I am using a Porter Cable 7310 laminate trimmer as a drill motor on a home made cnc machine. The motor runs at 30,000 rpm on 120VAC. Can I alter rpm to say 15,000 rpm by reducing the voltage to 60VAC? Is there a better way? Thanks

Hello Ed
Welcome to the forum.
Your post suggests that you have an induction motor operating at 30,000RPM. This sounds a little fast!! If it is an induction motor directly connected to the supply, then I would expect that the speed is 3000 (2 pole motor).

Reducing the voltage applied to the motor will eventually cause it to slow down, but unless it is a high slip motor, it would cause the motor to fail. An induction motor is a pseudosynchronous device, that is it wants to operate at the frequency of the supply. Reducing the voltage will reduce the torque and eventually it will stall and operate at the speed where the torque developed by the motor equals the torque required by the load.
If you used a high slip motor and reduced the voltage, the actual speed would vary both with load and voltage. Half voltage and no load would result in full speed operation.

I expect that you will want to have good control over the speed, and in this case you would need to control the frequency aplied to the motor. Unfortunately, to control the speed of an induction motor, you need to use a poly phase induction motor (i.e. a three phase motor) and an inverter. It is not very practical to control the speed of a single phase motor due to the start requirements.

Another option is to use a DC motor and controller, but I would expect that the three phase option would be cheaper.

Best regards,

This sounds like an AC spindle motor, which is common for CNC machines. These motors are a two pole induction machine, which are rated for high frequencies at the rated voltage. They must be used in conjuction with an inverter (VFD) to obtain the correct V/Hz ratio. Is there a VFD connected?
If not, the motor may be a permanent magnet servo motor. Again, if this is the case there will be an amplifier connected to the motor.

Hello theDOG

Good points. In those scenarios, the speed cannot be changed by simply altering the voltage. I guess we need more information to comment further!

;p;

Hi,

It looks like it's a simple series machine, so it can operate at high speeds. Operating series machines at lower voltages is possible but speed will depend largely on the load on the shaft.

;)

sorry for interrupting,
what is a series machine?

Hello weiwei2

Welcome to the forum

A series machine generally refers to a DC machine where the armature and field windings are connected in series. The other DC machine configuration is the shunt machine where the armature and field windings are connected in parallel.

Universal machines are brush motors that will run on A.C. but are not synchronous. These can also be series machines.

Best regards, ;d;

Most major manufacturers of routers and the like make a speed control unit for their products, that would likely be the simplest route.

Hi, go to www.mlcswoodworking.com, search "speed controls ", there you find two models, standard and heavy duty. You can go to www.siliconchip.com.au too, look the issue 169 " improved speed controllers " published 1 otc. 2002. good luck