Hi,

I saw a Variac on amazon and i want to buy it to control the speed of a drll, a grinder and the heat of my solder iron. I do not often use the drill, grinder and solder iron, so i'm afraid that the variac will sit around doing nothing for most of the time. Are there more useful things to do with the Variac at home or in electronic projects. I'm not planning to recover old amps and radios. .If i can't use it for many more things, i won't buy it, so please can i have your advice on this..

Thanks,
Rob

 

Well you can use it as a door stop, a boat anchor, or to crack open walnuts.

 

Best use ever. Use it to control the heating power of an electric fry pan. It's always been a pet peeve of mine that the thermostat on an electric fry pan is sooooo imprecise - and that it tries to control temperature instead of heating power. It is impossible to set it to do a simmer on anything. If you want to fry, fine set it at 350 and throw in the pork chops.

But what if after searing the pork chops, you want to toss in a can of Cream of mushroom soup or cover with Kraut and simmer for 30 minutes. NO WAY. There is no way you can set that thermostat for a simmer.

Instead, connect the fry pan to the variac. Turn the thermostat all the way up and control the heat with the variac. NOW you can easily adjust it for the simmer you want.

 

Thanks Ylli, i don't have a frying electric pan. So not much to use it for, so i won't waste my money.

 

Well you can use it as a door stop, a boat anchor, or to crack open walnuts.

Many thanks for you're best answer. Sure, you're the most funny family member at you're home.

 

A Variac is one of those things I don't use very much, but when I do, it's a happy thing!

 

A Variac is one of those things I don't use very much, but when I do, it's a happy thing!

and what do you use it for??

 

Among other things, I use it to control the speed of a floor box fan, since the minimum speed of the fan is still too fast for my preference.

 

Among other things, I use it to control the speed of a floor box fan, since the minimum speed of the fan is still too fast for my preference.

That means that the fan motor is an universal motor. I've read that you can't control the speed of an induction motor with the variac, because the speed of an induction motor relates to the frequency of the AC current, am i correct??

 

When repairing or designing a line powered device, use it to carefully increase the input voltage, so nothing blows up.

Use with a bridge rectifier to drive all types of DC motors, at variable speed.
(I have a bridge installed inside my variac, with an AC-DC switch for convenience)

Use to vary the output power of heating devices.

Use to vary the speed of fans and blowers
(yes, you can control induction motors, but not very well, they just "slip" more)

It's a must-have for a complete electronics lab.

 

That means that the fan motor is an universal motor.

No.

I've read that you can't control the speed of an induction motor with the variac, because the speed of an induction motor relates to the frequency of the AC current

In general that's true.
But there are often exceptions to any rule.
For example you can, within limits, control the speed of an induction motor driving a load, such as a fan, where the load torque increases smoothly with speed.
In effect the motor runs well below synchronous speed at the point where the motor torque just matches the required fan torque.
If the load had a more-or-less constant torque requirement with speed, or varies at the same speed, lowering the voltage would not work well to control the speed.
Try it with a fan and you'll see.

Make sense?

 

Variacs are commonly used to control heating. I use one for my big soldering iron that is not temperature controlled. They also work well for controlling "universal" motors, as are used in stirring motors and some electric drills. Some grinders (e.g., toolpost) may also have that type of motor, but I have found little need to control their speed. A bench grinder is unlikely to be that type.

I have also used a Variac to control the open circuit voltage of a MOT spot welder.

 

Years ago, working as a micro section technician I wanted to control the speed of a centrifuge. The one I had was hand cranked and I didn't want to spend 20 minutes cranking it just to drive all the air out of a sample cup. So I tried motorizing it with an old fan motor. Spun way too fast, so I tried to regulate the speed with a variac. Just as Crutschow said, an unloaded induction motor will spin based on the frequency of the power source. I found that to be true because the centrifuge was almost no torque. I ended up finding a DC motor and a DC variable supply to control the speed. That combined with a plug in count down timer I was able to set the speed and time for my centrifuge and go attend to other tasks.

You can use a variac to vary the speed of a fan, for the very reason Crutschow stated. I have one for my grinder / polisher. I use it when polishing the headlights on my car. Too fast and you just burn the plastic. You need slower, but there's a trade-off: You can't apply as much pressure at lower speeds because the torque is not there.

 

the heat of my solder iron

Sort of overkill on that last one. You can buy a cheap dimmer switch at any home improvement store for a few bucks and control a soldering pencil. Most hand held electric drills include a speed control built into the trigger.

Years ago a variac was popular for testing line regulation of the older linear power supply regulation but I seldom if ever see that anymore. The ones I have just sit around and collect dust. Unless you have a specific need and know a variac will suit that need I would spend the money on something else.

Ron

 

Hi,

I saw a Variac on amazon and i want to buy it to control the speed of a drll, a grinder and the heat of my solder iron. I do not often use the drill, grinder and solder iron, so i'm afraid that the variac will sit around doing nothing for most of the time. Are there more useful things to do with the Variac at home or in electronic projects. I'm not planning to recover old amps and radios. If i can't use it for many more things, i won't buy it, so please can i have your advice on this..

Thanks,
Rob

The Variac is basically a laboratory tool...but a very useful one, when a variable AC power source is desired. Free of harmonics and semiconductor switching noise, it is indespensible for research. Anytime a motor (or a process) needs to be closely controlled; pumps, fans, feeders (bowl & linear), mixers and blenders, the Variac is un-surpassed. My big tobacco leaf fan has to be run from a variac, unless you want to live in a gale. If you can't figure out what uses to put it, you surely don't need to own it.
Also, 120% Line is available so FWB rectification to 120 VDC is possible.

Cheers, DPW [Everything has limitations...and I hate limitations.]

 

A comment that may have no bearing on this thread whatsoever, I have several autotransformers. Rated for a few amps. So far I haven't had a use for any of them. I have plans to build a supply but it will be a dirty supply. I got them from a science lab at a school. They're old and operated off of a ferroresonant transformer. Students used them to supply AC or DC anywhere from zero volts all the way up to 135 VDC. I think they got rid of them because of the potential safety hazard of such high voltages.

 

I must have over 1000kg worth of old transformers, variacs, and CV transformers sitting in a cabinet all dressed up with no place to go.

DG Nova 2/10 minicomputers were built with a whopping big auto-transformer that delivered 5V 30A with no additional voltage regulation required. During brown-outs when the room lights dimmed, the Nova would keep on running merrily.

 

I have only used one (not unlike this one)

a few times and many years ago. They seemed convenient for "smoke" testing some things. I had considered getting one or at least keeping my eyes open for a cheap one that might come up.

I then read this thread on AAC https://forum.allaboutcircuits.com/threads/variacs-are-dangerous.143739/

@Robesim It did not scare me off getting one, but it did make me aware of some potential issues that I was not aware of (even though maybe I should have been). So, I thought I would pass it on to you as it is a good read.

 

Hello there,

A variac is a transformer with variable secondary winding turns. That means the applications are almost endless. Since in it's main uses it lowers power to AC devices, whether or not you need one depends on what devices you want to lower the power to.

For one quick example, you can lower the power to a microwave oven. Now this may not sound like much because almost all microwave ovens have power settings, but for a standard microwave oven the lower power settings are done using very low frequency PWM rather than turning down the actual continuous power, so the oven effectively turns on and off and on and off to get a lower power setting. That means that it bangs the food with maximum power for a time, then turns off, then bangs it again, and this repeats many times over a few minutes so the oven achieves an average power that is lower than the full power of the oven.
This causes two problems. First, the food is bang with full power for a time, second the INPUT power to the oven is always at maximum. So if you have a 1500 watt input power oven when it turns on it takes 1500 watts even though it wont stay on continuously over the whole cooking cycle. Drop the power with a variac though and that input power drops to whatever you want it to be just about. That means more gentle cooking and you dont draw as much power from the line. Set it down to 750 watts input power and you have about 50 percent cooking power.
If you already have an 'inverter' type microwave though then you may not find an advantage with a variac because the inverter types already do this internally with a power converter. But if you have a standard oven, then this works pretty well.

So there are two aspects of the variac:
1. The lower output power.
2. The lower input power to the variac.

#2 is more important when you dont want to draw too much power from a power source like a small generator.

Some facts...
People that make beer find a variac useful too, you can look that up if interested.
Most variacs are not isolated but you can get isolated variacs at a premium price.
Variacs do not REGULATE the output voltage or current. If the input line varies by 10 percent the output varies by 10 percent also.
Variacs are used a lot for testing other appliances because it allows the voltage to be raised little by little while testing.

 

That means that the fan motor is an universal motor. I've read that you can't control the speed of an induction motor with the variac, because the speed of an induction motor relates to the frequency of the AC current, am i correct??

Usually shaded pole and PSC fan motors can be controlled, generally they need to be started in high rpm.
1ph Capacitor start-run motors do not control well at all.
One use I have for the Variac is to stick a bridge rectifier on it and test DC motors of all sizes and voltages.
Max.