KE)is what limits the speed of your motor at no load (mV/rpm). You can estimate this by calculating:
mV/rpm=1000*(VT-INL*RT)/SNL
Where:
VT is applied terminal voltage
INL is no load current
RT is armature resistance
SNL is no load speed
When you double the number of turns the KE doubles so you reach your terminal voltage at a lower speed. You also double your torque constant (KT). This means that you have twice the torque per amp. If you go from a 12 volt 5500rpm motor to a 24 volt 5500 rpm motor, you will not halve the maximum output power (PMAX).
PMAX=(RT/4) * (VT/RT-INL)^2
Try this website for a pretty useful commentary.
http://w
RE: Rewinding PM motor to change voltage
"So the diameter would go up by about the square root of two. For instance, if the wire diameter is 1mm it would be 1.414mm for double the turns."
The diameter would of course be reduced. In this case if the wire diameter were 1.414mm it would be 1mm.
Monday half and double trouble.
RE: Rewinding PM motor to change voltage
RE: Rewinding PM motor to change voltage
I think you have it backwards. Bigjohn is requesting that his no-load speed at 24V be cut in half. This means that he wants to cut his back-EMF constant Ke in half. The torque constant Kt of a motor must always vary with Ke, so this must be cut in half as well.
The torque generated is proportional to the armature magnetic flux, and this flux is equal to the magnetomotive force (MMF) divided by the reluctance of the magnetic circuit. The reluctance does not vary with winding (barring magnetic saturation), so flux and torque are proportional to the MMF.
The MMF generated by the coils is just n*I, the number of turns multiplied by the current. This means that Kt is proportional to the turns count n, and Bigjohn should cut his turns count in half.
If he does this with the same diameter wire, reducing the size of his winding, he cuts his resistance in half. In this case his mechanical time constant Tm [=(J*R)/(Kt*Ke)] is doubled (assuming no significant J change).
If he does this with wire of double the cross-sectional area, keeping is overall winding size the same, he reduces his resistance by a factor of 4, so his mechanical time constant stays the same.
Curt Wilson
Delta Tau Data Systems
RE: Rewinding PM motor to change voltage
Never mind! (The perils of trying to post while doing other things...) Of course, for the back EMF voltage to reach the supply voltage at half the speed, the back EMF constant must double, and therefore so must the torque constant, and therefore so must the turns count.
With the same diameter wire (assuming there is room, as Bigjohn implies), the mechanical time constant is cut in half.
With wire of half the cross-sectional area to keep the same "fill", the mechanical time constant stays the same.
Sorry about that!
Curt Wilson
Delta Tau Data Systems
RE: Rewinding PM motor to change voltage
At first I thought you'd lost your mind.
RE: Rewinding PM motor to change voltage
I would just try it. If the motor does not have enough torque find a different motor, one of those 90 volt tread mill motors you find at surplus places may even work, I know they would if you jumped up to 48 volts.
Unless you just want to do the rewind for the fun of it, but the wire is going to cost as much as a surplus motor.
RE: Rewinding PM motor to change voltage
RE: Rewinding PM motor to change voltage