I'm converting a 1hp 115/230vac 3450rpm motor to a permanent magnet generator. If I remember how to math this correctly that means this motor is a two pole motor. I want to set the rotor up with alternating magnets every 45 degrees...essentially an 8-pole rotor. Would that work?
Information about how an asynchronous motor can be changed into a PM-generator is given in my public report KD 341 which can be copied for free from my website:
www.kdwindturbines.nl at the menu KD-reports.
If you want to use the original 3-phase motor winding, the pole number of the PM-armature must be identical to the pole number of the motor. So then you can only make a 2-pole armature. But if you provide the stator with a new 8-pole winding, it is possible to use the stator of a 2-pole motor for an 8-pole armature. You only get a simple 2-layers winding if the number of stator grooves is a factor three or six times the number of armature poles. So for an 8-pole armature, the stator should have 24 or 48 grooves. If the stator has 36 grooves, a winding is possible but then you will get double coils in the first layer and single coils in the second layer. A 2-layers winding is also explained in KD 341.
For most stator stampings of a certain frame size, the inside diameter of the stator increases with the pole number of the motor. The outside diameter of the stator is the same for all pole numbers of a certain frame size. The reason is that for a 2-pole motor you have only two big magnetic loops coming out of the armature and to prevent saturation of the stator stamping at the bridge in between the bottom of a stator groove and the outside of the stator, this bridge must be much wider for a 2-pole motor than for an 8-pole motor. So if you use the stator of a 2 pole motor for an 8-pole generator you have a bridge which is much too wide and so an inner diameter of the stator which is too small. This means that your armature volume is much smaller than for an 8-pole stator stamping of the same frame size. The maximum generator torque is proportional to the armature volume and so you can better use the stator stamping of an 8-pole motor if you want to make an 8-pole PM-generator. But if you accept the lower maximum torque level, it is possible to use the stator stamping of a 2-pole motor.
I use only 4-pole motors for my generators made from asynchronous motors. If you use neodymium magnets, a 4-pole armature can be made strong enough. Another advantage is that the original winding can be used for 24 V or 48 V battery charging depending on how the winding is rectified and on the diameter and design tip speed ratio of the rotor. If you use an 8-pole motor with the original winding, the voltage at a certain rotational speed will be about a factor two higher than for a 4-pole motor with the original motor winding.
To prevent a strong fluctuation of the sticking torque, the armature grooves in which the magnets are glued must have an angle such that there is just one stator pitch overlap in between the left and the right side of the armature. Use of stator stampings with four outside grooves in which small strips are laid to bind the stampings together, give four strong preference positions for a 4-pole armature but these stampings might also give eight less strong preference positions for an 8-pole armature.
