PDA

View Full Version : Stupid question time, electric motor

steverice
06-24-2009, 11:12 PM
What I wanna do is... get an electric motor and be able to control how fast it is turning with a knob or a foot pedal.

Would I be able to use a regular 120v. electric motor with a dimmer switch?

Will I need somthing more better. I am looking at trying to build a rotating table for welding purposes.

Or maybe a wheel on a shaft that I can rotate with my foot, kinda like an English Wheel.

s.r.

airsmith282
06-24-2009, 11:30 PM
dimmer switches do work to a certian point just watch the wattage the motor sucks might be to much for the switch to handel..

id look on the net for plans for a DIY motor control ,, pretty easy to build them,,

jb-mck
06-24-2009, 11:34 PM
AC motors turn at fixed RPM's, depending on the number of poles. It is a function of frequency (60 hz). DC motors are not fixed. You need either DC motor or AC motor with VFD.

jb-mck
06-24-2009, 11:42 PM
RPM= (120 x freq ) / # of poles

doctor demo
06-25-2009, 12:49 AM
AC motors turn at fixed RPM's, depending on the number of poles. It is a function of frequency (60 hz). DC motors are not fixed. You need either DC motor or AC motor with VFD.
Could You expand on that, as I have a whole house fan with an a.c. capacitor start motor that is variable speed and has what appears to have a dimmer switch . Could the dimmer switch actualy be some sort of V.F.D. ?
Steve

darryl
06-25-2009, 01:38 AM
Usually the fan motor speed controls are 'open loop', which means that you make an adjustment and the motor comes to a speed based on how much resistance the motor sees to turning the fan. They aren't speed controls in the sense that you can set the speed and the motor turns at that speed regardless of the load. A real speed control is going to sense how fast the motor is turning and vary the power to the motor to maintain the speed regardless of the load. This is the type of control that you would want for a lathe for example.

I'm not up on vfd's to any great extent, but it seems logical that they would give actual speed control because the motor is turning at a speed based on the frequency of the current pulses driving it. You might have a problem with a single phase motor driving at much slower speeds than normal, so if you need a wide range of speeds you might consider a 3 phase motor or a dc motor with the proper control circuitry. It's this latter thing which is of most interest to me since I have several treadmill type motors, some of which I have in use.

In any event, if you need high torque at low speeds, it's usually best to involve some kind of mechanical speed reduction, like gears or belts and pulleys. If you go direct drive and want low speeds, the motor will have to be oversized so it can deliver sufficient torque.

There's a lot of tradeoffs. A dc motor can be made to be powerful and efficient by running it at higher speeds than you can normally get from a single phase motor. Such motors are often fairly small, so you have to pay attention to heat rise with prolonged operation at higher speeds. Same goes for low speeds where you are asking the motor for significant torque output. If the motor has its own cooling fan, it will be deprived of effective cooling at low rpms. It may also draw a heavy amperage at lower voltages, so the power supply, or what you might call the motor speed control, has to be capable of safely driving the motor under all expected operating conditions without itself overheating.

PeteM
06-25-2009, 01:47 AM
Your welding table will need to run slow and have significant torque.

A DC motor with a DC speed control is one option. Might find one from something like a scrapped treadmill. You may have to gear it down a bit.

A 3 phase AC motor with a VFD is another option, but it will need to be geared down to spin the table near welding speed. Count on the VFD for maybe 1/2 to 2x rated speed. You can likely press it more if it's a modern motor, but put your sweet spot in that range.

A universal motor (with brushes) can run on either DC or AC. Someting like a beat up Milwaukee Hole Hawg (big drill) with a cheap speed control is another option. This is the only common "AC" motor that can be controlled with a dimmer switch. Better control might come from a cheap imported router speed control.

You'll also want to start thinking about what you're going to use for a slip joint for the ground lead.

Paul Alciatore
06-25-2009, 03:06 AM
You can get cheap variable speed drills in almost any home supply place: one piece motor and speed control. Add a good gear-down, not literally with gears, but perhaps a belt drive and you should have it.

J Tiers
06-25-2009, 09:55 AM
Could You expand on that, as I have a whole house fan with an a.c. capacitor start motor that is variable speed and has what appears to have a dimmer switch . Could the dimmer switch actualy be some sort of V.F.D. ?
Steve

You may have a "PSC" motor, with a single start/run capacitor. Yes, they can be dimmer controlled even though they are single phase.

Very common in HVAC systems, usually for the fan, since they are really "high slip variable torque" motors...... not really 'variable speed". The speed is load dependent, and a fan is perfect for them

Those, and shaded pole motors, are virtually the only AC motors that can be dimmer controlled (I don't count "universal" motors as AC, since they run on DC also).

otherwise you generally use a VFD with a 3 phase motor, which actually adjusts the AC frequency fed to the motor, directly controlling the motor speed at any load.

There ARE VFDs made to control PSC motors, but they are generally rare. Invertek makes them, for one.

The OP might use a DC motor and controller, or a VFD and motor, etc, depending on power level, cost requirements, etc.

johnnyd
06-25-2009, 11:46 AM
Why not just rig up something along the lines of a glorified "potters wheel"?

johnnyd
06-25-2009, 11:47 AM
Oop's....duplicate post

gary350
06-25-2009, 01:28 PM
If you buy a eddie current drive or variable frequency drive you can do it they cost about \$900.