For a grinder, you are fine with a Totally Enclosed Fan Cooled motor, but Totally Enclosed Non-Ventilated motors are able to run at full torque at lower speeds without overheating, because they are not reliant on a fan for cooling. As a TEFC motor slows down, so does the cooling fan on the motor shaft, reducing the cooling airflow. If you find a TENV motor, don't dismiss it for not being TEFC.
It's the "Totally Enclosed" that's the must-have for a grinder.
I'm in the UK, where mains power is 230V 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, less "slip".
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 most cases, the electrical characteristics of the motor will limit the usable maximum speed to less than this on anything but a 2-pole motor.
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). If the motor has ratings for both 50 Hz and 60 Hz, it's worth doing the sums to see which set of values gives the most area under the curve and using that one. I've tended to find the 50Hz values are usually, but not always, the ones to use.
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 often wrong. There used to be a premium for SV 2 or 3 years ago, but I don't really see it now.
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 on a 4-pole motor for continuous operation. Below that, it is necessary to reduce the motor load, reduce the duty cycle or provide additional cooling. In many cases all that's needed is for the user to be aware of the potential problem and have a bit of mechanical sympathy. If needed, 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.
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.
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.
