variable speed???

[turmite]

Ok folks I have a ? and hope this is the place to post it.

I want to build a non-cnc sanding/buffing machine and would like to be able to control the speed of a 1.5 to 3 hp ac motor. I would like the motor to be 220v single ph with max rpm of 1750 but I would like to be able to reduce the speed down to about 600 rpm without loss of torque or damage to the motor. Where do I begin to start looking and what do i look for?
Mike

[Urgundiz]

If 1hp~746W we are talking about 1150 to 2300W.. There are variable speed boxes for drills and similars, but do not know if they support that power.

If you do not find it, maybe a lamp dimmer with support for the Watts your motor eats could work, but again do not know if they exists for that power.

If you do not find it you will need any AC voltage regulator with a triac able to support at least 2300W/220V=10.45A. The SM12J45 triac for example is able to drive 12A of AC.
hope it helps
/U

[Urgundiz]

Um, I've done a little google browsing and I've found these little collection:

http://www.epanorama.net/documents/l...ghtdimmer.html

http://us.st.com/stonline/books/pdf/docs/3566.pdf (look at fig. 11)

At least will provide some ideas..

hope it helps

/U

[HuFlungDung]

Mike,

Your best bet is to go with a 3 phase motor. Whoa, you don't need a 3 phase panel to get some juice . Purchase a variable frequency AC drive. Many of these will allow you to input from a single phase source, yet will produce 3 phase out. However, you do need to buy an oversized drive to accomplish this. In your case a 5hp AC Variable Frequency Drive.

Naturally, the VFD takes care of your variable speed requirement. And, they are constant torque up to the full nameplate rpm of your motor, after which the torque declines, because the current must be reduced to prevent burning out the windings as the speed increases. In other words, your motor maxes out at rated hp no matter how fast you dare to run it.

You should take care to run the motor at a decent speed to assist in fan cooling. This might mean using a belt drive to put the motor in the 1000 to 3600 rpm range, if it is working hard. Appropriate pulleys will then get your spindle into its desired range. You might be able to reduce the motor size (and VFD size rating) if you do this. This is the reason that a lot of CNC machines have huge motors on them: so they will have enough power to do heavy work at the lower speed range. There is a lot to be said for mechanical gear or belt ranges.

[balsaman]

Originally posted by Urgundiz If 1hp~746W we are talking about 1150 to 2300W.. There are variable speed boxes for drills and similars, but do not know if they support that power. If you do not find it, maybe a lamp dimmer with support for the Watts your motor eats could work, but again do not know if they exists for that power. If you do not find it you will need any AC voltage regulator with a triac able to support at least 2300W/220V=10.45A. The SM12J45 triac for example is able to drive 12A of AC. hope it helps /U

Varying the voltage does not vary the speed of an ac motor (unless it's a "universal motor", such as a router or drill). You need to vary the line frequency. See Hu's post above. The problem is VFD's are not cheap.

Eric

[HuFlungDung]

Yes, there's a good project for the do it yourselfer with the knowhow: design a DIY VFD drive. I don't really know why these things command the price that they do. They don't seem to be particularly smart devices.

[turmite]

Thanks U HU Ba! Wow that sounds like a great screen name !!

Hu I have a 8 hp ph converter. Would that lessen the cost any?

Balsaman when you say universal what exactly do you mean? Are you talking a std x-frame electric motor or something else like an ac servo motor?

/U I will check the links and thanks for the help.

Any more ideas??

Mike

[Urgundiz]

Well, as Hu and Balsaman said, if is a true AC motor, my post is not useful but really I was thinking in the kind of sanding machine you can buy at any hardware store

/U

[Swede]

Mike, a phase converter isn't a VFD, although you probably knew that. Most "lower" HP VFD's can input 220 1ph, output 3ph variable frequency. These are nice if you don't have available three phase. Other VFD's require a 3ph input, then output a variable freq. 3 phase. In this case, you can use your static phase converter to create the input for the VFD.

I guess expense is relative. To many, a $250 VFD is too costly, but if you have $5,000 or more in your mill and motion control hardware, then $250 ain't so bad to get a very solid and reliable variable speed drive.

I have experimented with a small VFD, a Hitachi, and am impressed with its capabilities. Most of them have digital inputs for spindle ON/OFF and speed, pretty handy for a CNC controller, and necessary for ATC.

[Bloy2004]

Swede,
Could you elaborate on "a small VFD" ? I'm relatively a dunce when I see "VFD" . I'm still unraveling its mysteries. But sometime I want to implement a variable speed spindle with CNC and want to know the correct route.

[balsaman]

Balsaman when you say universal what exactly do you mean? Are you talking a std x-frame electric motor or something else like an ac servo motor?

A universal motor is a "brushed" ac motor like the kind you find on a router of drill. They are usually high RPM, and generally noisy. Called universal because they will run on AC or DC (altho usually optimised for one or the other, so don't try and run your router on DC).

A standard ac motor is one like you have for the fan on your furnace. Quiet, and usually 1750 rpm or 3600 rpm, and can not be slowed down by reducing the voltage, as thier speed is a function of the AC line frequency.

Eric

[turmite]

Thanks for the replies guys. The sad thing is, I had two brand new vfd's a few yrs ago and sold them for pennies on the dollar because "would never need them". My wife complains that I am a pack rat and now I wish I were one!

I think I'll do some sufring now since I kinda know what i am looking for.

Thanks again.

Mike