Hi guys,
I have a 0.5HP 230V single phase motor that I want to use to make an electric hacksaw. I'd like to vary the speed of it if possible (in additon to conventional gearing) and as simply as possible. Could I get away with using a light dimmer for this or is the inductive nature of the load likely to cause an early "phut!"? If so, what's a workable alternative?
cheers,
cd
It should work.
Try and see - at the worst you will destroy the dimmer triac.
Ummmm - if it's an induction motor, which you would be able to see by the p late on the motor showing an 'rpm', you could destroy the controller and po ssibly the motor. Induction motors are mains synchronous and don't like 'd ropping out' of that with overload or low voltage.
Is that a definitive method of identifying such a motor, then? A fixed RPM shown on the plate?
No commutator.
And a capacitor starter. brushed motors are in general NOT induction motors.
So is it the case that to know for sure, you have to take the damn thing to bits?? That seems daft. They really should put something on the plate to save aggro and for the avoidance of doubt!
A universal motor - the type that can be easily speed controlled, will have brushes. Assuming no gearbox, the spindle speed will also be much higher than an induction one. Induction motors usually around 2000 rpm - universal 10,000+
Induction motors are usually pretty quiet - universal pretty noisy.
Well, in the mean time I've found some sellers on ebay advertising mains motor speed controllers up to around 2kw for only around 5 quid inc. so couldn't resist ordering one. If the motor turns out to be unsuitable when I've had a better chance to examine it, I'll just swap it for a better one. Can't beat 5 quid!
I'm sure I've seen drill speed controls being sold, and I'd imagine they work as you require. Most dimmers seem unable to manage the inductive loads, probably the reverse current on parts of the waveform. Seem to recall drill speed controllers came in two types, what I call brute force, ie they were protected against the phase issues of inductive loads by being just plain big, and zero switching types, which to me at least nearly always seemed to puldse the motor erratically at slow speeds, but much water has passed under many bridges since then, so hopefully the issue is solved now.
Brian
No induction motors have no brushes! I'm assuming brushed motors here. Induction motors need to be driven with some sort of variable AC frequency, not something trivial to build. Brian
In message , Brian Gaff writes
I think you can get away with a variable voltage speed control if the load torque increases more than that available from an increase in supply voltage.
Hence fan speed control where the fan load is a square law.
Can have slip rings in some types.
Good point, I'd guess.
I've had a chance to look at the motor plate now and it seems a bit contradictory. On the one hand it states a speed of 2850rpm which implies from what someone wrote above that it's induction type, yet it also states in big letters that it's an asynchronous motor, which I'm guessing implies it isn't. Have to confess to a certain amount of confusion as a result. Can anyone clarify if this is an induction type or not?
A lathe I used to have, which had stepless speed control, used a 5HP DC motor to achieve it.
I'm much more comfortable with DC motor speed control, but am guessing that a DC motor of any decent power (from a couple of HP up) is going to be considerably more expensive than its AC counterpart?
No idea. It came with the CNC lathe :-)
Probably not as they're smaller and lighter. The main difference in use is an induction motor is fine for running more or less continuously. A universal one (as used on mains drills, routers, vacuum cleaners etc) is more designed for short periods of use, as it gets hot in service.
Three types.
Universal. DC or AC. Brushes and commutator and for AC wound field and rotors.
Induction. AC only and generally has wound rotor and stator and my have slip rings but no commutator. On single phase generally has a starter capacitor.
Synchronous. Can have PM rotor or stator (unwound) and will operate at mains frequency.
Not really at all.
In fact a universal motor (brushes and commmutator) is called that because it works on AC and DC ...
They may be designed to pack a lot of power in a small package and thereby get hot, but that's not intrinsic to the design.
Induction motors are not good at starting torque and wide speed ranges and power ranges. Nor are they compact.
But they are simple and they are cheap.
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