Check my last posts/replies. Instead of a Variac, can i use a transformer with a Rheostat.?

Check MrAl's last post. He answered your question. It's maybe not the straight forward answer you're looking for but it is spot on. A variac is a transformer with a variable tap. You're not producing much extra heat. But a rheostat is basically a resister / divider network meaning it's got a lot of voltage across it to drop. And that's just the voltage. Wattage is another key factor you need to consider. Watts is heat Lots-0-watts, lots-0-heat. WILL A RHEOSTAT WORK WITH A TRANSFORMER? Yes. IF it's the right size for the job. WILL IT WORK WELL? No, not really. Even if you get the right size based on the supply and demand. The variac is the better choice.

 

A variac is a variable Autotansformer. it's single winding transformer that offers no isolation.

A non variable autotransformer is typically used to buck/boost the line voltage.

For bench trouble shooting, the device is usually combined with an isolation transformer.

Back in the day to save some costs, and no move 6.3V filament transformers, they made all of the tubes add up to line voltage. Thus you had 50V (50C5) and 35 V (35W4) filaments. the TV's had polarized plugs and they were directly connect across the line.

The isolated Variac was a necessity. Actually Variac is a brand name.

The Variac is a necessity when fixing high power audio amplifiers. I worked on one amp that had a bad transformer where the chassis ended up being live.

 

Bottom line is, there are a lot of appliances, but you have to check the previous posts to see which of them suit you.

 

Check my last posts/replies. Instead of a Variac, can i use a transformer with a Rheostat.?

Sure a rheostat will provide a variable voltage. BUT, it is not very efficient. The power that is not used is converted to heat. Let's assume you drive a 2:1 reduction transformer with a rheostat from 110V and you want the output to be 30 V at 10A. Assuming 100% efficiency of the transformer, you need 60V at 5A input. Reducing 110V at 5A to 60V at 5A with a rheostat will waste approximately 50*5 VA as heat (250W approximately). That is a lot of heat and a large rheostat.

 

Thanks guys. I'm better off with a Variac. I'm going to get a 20 amp Variac.

 

Interesting decision to go with the 20 amp Variac. It too is a lot of power. And if you're using it to control a motor that is running on 2 amps at 110 volts, you're wasting a lot of energy. And at start up - as others have already mentioned, the startup current can be enough to trip a breaker. It's kind o like trying to start a diesel engine using a weed whacker as a starter motor. Maybe that's a bad analogy, but the point is this: Why get something that big when you only need something large enough to handle your largest need? If your need never exceeds 7 amps (typical vacuum motor might draw that much then why build for bigger? Unless you're going to be starting up large machinery, which typically runs on much higher voltages, I don't readily see a need for a 20 amp variac. Not saying there is never a need for one that large or larger. I've seen them in the shop, naval power systems, old school movie projectors such as are at a movie theater, or welding shops where control of huge diode pack welding units need to be controlled. And many of these examples are old school systems.

Still, if 20 amp is what you want then by all means - purchase one. I can't imagine a use for such a large unit. I have a table saw that's on a 15 amp breaker circuit, so that's not going to draw 15 amps. Dryers and stoves (electric) are on 30 and 50 amp circuits, but they're also on 220 volt sources. The stove can run up to 4 burners and an oven at one time, and still, I don't think any single part of the stove is going to draw anywhere close to 50 amps. Probably not even 20 amps. Me? I have several autotransformers that are rated at 3 amps. I wouldn't use one for more than 2 amps service at any voltage.

Using a transformer to knock down the voltage first, yeah, you can. But it's like using a 2:1 transformer coupled to a 2:1 transformer coupled to another 2:1 transformer (three transformers) because you want 15 volts. Yeah, you'll get 15 volts. But you lose a lot in efficiency doing it that way. You're better off getting a transformer that is 10:1.5. That way you convert 120 VAC down to 18.2 VAC (RMS). You still have head room.

 

We purchased six 20A variable autotransformers for our labs. In testing them we found that all of them would trip 30 breakers about half the time. After contacting the vendor and "VARIAC" (not our brand) , they indicated this was likely in that size autotransformer. We sent them back and went with 10A units. The 20A ones were overkill for our current purposes.

Ken

 

Interesting decision to go with the 20 amp Variac. It too is a lot of power. And if you're using it to control a motor that is running on 2 amps at 110 volts, you're wasting a lot of energy. And at start up - as others have already mentioned, the startup current can be enough to trip a breaker. It's kind o like trying to start a diesel engine using a weed whacker as a starter motor. Maybe that's a bad analogy, but the point is this: Why get something that big when you only need something large enough to handle your largest need? If your need never exceeds 7 amps (typical vacuum motor might draw that much then why build for bigger? Unless you're going to be starting up large machinery, which typically runs on much higher voltages, I don't readily see a need for a 20 amp variac. Not saying there is never a need for one that large or larger. I've seen them in the shop, naval power systems, old school movie projectors such as are at a movie theater, or welding shops where control of huge diode pack welding units need to be controlled. And many of these examples are old school systems.

Still, if 20 amp is what you want then by all means - purchase one. I can't imagine a use for such a large unit. I have a table saw that's on a 15 amp breaker circuit, so that's not going to draw 15 amps. Dryers and stoves (electric) are on 30 and 50 amp circuits, but they're also on 220 volt sources. The stove can run up to 4 burners and an oven at one time, and still, I don't think any single part of the stove is going to draw anywhere close to 50 amps. Probably not even 20 amps. Me? I have several autotransformers that are rated at 3 amps. I wouldn't use one for more than 2 amps service at any voltage.

Using a transformer to knock down the voltage first, yeah, you can. But it's like using a 2:1 transformer coupled to a 2:1 transformer coupled to another 2:1 transformer (three transformers) because you want 15 volts. Yeah, you'll get 15 volts. But you lose a lot in efficiency doing it that way. You're better off getting a transformer that is 10:1.5. That way you convert 120 VAC down to 18.2 VAC (RMS). You still have head room.

Hi,

Just wondering why you said that using a 20 amp variac "you're wasting a lot of energy".

 

We purchased six 20A variable autotransformers for our labs. In testing them we found that all of them would trip 30 breakers about half the time. After contacting the vendor and "VARIAC" (not our brand) , they indicated this was likely in that size autotransformer. We sent them back and went with 10A units. The 20A ones were overkill for our current purposes.

Ken

Hi,

Wow, that's interesting. I've had one for years now and never had that problem. Maybe the brand has something to do with it?

Did you ever test anything else that has an inrush current?
Did you try any other breakers?

 

wondering why you said that using a 20 amp variac "you're wasting a lot of energy"

Kind of mis-spoke. It IS overkill, but a 20 amper will waste more than a 5 amper will simply because of inefficiencies. The larger will be warmer. More energy wasted. Well, that's my opinion and I'm sticking to it until I change my opinion. Besides, there's a thunderstorm outside and I can't get DirecTV.

 

Hi,

Just wondering why you said that using a 20 amp variac "you're wasting a lot of energy".

So this got me curious. I have a few laying around with a 7.5 Amp and a 10.0 Amp being handy. I do have a 240 Volt 20 Amp but it is open frame and I am not that curious. So with no load applied I plugged in each and took a few measurements.

7.5 Amp Maximum

• Voltage: 121.1 VAC
• Current: 0.13 Amp
• Watts: 4.0
• Volt Amps: 16
• Frequency: 60 Hz.
• Power Factor: 0.27

10.0 Amp Maximum

• Voltage: 120.8 VAC
• Current: 0.12 Amp
• Watts: 3.0
• Volt Amps: 14
• Frequency: 60 Hz.
• Power Factor: 0.26

So less any load neither unit really uses any power to speak of and the difference between a 7.5 Amp and a 10.0 Amp version is really not that great when sitting here doing nothing. Obviously the 10.0 Amp version is heavier and short of opening it up I would guess wound with a heavier gauge wire. If I were to load them I would guess any additional power draw would only be that of the load. I don't see much if any wasted power differences between the smaller and larger.

Ron

 

OK, my opinion's being changed.

 

OK, my opinion's being changed.

I would never try to sway an opinion. What it comes down to is retired people have a surplus of time.

Ron

 

The 20A ones were overkill for our current purposes.

Was that pun intentional?

 

So this got me curious. I have a few laying around with a 7.5 Amp and a 10.0 Amp being handy. I do have a 240 Volt 20 Amp but it is open frame and I am not that curious. So with no load applied I plugged in each and took a few measurements.

7.5 Amp Maximum

• Voltage: 121.1 VAC
• Current: 0.13 Amp
• Watts: 4.0
• Volt Amps: 16
• Frequency: 60 Hz.
• Power Factor: 0.27

10.0 Amp Maximum

• Voltage: 120.8 VAC
• Current: 0.12 Amp
• Watts: 3.0
• Volt Amps: 14
• Frequency: 60 Hz.
• Power Factor: 0.26

So less any load neither unit really uses any power to speak of and the difference between a 7.5 Amp and a 10.0 Amp version is really not that great when sitting here doing nothing. Obviously the 10.0 Amp version is heavier and short of opening it up I would guess wound with a heavier gauge wire. If I were to load them I would guess any additional power draw would only be that of the load. I don't see much if any wasted power differences between the smaller and larger.

Ron

Hi,

Oh thanks for doing that. That shows in detail what we can expect.

Very nice of you to take the time and effort to do that.

 

Hi,

Oh thanks for doing that. That shows in detail what we can expect.

Very nice of you to take the time and effort to do that.

Most welcome, as I mentioned, I have a surplus of time on my hands.

Ron

 

I would never try to sway an opinion.

If a man can't change directions when he discovers he's headed in the wrong direction then what is he? A politician?

 

No, He's being pushed.