Copy and Paste from Northridge grinders of which I totally agree.
Electric motors
"The motors we supply are 2 horse power 1725/1800 rpm three phase 56c face mount 4 pole electric motors, and use a variable frequency drives (VFD) to convert from the 110v or 220v single phase power of a normal house or shop wiring to the 3 phase power the motor requires. At the same time the VFD gives smooth and accurate variable speed control. We chose the 4 pole motors despite there lower rpm because we can double their speed by increasing the hertz from 60 to 120 with the VFD and maintain full torque up to the original 1800 rpm of the motor with a slow drop off as it increases to 3600 rpm. But as you increase speed you gain inertia in the form of rotational momentum to help offset the loss of torque. Compared to a 2 pole 3600 rpm motor where you would lose torque throughout the entire rpm range rather than just half of it.
2 pole vs 4 pole
The standard AC motors that everyone is familiar with have 2 poles, that is two electromagnets on opposite sides of the motor. At any single point in time while the motor is running, one pole is north and the other is south. By alternating the poles back and forth, the changing magnetic field causes the motor to spin. The fields are switched twice per rotation, always trying to repel the close side of the motors armature while at the same time attracting the opposite side, in a constant game of tug-a-war. A four pole motor has twice as many magnetic poles, one every 90° instead of 180°. The same basic principles apply but instead of switching twice per rotation, the magnetic field is switched 4 times. This is like a game of tug-a-war with four ends to the rope instead of just two. Just like a 4 cylinder engine vs a V-8, the more poles a motor has the more torque it produces.
If 4 pole motors are so great, why dont we use them everywhere? Well thats because standard household current is only single phase, which means its only able to directly operate a 2 pole motor. To operate a 4 pole motor you need industrial 3 phase power which is not available to most people, which is why most consumer motors only have 2 poles. But there is a loophole that allows you to run a motor with 4 or more poles on single phase, and that is the Variable Frequency Drive. Through sophisticated electronics, a VFD can take single phase 120v or 220v current and convert it to the three phase power required to run a 4 pole motor."
A 3600 rpm motor is always cheaper and there's a reason for it!
