Not really audio related but I figured I will ask here. I have an AC motor that runs on 115V and draws 1.3A. I need to drop the voltage going to the motor (to slow it down) by using 10 Ohm wire wound resistor. What wattage of the resistor should I use so it is safe for a continuous use?
Last edited:
If there is 1.3a rms flowing through the 10 ohm resistor, that would amount to 16.9 watts of dissipation, so a 20 watt resistor would be adequate. If that's the current without the resistor, then the current will be lower when the resistor is installed, and the 20 watt size will still be fine.
You always want to oversize power resistors. The 20W rating is a bare minimum.
They are cheap, so I'd use two 20 ohm resistors in parallel, or else two 4.7 ohm resistors in series, all at 20W each.
They run hot, so keep them in free air, and not touching anything.
They are cheap, so I'd use two 20 ohm resistors in parallel, or else two 4.7 ohm resistors in series, all at 20W each.
They run hot, so keep them in free air, and not touching anything.
They will be inside a cast iron enclosure so I can mount them to it to dissipate some heat. Would i.e. a 100W resistor be better for this application or the heat will be the same from 100W resistor as from the 20W one?
The heat will be the same, no matter what combination of resistors, as long as it is the same total resistance.
The cast enclosure will not have a properly flat surface. Also the resistors are not intended to be surface mounted,
unless you use this type of case: https://www.digikey.com/en/products/detail/ohmite/HS100-100R-J/5306231
The cast enclosure will not have a properly flat surface. Also the resistors are not intended to be surface mounted,
unless you use this type of case: https://www.digikey.com/en/products/detail/ohmite/HS100-100R-J/5306231
Maybe you can sand or machine the mounting surface,
but if not the resistor(s) must be properly derated according to the mfr.
but if not the resistor(s) must be properly derated according to the mfr.
The AC motor is from a 1940s desk fan. There used to be a speed coil (I guess something similar to a wire wound resistor) but it is gone now. I want to add at least one speed to the fan so that's why I was thinking of using the current limiting resistor. But I am open to suggestions for something else that will work in this case. I will research the triac regulators.
Just use several resistors in series, and switch them in or out.
Watch the resistor power dissipation on the slower speeds,
and don't let the motor stall from too slow a speed.
Watch the resistor power dissipation on the slower speeds,
and don't let the motor stall from too slow a speed.
I do have measurements from the same fan in working condition so I know that the original speed coil had two resistance values 10 Ohm for medium speed and 15 Ohm for slow speed. I can get both resistors and try. I may only wire the 10 Ohm for medium speed if the slow speed generates too much heat. Afterall, the resistors are going to be in the base of the fan that's fully enclosed.
20 watts is adequately oversized for that application. Going larger in power dissipation capability won't hurt, and it won't help either. A competently designed resistor with a power rating specified will reliably dissipate that specified amount of power, given adequate ventilation.
Generally the power capability of the resistor will be for ambient temperature of 25C. If the resistor heats up, its power dissipation has to be derated.
Check the data sheet; it's not uncommon to see the power spec derated above 25C to zero at 100C.
If the resistor is inside the fan base, that space will surely become hotter than 25C and the 20W must be derated.
For instance, if the fan base becomes say 40C, you may need to start out with a 30W resistor to handle 20W at 40C.
Just some example values - check the data sheet.
Jan
Check the data sheet; it's not uncommon to see the power spec derated above 25C to zero at 100C.
If the resistor is inside the fan base, that space will surely become hotter than 25C and the 20W must be derated.
For instance, if the fan base becomes say 40C, you may need to start out with a 30W resistor to handle 20W at 40C.
Just some example values - check the data sheet.
Jan
Would a buck transformer do the same job without heat?
Say 2x15V secondaries for full speed & 2 slower speeds.
Please post pictures of this vintage fan.
Say 2x15V secondaries for full speed & 2 slower speeds.
Please post pictures of this vintage fan.
Fan
And here is the AC motor. It only has two leads so the speeds are controlled by dropping the voltage.
And here is the AC motor. It only has two leads so the speeds are controlled by dropping the voltage.
AC induction motors are not very good at speed control - the mains frequency is fixed. Add too much slip and the current consumption and internal heating rises rapidly (depending on the rotor magnetic construction). Adding series resistance will limit the current, but that may lead to a tendancy to stall or not spin-up from standstill (higher currents are pulled at startup).
Note that at stall or during startup the current could be a lot more than the 1.3A in normal operation.
This is not a simple situation. The best way to slow down an induction motor is using a V/f drive, and you can still have over-heating issues at low speeds.
Erm, in theory yes, in practice no, that will run extremely hot - derate by at least 50% I suggest.
Note that at stall or during startup the current could be a lot more than the 1.3A in normal operation.
This is not a simple situation. The best way to slow down an induction motor is using a V/f drive, and you can still have over-heating issues at low speeds.
Use a bucking transformer to drop the suply voltage. Valve heater transformers are ideal for this sort of thing: switchable for 6.3 or 12.6Vac drrop, high current/ v. low output impedance, and widely-available. Put it in an earthed box, with a n outlet socket, and use that in-line from the wall socket.
It'll be a lot more efficient too.
It'll be a lot more efficient too.
I am not an expert on vintage fans but they used something like this:
Or this back then:
All that was stuffed in the fan's base and somehow worked without overheating the fan. I am hoping that a resistor will do the trick as I am trying to avoid having extra boxes on the power cord. I can get a 100W resistor just to be safe.
Also, I think each fan with multiple speeds started at the highest setting and then you could switch to a lower speed. This is I believe how they got around stalling issues when starting the fan.
Or this back then:
All that was stuffed in the fan's base and somehow worked without overheating the fan. I am hoping that a resistor will do the trick as I am trying to avoid having extra boxes on the power cord. I can get a 100W resistor just to be safe.
Also, I think each fan with multiple speeds started at the highest setting and then you could switch to a lower speed. This is I believe how they got around stalling issues when starting the fan.
