OK guys here is a question that I have not been able to get a concurrence on.

I have been told by a couple of people that I could use a VARIAC to control the speed of my grinder, but I have some people telling me that they think it won't work and that I would only burn up my motor.

Here are the particulars: 1) 1.5hp 20a. leeson motor (not variable speed); 2) used with a Burr King 960.

so what do you guys think?

Mike

It is my understanding that unless the AC motor is a true varible speed, using a variac slows or speeds the motor by varying the voltage which will cause the motor to overheat and eventually burn out. I have a variac but do not use it on my Burr King for this reason

Is your motor AC or DC?

DC is made variable quite easily, the AC motors weren't designed to change speeds like that.
If you thought AC motors were expensive, wait till you look at the DC motorsPLUS the controllers.(they work as a team)
This is why I advocate the step pulleys.
Same effect at a fraction of the price.
It takes longer to change speeds, but for several hundred dollars difference in price, I can spend the couple extra minutes changing gears,so to speak.

Geno,

Can you convert a variac to DC with the bridge rectifier and then use it to slow down the DC motor? What is the controller doing other than reducing voltage?

Bob, the commercially available controllers do more than just control DC voltage. There is some pretty sophisticated circuitry, with pots controlling max RPM, min RPM, and torque, among other things.

Thanks guys. I kept getting a lot of different answers, but as usual I should have listened to my father.... the old dog is usually right.

Now I just have to figure out how to put pullies on my grinder that on the pedistal.

Oh BTW were can I get the pullies and bearings?

Mike

Mike, here is one place.www.mscdirect.com (http://www.mscdirect.com)

Ok guys so if I do get a variable ac motor can I control the speed with a simple reostat. I know I'm looking for the cheap way out but if you don't ask you'll never learn.

Mike

Mike,no, you have to get a DC variable speed motor and the correct control,then all you have to do is reach out and twirl the dial. Warning; once you use this type of setup, you will never want to go back to what you have now.;) ,Get a catalog from these people, surplus center,1-800-488-3407, you can get started for a reasonable price. Email me when you get the catalog and I will give you the P/Ns you will need.

Mike H.

Quick edit from yes to no in your post. I was getting ready to say something and it changed from the time I viewed it and the time I went to the reply window.

There are resistance variacs that will work on AC motors, however, they will eventually burn up the motor because it starves the motor of power in order to slow it down. This causes the motor to heat up and burn the shlack from the winding inside the motor and eventually cause a short.
It was designed for a router type motor, not an external one.
DC motors and controls are expensive, but that is the best way to go electronically.
The step pulleys are hard to install on a fixed unit like the burr king. Most belt drives are seperate, not inside.
If you find a way to put step pulleys on a burr king, let me know.There is just no room for them inside.

Thanks for the education.

Mike I made that call and the catalog is on the way.

Geno I will be seeing a friend of mine this weekend that also has a Burr King and I will pose the same question to him. Maybe between the 2 of us we will be able to figure something out.

Mike

I hesitate to enter a conversation on something I know very little about. However here goes. I have an AC motor controller that was given to me. The way that it was explained to me was that this controller varied the speed of AC motors by varying the "cycles". I have not used it because it is 3 phase and my electrical service is single phase. I have looked these controllers up in catalogs and the cost is outrageous.
To me $800 to $1200 for a controller is outrageous. Anyway, maybe someone else out there knows about these units.

You can hook up the three phase by adding another motor on the other leg of power that just runs by itself, It acts like a big resistor. I havn't seen a three phase variac, but it would have to work the same.(laws of power)
DC is the best way. H/P is the variable that costs.
A small sewing machine motor works the same way but has little power to run anything but a sewing machine.
1 H/P and up is gonna cost you somehow.
Just get an extra motor if you want to run the three phase system.(again NOT cheep,I know)
Used three phase motors are cheeper than single phase motors for some reason.
Hope this helps.
DC is still best for longevity.

David,
That is a VFD (variable frequency drive)that you are referring too and there purpose is to reduce cost of operating large and power hungry equipment such as large air conditioning equipment(air handlers and such).If the depend on the equipment is increased then the VFD compensates and makes necesarry changes in voltage and power.
If someone used this on a grinder and you loaded the grinder it would more then likely speed things up.

Sounds like a Turbo charger on demand. :)

Don't use a variac on or reostat on AC motors with any real load, it will shorten up the life.

They do make an AC controller that uses "SCR" control, this will also burn up some motors but is somewhat less expensive than a VFD, but it must be rated for the Volt & Amp of your motor. Cheap SCR controllers will put "noise" back on the AC line and screw up the TV, Stereo and might drive your computer to an early grave. Most single speed AC motors just don't like running slower than the plate rating.

Running a variac through a bridge rectifier will work for a DC motor but be carefull, most variacs can put out more volts than the line voltage when turned all the way up. Use a volt meter on the output! The bridge needs to be rated for the highest wattage (watts = volts times the amps. 120v X 5a = 600w) expected. And if you load down the motor real bad, you don't have a current (amps) limit circuit to protect the motor.

3 phase stuff is always expensive.

With all things electric, once you let the smoke out of them, they just don't work. <img src=http://www.ezboard.com/intl/aenglish/images/emoticons/embarassed.gif ALT=":o">

PaulD

Thanks Paul,
Stick around, I bet you can help with a lot of electric info, huh?
I'm no specialist. I just been around tools most of my life. I don't have the formulas and such. I know the ohms law and a couple others, that's it.
I appreciate all the help I can get around here.
We get all kinds of questions.
None of us knows it all ya know.Thanks for sharing.

Thanks Geno,
If it's something I have good info or experience with, I try to help. If I'm not sure, I'll say so.

Most of the used 3 phase motors are cheap because they got replaced with single phase ones. If you have 3 phase power they work great. There are inverters to run 3 Ph. off of a single Ph. line but they waste a lot of power and some motors don't like it. They are expensive to buy and the power company will send you a Christmas card.

Even those VFD controllers will cook a motor that was not designed for it.

I have seen the SCR type "speed control" units in hardware stores but they would only run a big hand drill. Putting too big a load on a "cheapie" will cost you the controller and the motor.

The best way to go is with a matched motor and controller (AC or DC) If you can't afford it, those stepped pullies can be had with 5 steps and two (on the motor and the arbor) of them
will give you more choices and less risk for very little cost.

Thanks for all the input guys, I think I will just save up for the varialble speed motor. I think it will be the cheapest thing to do in the long run.

Mike

should i order pulleys,Here,s how i have a grinder set up 11/2hp a.c. motor Permanate magnet type, brush type 14 amp i have been useing for a year or so with no problems but now you have me concerned,I hate to burn it up its works so well..Oh;I am using a large reostat to control it..Any input on this would be appreciated...

Allen,
&nbsp &nbsp &nbsp &nbsp A 1 &1/2 horse AC motor, with permanent magnets? How old is this thing? If it truly has magnets, it may explain why it runs well. Are you sure you have a rheostat? A rheostat is just a variable resistor, like the volume control on the TV, only heavy duty. To handle 14 amps, it would need 10 gauge or larger wire windings and be very large and heavy.

&nbsp &nbsp &nbsp &nbsp More details or pictures are needed to be sure of what you have.

PaulD

Paul,thanks for your intrest,The plate on the motor says permanent magnet A.C.motor..the plate on the rheostat reads Varistat-Dimmer 2500 watts Max. rated 20 amps..it is about 6"dia and 3"thick wire winding is about 1/16" .Re motor ..Don't know how old it is but it looks modern,I salvaged it out of a commercial floor ..polisher...

You don't have a rheostat, it's a variac or a SCR type controller. I didn't take the time to look up the brand name but if the motor isn't heating up past being able to leave your hand on on it for 10 seconds, after normal use, every thing should be OK. Motors do run hotter than most people think is OK, the 10 second test is valid for most motors. If you run one hard, let it run with a light or no load for 2 min before turning it off. Do it every time you use it and it WILL last longer.

PaulD

Paul ,thanks for the infro,it dosn't heat up so i'll leave it as is .thanks again..

Speed control for a DC motor can be done with PWM (Pulse Width Modulation). What this does is change the amount of time the motor will see a voltage present. The voltage is not changed, only the time interval that the voltage is applied. These devices can be made for a reasonable price depending on the current requirements of the motor in question. An example of one for a small motor can be found HERE (http://www.electronickits.com/kit/complete/motor/ck1400.htm)

Speed control for what is called a AC " Shaded Pole Motor " can be done with a Triac. Also motors that use brushes can be controled using this same type of device. Be sure to check the current rateing before you purchase one so you don't get one that can't handle the amount of current your motor draws. These are most commonly seen as the device used to control ceiling fans. I use a variac now to control two 120mm shaded pole fan motors that I installed into my computer. I usually have the variac set to around 70 - 80 volts. If you add a MOV (Metal Oxide Varistor) accross the lines input supply to the fans it will prevent the transient voltage spike that occures as the motors try to start up. More information about resistive loads and inductive loads can be found HERE (http://www.act-solutions.com/kingery02.htm) The main page for this information resource is found HERE (http://www.act-solutions.com/uncle.htm).Good luck and I hope this helps you some.

I have been running a DC motor for the past dozen years or so with a Variac coupled with a bridge rectifier, very simple circuit. Two AC lines from the variac to the AC poles of the rectifier and the two DC output lines to the DC motor. The circuit used to have a double pole double throw switch to reverse the motor but I cut that out long ago.

Here it comes again only because I still need help and I believe this is the best place to try to find it. Iwant to buy a variac for anodizing titanium. When I asked a supplier about availability he asked me what sort of resistance it would have to handle. I told him I didn't know but had been told that if I got a 4 amp 110 volt A.C. input model that should do. His reply in wanting to help me was this resistance thing .Do I really need to know that? Bob Warner says he didn't worry about that and his variac does just great. Help PLEASE. Frank Niro

Frank,
A variac is short for variable resistance transformer. It is a voltage reducing tool. Most go from 0-120 volts and need to be changed to D/C from A/C. A radio shack bridge rectifier will change the polarity for you, a volt meter and power switch, and you are ready to roll.
The amount of resistance depends on where the dial rests at that time.
I dial mine into 36 volts for a blue/purple color, and gold is around 12 volts. I can dial the voltage to what I need before I dip it in the solution.(I use 10 second interavels on the dip)

I think we should re-name the variac to a "dial a volt" switch.
The resistance IS what is variable. Hope it helps.

George,
Your set up is ideal, the variac, bridge, motor, the settings you use and the loads you apply are all variables. You are obviously within the expected operating range of them all. If you could please list the specs on every thing you have, and how it is used, it might be a good indication of a starting place. I might be able to extract anything missing if you don’t have it all.

Frank,
The guy is just attempting to make sure you don’t get a unit that is too light for your use. I don’t have any experience with small anodizing baths, but I am assuming a 1-gallon or smaller tank would not overload a 4 amp variac and bridge rectifier. Perhaps someone out there has (or can hook up) an ammeter to their existing setup and list the pertinent data here.

Some variacs can output higher than the line voltage, any new setup should include a voltmeter for insuring the proper settings regardless of the use, I personally put an ammeter in the circuit, at least temporally, to insure safe operating limits on any new setups.

Any one, who needs help sizing these things can also e-mail me for any help I can supply, I would need every bit if info you can provide to fill in any blanks. The address is atfab@bestweb.net

PaulD

Just in case someone wants a factory unit.
www.elect-spec.com/variac.htm (http://www.elect-spec.com/variac.htm)

I sure do appreciate the help people!! I will probably end up going to eBay for that variac as Bob Warner sugested. The prices there especially on one unit sold by a company in California was most reasonable compared to what I would pay here in Canada even though my cost in Canadian funds would end up being more than double the listed price. And yes I have a fine bridge rectifier that will handle up too 240 volts A.C. in. Thanks again. I knew this just had to be the right place to ask. Frank Niro.

Frank,
Make sure the amperage (amps) rating of your bridge is higher than the load rating also. For your anodizing 4 to 6 amps should be more than enough. Those small "wet wand" plating systems use less than 1 amp power supplies.

George's DC motor works well because the voltage settings he uses are correct(not more than 10% higher) and amperage ratings of the variac and bridge are higher than the the motor needs(should ALWAYS be at least 10 to 25% higher).

I wish I had more experience with the small stuff, 99% of it is with large, automated plating lines that used 100 amp and bigger power supplies. My motor experience is mostly with precision stepper motors. I do know all the "Ohms Law" stuff so I can advise on specs if I have enough info.

PaulD