I want to speed control my lathe at home. I am not a wizz in power electronics. Other than gating a SCR or something like that is there a more intelligent way to control the speed of an AC motor, or would it be better to rather look for a DC replacement?
AC motor control is alittle coplicated but can be discussed in 3 steps:
1.rectify and filter input AC voltage by a bridge and a capacitor.
2.use a full bridge cunstracted with IGBTs and use opto-coupler/driver or bootstrap driver for driving top switches (they require floating ground)
3.controll the speed by PWM generator driving the full bridge
refer to applicatin notes available @ www.irf.com for more infos!
BEST!
If you have a 1-phase AC motor and use feedback from the axle it can be simple if it’s the right kind of motor. If it is a 3-phase motor it’s a lot more complicated. What kind of power (Watts) are we talking about?
I have tried commercial drives for synchronous single-phase motors, i.e., not AC/DC universal motors. You will lose torque and the range of useful control will probably be too small. As a result, I switched to DC motors with AC-powered drives and three-phase motors with variable frequency drives (VFD). 3-phase VFD's running from single-phase are not very expensive and make 3-phase quite practical for home use.
It happens that I used DC for my lathe and VFD for a drill press, molder/shaper, and scroll saw. Both methods give a range of control from a walk to full speed with good torques across the full range. The main difference that I have seen is that VFD's take awhile to get going (like a second or two). DC is almost instantaneous. So, if I were to recommend anything it would be to use DC on the lathe because I know that works well. That is not to say in the least that a VFD with 3-phase wouldn't work as well. I just haven't tried it.
I have used KB Drives (KB Electronics AC Motor, DC Motor Drives & Fan Speed Controls), Hitachi, and another brand that was on sale. **broken link removed**, if you are in the US, has good prices on those brands. The DC drive for my lathe is KB and was only about $125 USD new.
John
Edit: I forget to add that DC motors also provide easy braking. The lathe mentioned above is a Prazi SD300 and does not have half-nuts for threading. The instant stop comes in very convenient when threading on that machine.