Re: Induction motor design for PWM
- From: pentagrid@xxxxxxxxx
- Date: Fri, 01 Jul 2005 12:30:49 +0100
On Fri, 1 Jul 2005 03:36:36 -0400, "Paul E. Schoen" <pstech@xxxxxxxxx>
wrote:
>My project is to make a three phase AC induction motor that will be powered
>from batteries by means of a PWM sine wave inverter. I have successfully
>rewound a 120 VAC 1/2 HP single phase capacitor start motor into a three
>phase, four pole motor that operates on about 6 VAC at 60 Hz. I also made a
>simple controller with a Microchip PIC and six MOSFETs that were able to
>drive the motor from a 12 VDC battery. I have several questions and invite
>comments:
>
>(1) I want to wind a motor with 6 poles on a 36 hole stator, which should
>run at just under 1200 RPM at 60 Hz. Using the PWM control, I want to
>increase the frequency to 180 Hz, for 3600 RPM.
* OK
>
>(2) For the same size motor, I would expect torque to be proportional to
>number of poles, so twice the poles and half the speed would be the same
>horsepower. Ideally, I would like to make a motor with 12 poles, for 600
>RPM, and run it at 360 Hz for 3600 RPM. I should get six times the original
>HP. However, motors with more than 4 poles seem to be much larger and
>heavier than two pole motors of the same HP. Can anyone explain this?
*Not true
Torque is limited by maximum permissible flux density. For the
same flux density, in a given frame size, HP is directly proportional
to full load speed. This is slightly modified by various second order
effects this but it is the basic relation.
Doubling the number of poles will make little change to maximum
torque so half the full load speed means roughly half the horsepower.
>(3) How much effect does the number of slots on the rotor vs that of the
>stator have on performance, and what about rotor skew? I was concerned about
>this, especially when I rewound a single phase motor for three phase, but it
>seems to run OK. However, I have not tested it for torque, HP, or
>efficiency.
* If the back EMF generated within a motor is not a perfect sine wave
the waveform difference results in circulating currents at harmonic
frequencies. These do not produce useful torque so the efficiency
drops.
This suggests the use of a sinusoidal winding distribution in an
arbitrary large number of slots to achieve low flux ripple. In
practice the number of slots in standard stator and rotor laminations
is already chosen to keep these losses acceptably low. For minimum
ripple the number of rotor slots should not be an integral multiple of
the stator slot number and the rotor or the stator should be skewed by
at least one slot pitch.
This is not a critical variable and even large differences will
still result in a usable motor. A motor designed to run from a sine
wave supply will still run (but a bit hotter!) from a square wave
source
>
>(4) I am now rewinding a motor that was originally a 120 VAC capacitor run
>motor at 1120 RPM, for three phase. It has a 36 slot stator and a 48 slot
>rotor, with a skew of about 150%. By this I mean that the rotor pole piece
>at one end is skewed 1.5 slots on the stator. Could there be any problems
>with this rewind?
*No problem
>
>(5) I have found various motor design software packages, but they are
>several thousand dollars, and I don't know if they will work for my special
>design
* Patient experiment can work wonders!.
Jim
.
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