Generally speaking, motor speed is controlled by the voltage and the torque is controlled by the current. If you lower the voltage, power output drops, but that's because the speed drops. As long as you haven't saturated the transformer, the current should remain more-or-less constant for a given load. Speed will drop off a bit with load on a real motor due to resistive losses in the windings.
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So is a 240v Variac considered a tool as well?
- Thread starter silverforgestudio
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My 2 cents worth, for what that's worth:
The "variac" is an "auto-transformer". You may use a lower voltage than rated, however you may NOT USE A HIGHER voltage.(at 60 Hz AC) I doubt you would ever face that problem, with exceptions of course. The bottom line: you can safely connect to 120 VAC, just remember that current (amps, I, KVA) does not double. It is the limiting factor. If connected to 120 volts, the output would be ~140 volts max at rated current. That would give you a safe output of 0-100%=0-120volts for use with a grinder or other "sewing machine motor". I HAVE seen an induction motor used with a lamp dimmer as a speed control, but it was an industrial application and only "sorta-kinda" worked. OK for the application but not recommended.
Your clutch problem is a whole different issue. The determining factor there involves the manufacturer's spec sheet. I can only say for sure that DC would far and away do a better job than AC. A good strong DC field would be much more desirable for a clutch. (chuck) Use a full wave bridge and a good strong capacitor. A 1000MFD cap will suffice for 1 amp at power line frequency. With AC, you would get "zero crossing" at 120 times per second. Beyond that, you get into electrical theory serious and I don't have the 10 years or so to make you into an electrical engineer.
Oh, and by the way, when my name gets posted with this entry; you can add BSEE1974 in your mind. I posted an artical a few years back on the subject. But as a newbie I don't think the moderator will allow a link. Maybe later...
Try www(dot)hudsontelcom(dot)com and home shop electrics for a general overview of the subject.
The "variac" is an "auto-transformer". You may use a lower voltage than rated, however you may NOT USE A HIGHER voltage.(at 60 Hz AC) I doubt you would ever face that problem, with exceptions of course. The bottom line: you can safely connect to 120 VAC, just remember that current (amps, I, KVA) does not double. It is the limiting factor. If connected to 120 volts, the output would be ~140 volts max at rated current. That would give you a safe output of 0-100%=0-120volts for use with a grinder or other "sewing machine motor". I HAVE seen an induction motor used with a lamp dimmer as a speed control, but it was an industrial application and only "sorta-kinda" worked. OK for the application but not recommended.
Your clutch problem is a whole different issue. The determining factor there involves the manufacturer's spec sheet. I can only say for sure that DC would far and away do a better job than AC. A good strong DC field would be much more desirable for a clutch. (chuck) Use a full wave bridge and a good strong capacitor. A 1000MFD cap will suffice for 1 amp at power line frequency. With AC, you would get "zero crossing" at 120 times per second. Beyond that, you get into electrical theory serious and I don't have the 10 years or so to make you into an electrical engineer.
Oh, and by the way, when my name gets posted with this entry; you can add BSEE1974 in your mind. I posted an artical a few years back on the subject. But as a newbie I don't think the moderator will allow a link. Maybe later...
Try www(dot)hudsontelcom(dot)com and home shop electrics for a general overview of the subject.
Generally speaking, a variac makes a very poor speed control. The torque developed by an ac motor is proportional to the square of the voltage. So, if you reduce the voltage by 20%, the motor will only produce 64% of rated full voltage torque. In addition, running ac motors act like a constant volt-ampere load, so a reduction in voltage will draw additional current proportionally. The best way to kill a motor is to feed it with lower than rated voltage.
No. AC motor torque is a function of voltage, actually the square of the voltage, not the current. That's why voltage reduction, regardless of how it is reduced, is such a poor way to control speed. See my previous post in response to f350ca! In addition, when you reduce the voltage to an ac motor, the motor will try to maintain a constant volt-ampere, but the power factor will decrease, motor curre The speed of the motor
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Good to know! I'm more-or-less a "seat of the pants" electricker myself, and try not to post info I'm not sure of. But I do try to be of help. Glad to have you here. I'm sure you'll be valued by many of us!
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Thank you! I am WAAAAY under the learning curve now- and I will hopefully be pulling the books out way before using this thing. good in fo and I appreciate the thoughts and advice to ALL that have posted- THANKS GUYS
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Routers and die grinders use universal motors that will operate on AC or DC. The motor speed is directly proportional to the applied voltage.
Greg
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Just be sure to account for loss of torque as you account for loss of speed. Torque will fall off dramaticly as speed falls off. That comment is for series universal motors, such as drills, routers, die grinders, sewing machines, and the like. Induction motors like on a drill press, lathe, mill, etc. are a whole different subject. As said, if you try to run an indiction motor on a variac, it will try to maintain speed and get very hot in the process.
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Thank you Bill! Good to know-
AC motors have problematic characteristics, in particular they're locked to line frequency and you can't change the speed without changing the frequency, but torque is still directly proportional to current. That's basic physics.
If you start varying the voltage into an AC motor, you'll get reduction in current, and hence torque, but that's a fairly limited behavior because the back EMF is proportional to speed, and speed in turn is locked to line frequency. Once you start dropping the voltage below that level, the motor quickly stalls out.
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