Difference between variac and a rheostat

[tkvenki]

Hi all,

I have seen some posts in this forum...which will tell about designing a variac.

some say that we could use a rheostat as a crude method for a variac.
So i just want to know what are the full functionalities of a variac??
Kindly refer me some link so that i could study them.

And also i need a 3-phase variac.
I must be able to vary 0-220Vac.
The frequency must remain constant.
Should i use 3 rheostats??
Or is there some other simple method??

Kindly guide me....
Thanking you
Venkatesh T.K

 

[JimB]

A rheostat is just a variable resistor which will limit the current (and hence the voltage) at the load.
It will get very hot and is not efficient.

A variac is a variable voltage transformer.
It has one winding, usually on a large toroidal core. A "wiper" taps the output voltage from where ever it is set on the winding.

A three phase variac is just three variacs ganged together on a common shaft, so that they are adjusted together.
The frequency is not changed by a variac.

A rheostat is not a substitute for a variac if you want to vary the output from 0 to 220 volt.

JimB

 

[tkvenki]

THanks a lot

THanks a lot ...

I thank you for giving me the information.
Do you know the approximate cost of a variac??just in case??


THanks a lot for giving me some time....
Venkatesh T.K

 

[Nigel Goodwin]

Re: THanks a lot

THanks a lot ...

I thank you for giving me the information.
Do you know the approximate cost of a variac??just in case??

Variacs are expensive, and as you need three of them, that makes it three times as expensive.

But as already suggested, a rheostat (large potentiometer!) isn't a replacement for one.

You also don't mention what power you need?, presumably (as it's three phase) it's considerable? - again, this adds greatly to the cost.

What are you trying to do?, and is a variac the best way?.

 

[4electros]

A variac is a variable voltage transformer.
JimB

I know that variac is variable capacitor so variac is an abbreviation to it!

Could it be or not?

 

[JimB]

A variac is a variable voltage transformer.
JimB

I know that variac is variable capacitor so variac is an abbreviation to it!

Could it be or not?

As I just said, a Variac IS a VARIABLE VOLTAGE TRANSFORMER. Capacitors have nothing to do with it.

JimB

 

[tkvenki]

THanks

Thanks for the reply Nigel and electros......

I need the variable ac supply for testing my energy meter.
So Power is not a major concern.
I'am using ade7754 evaluation board for which i will be connecting...
the 3 phase voltages and currents.

Thanks for the concern....
Venkatesh T.K

 

[4electros]

I don't know whether you can find what you need in this site or not but you won't loss anything if you try!

http://www.discovercircuits.com/P/pwr-variable.htm

tell me about what will have happened with you later!

good luck!

 

[Nigel Goodwin]

As usual the OP hasn't bothered searching on google!, a VERY brief search finds https://www.variac.com/staco_240_3ph_y.htm for varying sizes of 3 phase variacs - plus loads of information and useful links.

And to be honest, you hardly need google to find www.variac.com do you?.

 

[Gorgon]

Hi,
If you need this variac just for testing / calibrating your design, why not try to rent one from an instrumentation rental company/ service.

TOK

 

[nwhurlburt@verizon.net]

variac vs. rheostat

Please forgive my lack of knowledge but I also have another question re: variac vs rheostat.

Question: I want to slow down a makita die grinder, 1/4" model GE0600, 115V/2A/60Hz, running at 25,000 rpm. I do wood carving and use the die grinder to hog waste. I want to slow this down to 1700 to 3000 rpm. What is the best and most economical way to do this, if even possible, without harming the motor?

Thanks for your help.

 

[Nigel Goodwin]

Best way is to use mechanical gearing!.

If you slow the motor down it may very well overheat, unless it's designed to be operated in that way?.

A rheostat is a completely useless way of doing it!, electronic speed control would be best, a variac would work pretty well - but may be a bit expensive?.

 

[nwhurlburt@verizon.net]

I have the chance to purchase two used variac's for $20 and change. I can get by operating in 5 or 10 minute bursts. Is this OK?

 

[Nigel Goodwin]

I have the chance to purchase two used variac's for $20 and change. I can get by operating in 5 or 10 minute bursts. Is this OK?

Try it and see how hot the motor gets - electric motors have cooling fans inside, as it runs slower there's less air forced through it.

 

[Hero999]

If it's an induction motor then forget about it.

As it slows down it will draw more current until it overheats.

 

[Captian Keyboards]

New question - I need to control the output of a calrod (infrared) heater. Which is better for his application a rheostat or a variac?

 

[MikeMl]

A Calrod is just a pure resistive heater. The simplest way is to use thermostat. The best way (most efficient, tightest temperature control) is to use an SCR proportional phase-control. A Variac is ok for an experiment, but you will be chasing the final temperature by manually diddling the knob. A reohostat gets stinking hot and dissipates lots of Watts, so will need to be very big.

 

[JimB]

Wow, a very old thread is revived!

And the answer is still the same, a variac.

JimB