Quote Originally Posted by malb View Post
Hi CD.
Mains motors fall into 2 broad categories.

Universal (brushed) motors as in portable drills, circ saws, routers, vacuum cleaners, etc and sewing machines. The speed is dependant on the effective applied voltage and independant of supply frequency. Speed can be varied by "chopping" the mains supply so a user determined portion is supplied, or by reducing the voltage with a variable transformer or high power variable resistance system. Old style sewing machine motors with foot or knee speed control had a chain of carbon buttons wired in series with the motor. The foot pedal or knee control compresses the buttons reducing the resistance of the chain, increasing the applied voltage and motor speed. In the fully released position, the buttons seperate, breaking the circuit and switching the motor off.

Induction motors, as found in many lathes, mills, drill presses and general industrial machinery. These are synchronous motors (speed is dependant on input frequency) and mostly independant of voltage. These are your 3 phase motors and the bulk of the 1HP+ single phase motors,the exception generally being high speed motors. 50Hz supply motors needing to operate above 3000RPM will be some form of universal motor as it is impossible to get a 50Hz supplied induction motor to run above 3000RPM, but 1500 and 1000 RPM variants are also possible by adding more magnetic poles in the windings. The motors will run slightly under these speeds,or slip. The degree of slip will vary with the load (torque) required of the motor. Typically motors operate at rated power with about 4-5% slip giving nominal speeds of around 2850, 1440, or 960 RPM.

Three phase motors will determine the direction of rotation from the sequence of the three phases in the supply and will self start. This makes it possible to employ a variable frequency drive which takes a single or 3 phase supply, converts it to a DC voltage, then converts it to a variable frequency three phase at the voltage similar to the supply voltage. With a well matched combination of drive and motor, it's practical to create a variable speed drive with a wide speed range extending to at least 22,000RPM, as used in CNC routing spindles.

A basic single phase induction motor cannot establish it's operating direction naturally and will rock a few degrees either way and hum on application of voltage but not rotate. To make them usefull, a second 'start' winding is added, usually with a speed sensitive switch a starting capacitor. This produces a temporary second pole in the motor giving it a kick start in the required direction and supplying additional starting torque. Once the motor gets to about 80% of rated speed, the speed sensitive switch switches the start winding and capacitor out of circuit and the motor continues to run in the same direction and gets to its nominal speed. If the motor is slowed below the switching speed, the switch will re-energise the starting winding and capacitor. However the start winding is designed for very short term use, and will overheat and burn out within seconds if kept energised. This is what limits the application of variable speed drive electronics to single phase induction motors, once the motor slows, the speed sensitive switch re-engages the start windings and burns them.

Your lathe upgrade is possible under certain conditions. You need to use a 3 phase motor, and unless you have 3 phase power available, the motor, needs to be capable of operating on 240V, needs to be modified to make it work on 240V (accessing internal winding connections, not always easy/possible), or you need to import a specific VFD unit from UK that takes 240V single phase input and outputs 415V three phase variable frequency. If you have a three phase supply available, then any 3 phase 415V motor and compatible 415V VFD will do the job.

Hope this helps helps you understand the factors involved.
thank you MALB, yep it dose help CD