I posted most of this a while back on the British Blades forum when a similar question came up:
" Quick explanation of 2 poles, 4 poles, etc.
UK mains power is AC at 50 Hz (50 cycles per second). This equates to 3000 cycles per minute.
A 2-pole motor has one pair of poles, so turns once every cycle, giving 3000 RPM (less "slip").
A 4-pole motor has 2 pairs of poles, so takes 2 cycles for a full revolution, giving 1500 RPM (less "slip").
A 6-pole motor takes 3 cycles per revolution for 1000 RPM.
North America is on 60 Hz mains, giving 3600, 1800 & 1200 RPM.
Pretty much any motor you buy will be physically capable of running at 3600 RPM because manufacturing economics mean that standardising as much as possible gives the lowest cost.
The bearings, shaft, stator, etc will all be good to 3600 RPM, but in many cases, the electrical characteristics of the motor will limit the usable maximum speed to less than this.
The way a Variable-Frequency Drive works is by taking AC in, rectifying and smoothing it to DC, then using little slices of DC to synthesise a fair approximation of an AC sine-wave output with a frequency and voltage that is independent of the mains input.
The clever bit is that the frequency and voltage are variable.
Below the rated speed of the motor, both voltage and frequency usually vary linearly and the motor operates in "constant-torque" mode.
If we take a motor rated at 2HP, 1450 RPM, 50 Hz, 240V, and run it through a VFD, at 25 Hz, it will be turning at about 725 RPM at a voltage of 120V, and will be able to output 1 HP. The current, and with it the torque, will remain at the rated value, assuming the motor is loaded accordingly. Power is basically Volts x Amps x a constant.
Above the rated speed of the motor, the voltage is unable to increase further and the Volts x Amps is fixed. The motor therefore runs at "constant power", with the available torque falling off as the speed increases.
Usually, motor manufacturers recommend using 4-pole motors with VFDs and give a speed range of 10 Hz to 100 Hz. Of this, 10 Hz to 50 Hz is "constant torque" (about 300 RPM to 1500 RPM) and 50 Hz to 100 Hz is "constant power" (about 1500 to 3000 RPM).
There are "sensorless vector" drives available which employ some fairly fancy electronics and will allow smooth operation at lower speeds than 10 Hz. Offhand, I can't think of a knifemaking application that would need such low speeds, but I'm new here and have much to learn.
Most 3-phase motors are Totally Enclosed Fan Cooled. They use a fan mounted on the shaft to draw cooling air over the motor. As the shaft speed drops, the cooling airflow drops with it and a heavily-loaded motor running at low speed can overheat. Most of the stuff I've seen says you are fine down to 25 Hz for continuous operation. Below that, it is necessary to reduce the motor load, reduce the duty cycle or provide additional cooling. It is possible to use a thermistor to sense the motor winding temperature and cut the power if it overheats. Some drives have the facility to take a thermistor input, otherwise a thermistor relay is needed.
Quite a few of the guys on the various lathe forums use axial fans similar to those found in computers to provide low-speed cooling. There are many variants available, some mains-powered, some DC."
Note: In many cases, the derate for low speeds is a non-issue. For grinders, as an example, the operator tends to work the machine less hard at low speeds and the problem simply doesn't arise.
"As far as the drives themselves go, the best advice I can give is to do your homework and to go for a basic drive."
Note; The KBAC drives are basic drives with the huge advantage of physical protection to NEMA4/IP55. They are not available in the UK, so I didn't mention them in my original BB post.
"The high-end drives have lots of bells and whistles aimed at interfacing with industrial control systems. It's a fairly safe bet that you yourself will be the control system and that you'll interface with a start button, stop button and speed control knob. Basic drives do this very well and save you having to go through complicated menus, switching off the bells and whistles.
If you buy used on e-bay, make absolutely certain you know what you are getting and that it will do the job. Operator panels are a common problem. Many industrial drives come without them, mainly to stop the customer messing with the drive. Without one, you can't programme the drive.
Another thing to watch for is fan drives. Fans have Quadratic torque characteristics. This means that halving the fan speed only takes one eighth of the power. A drive specially designed for fans is utterly useless for anything else.
Because 3-phase motors have no capacitors or centrifugal switches to worry about, they tend to be much more reliable than single-phase motors. Even a cheap industrial motor should be good for 40,000 hours continuous industrial use. VFDs have similar life expectancies."
Note: I compared the outputs of 2 different 2 HP motors; a 2-pole and a 4-pole. I assumed that the frequency range used would be as recommended by the supplier, who told me 10-60 Hz for the 2-pole and 10-100 Hz for the 2-pole.
It's worth doing the math on both 50 Hz and 60 Hz ratings if your motor is rated for both; quite often, the power curve is "better" when based on the 50 Hz rating.
"Driven by a basic "V/Hz" VFD, the 2-pole motor will be able to run between 600 and 3600 RPM.
By contrast, the 4-pole will be able to run between 300 and 3000 RPM.
Taking performance at specific speeds:
At 300 RPM, the 4-pole will produce 0.4 HP at a frequency of 10 Hz.
At 600 RPM, the 2-pole will produce 0.4 HP at a frequency of 10 Hz
At 600 RPM, the 4-pole will produce 0.8 HP at a frequency of 20 Hz
At 1000 RPM, the 2-pole will produce 0.66 HP at a frequency of 16.6 Hz
At 1000 RPM, the 4-pole will produce 1.33 HP at a frequency of 33.3 Hz
At 1500 RPM, the 2-pole will produce 1.0 HP at a frequency of 25 Hz
At 1500 RPM, the 4-pole will produce 2.0 HP at a frequency of 50 Hz
At 2000 RPM, the 2-pole will produce 1.33 HP at a frequency of 33.3 Hz
At 2000 RPM, the 4-pole will produce 2.0 HP at a frequency of 66.6 Hz
At 2500 RPM, the 2-pole will produce 1.66 HP at a frequency of 41.6 Hz
At 2500 RPM, the 4-pole will produce 2.0 HP at a frequency of 83.3 Hz
At 3000 RPM, the 2-pole will produce 2.0 HP at a frequency of 50 Hz
At 3000 RPM, the 4-pole will produce 2.0 HP at a frequency of 100 Hz
at 3600 RPM, the 2-pole will produce 2.0 HP at a frequency of 60 Hz
Over the range of speeds common to both motors, the 4-pole motor is ahead on power everywhere below 3000 RPM.
At the low speeds, up to 1500 RPM, the 4-pole produces double the power of the 2-pole. Both motors are in their "constant torque" range.
Between 1500 RPM and 3000 RPM, the 4-pole is in its "constant power" range, but the 2-pole is still on "constant torque". The advantage of the 4-pole tails off progressively from double the power at 1500 RPM to equal power at 3000 RPM, but it is still ahead all the way.
I hope it's reasonably clear.
In many cases, it will be possible to extend the working speed ranges beyond those I've indicated, but the general principles still hold good."
Note: I have run my lathe down below 10 Hz using a "Sensorless Vector" drive. Using a basic V/Hz drive, the motor felt "coggy" below about 10 Hz, but on the SV drive, it was smooth at 3 Hz. I didn't try any slower.
I've also run a (4-pole) grinder to 120 Hz. Although it was entirely subjective, there seemed to be a lot of reduction in available torque between 100 Hz and 120 Hz; more than the 20% that would be expected from "constant power".
Tail-covering note: Here in Europe, we tend to use motors built to the IEC (metric) standards. In North America, most motors are to NEMA standards. I have no first-hand experience of NEMA motors, but have seen a number of references to 56-frame motors for "our" applications. I understand the 56-frame was originally specified for single-phase, fractional HP motors, so it may be that the degree of redesign required to get the sorts of outputs currently being obtained from 56-frame motors, means that it is no longer practicable to design them for multiple speeds; please check with your manufacturer/supplier if in any doubt.
Regards
Tim
. Good explanation Tim. I'm saving that.