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    Default 240v rheostat

    Hi, I have a 110v drill. I live in the UK, so have a 240v to 110v transformer. I want to be able to control the speed of the drill with a rheostat. I ordered a 110v rheostat and a 240v has arrived. I know very little about how this works. What would happen if I used the 240v rheostat for my 110v drill? Many thanks, Ian

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    "Drill" cover an awful lot of ground.

    We need to know what kind of motor you have.

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    An actual rheostat, or a "motor speed control"?

    I presume this is a standard hand held drill motor, in which case it should be a "universal" motor. Regular motor speed controls will work, but the 240v type may be kinda rough at slower speeds.

    If it is a drill press type drill, with an induction motor, a "speed control" will not work.

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    Quote Originally Posted by Ian117 View Post
    What would happen if I used the 240v rheostat for my 110v drill? Many thanks, Ian
    A rheostat has a control knob that when turned clockwise the voltage increases at the output terminals. On say a Powerstat the knob would be turned approximately half way. I have a 110v Powerstat which can dial the output from 0 to 140v. The Powerstat is a variable transformer. Simple as that.

    Does your 240v rheostat have a knob that you can turn half way to get 0 - 110v output?

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    Quote Originally Posted by Ian117 View Post
    Hi, I have a 110v drill. I live in the UK, so have a 240v to 110v transformer. I want to be able to control the speed of the drill with a rheostat. I ordered a 110v rheostat and a 240v has arrived. I know very little about how this works. What would happen if I used the 240v rheostat for my 110v drill? Many thanks, Ian
    Rheostat is just a variable resistor so it should work either way but if it's some kind of device that needs 240V input simply put the rheostat before the transformer

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    This is a better way to vary the voltage. I have the 110v version of this:

    3PN217C - SUPERIOR ELECTRIC - Variable Transformer, 24 VAC, 24 VAC, 5 A, 5 A, CW / CCW | Newark element14

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    Quote Originally Posted by Ian117 View Post
    Hi, I have a 110v drill. I live in the UK, so have a 240v to 110v transformer. I want to be able to control the speed of the drill with a rheostat. I ordered a 110v rheostat and a 240v has arrived. I know very little about how this works. What would happen if I used the 240v rheostat for my 110v drill? Many thanks, Ian
    Since "rheostats" do not have voltage ratings - being a variable resistor it usually have only the value of the resistance and power ratings - you probably have a variable transformer. To operate a 110V device from 240V source this is exactly what you need, provided the current rating is adequate.

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    Show us a photo of the 'rheostat' including any data plates, or a reference to the specs for the equipment. It is very hard to give any clear advice without knowing a bit more about what you have bought. It would also be useful to have the same info about the drill you will be using.

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    A rheostat is just a fancy name for a variable resistor with a high power rating. The Voltage rating for such a resistor is a number that should not be exceeded if you do not wish for it to arc or short out. So, a 120 Volt unit should not be used for 240 Volts as smoke would be expected, sooner or later.

    But one that is rated for 240 Volts can easily be used in a 120 Volt circuit.

    The real question is how are you going to use it. You are in the UK and I assume that means that your power is supplied at 240 Volts (220 to 240 range). But you say you have a 110 (120) Volt drill. If you apply 240 Volts to a 120 Volt drill, again I would expect smoke. You will need to limit the Voltage to that drill at 120 Volts and NO MORE. I would think that the normal way to do that would be with a 240 to 120 Volt transformer. Thus is the preferred solution because it does not waste nearly as much power as a rheostat would.

    But you say you want to control the speed. That may or may not be practical or even possible. It depends on the type of motor you have in that drill. Induction motors will run at a speed that is determined by the frequency of the power, and not the Voltage. If you have an induction motor, you will just be wasting half the power and will not be able to actually control the speed. You would probably be better off getting a small VFD (Variable Frequency Drive).

    Other types of motors will behave differently. So it is critical to know what type of motor you actually have.

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    Yeah, I guess I HAVE seen variacs called "rheostats" before.

    A 240V type will work at 120V, just not as well as a 120V type. But if used on the 240V side, would be fine for a "universal" type motor..

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    Sure would be nice if the OP would come back and 'Splain just what kind of
    "Drill" they have, and what they are trying to do with it.

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    A true rheostat, i.e. a wire wound power potentiometer (variable resistor) or a variac (variable aurotransformer), also a wirewound device but with an iron core vs. a ceramic support. The former varies in resistance, the latter in transformer ratio.

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    An actual rheostat* (NOT same as potentiometer) would pretty much suck for the purpose, since the series universal motor draws a widely varying current, and would slow down and likely stall at settings where the no-load speed would hardly be affected.

    The variac will work decently, it varies voltage and is less load sensitive. I bet the OP means one of those.

    * An actual "rheostat" is a variable resistor, but only has two terminals (not three like a potentiometer). It goes in series with a load, such as a generator field, etc. Far better for loads that act like a resistance, not very good for motors.

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