Rheostat Question

[MarkV]

Happy New Year everyone. I have a fireplace insert with a built in blower that is controlled by a rheostat. What I would like is for the blower to run at about half the speed of its current lowest setting. Can I add another plug in rheostat at the wall socket to reduce the fan speed? Are there better ways to do this?

Thanks,
MarkV

 

[kenstrac]

Not knowing if you have AC or DC mtr I would not since some motors must turn at enough RPM to cool themselves.
You could restrict either inlet or outlet to reduce CFM,but if MFG. has set a certain min. limit that the unit has to run to safely dissipate the heat and not cause any overheating or OT shutdowns I would not do it without contacting the MFG.

 

[Redneck in training]

What you call rheostat is most likely electronic regulator. It controls the time (several times per second) the power is applied to the motor. If you want to lower the volume of air you can throttle the suction side of the blower. In example use a piece of sheet metal and inserted behing the suction grid. Another solution would be a thermostat that would switch the fan on and off based on the temperature in the room.

 

[kenstrac]

As I said in my initial post I would NOT do anything until you contact the MFG of the unit,if you lower the CFM the unit might overheat causing buckling of the steel or weakening the welds or overheating some other component.

 

[MarkV]

Thanks for the replies. I have no idea if the motor is AC or DC. It isn't in a place that is easy to look at. I hadn't thought about the high speed to dissipate heat in the motor. Given that it is part of a wood stove insert that is logical. Guess I'll leave it alone. The darn thing is like my leaf blower the way it runs now though. Besides the noise level it moves enough air over the iron insert that it brings the burn temps down below the level we like them to stay.

MarkV

 

[heehaw]

its really been a long time since i did this: but at one time i wired a light socket in series with a small motor: then the larger the light buld, the faster the motor would turn..to slow it down, put in a lower wattage bulb..cheap to try..
heehaw

 

[Redneck in training]

Thanks for the replies. I have no idea if the motor is AC or DC. It isn't in a place that is easy to look at. I hadn't thought about the high speed to dissipate heat in the motor. Given that it is part of a wood stove insert that is logical. Guess I'll leave it alone. The darn thing is like my leaf blower the way it runs now though. Besides the noise level it moves enough air over the iron insert that it brings the burn temps down below the level we like them to stay.

MarkV

There is almost certainly induction AC motor.
I wouldn't worry about heat dissipation in the motor. Here is why:
The relationship between fan speed and required power from the motor to achieve it is cubic. To put it in plain English to double the speed of the fan the power has to be increased eight times. In other words if you half the speed the amps will be one eights of original value. Since the heat generation in the motor is function of the motor current as it slows down there will be less heat to remove from the motor. Beside that most motors are thermaly protected by a special component embeded in the winding.

 

[Soundguy]

What you call rheostat is most likely electronic regulator. It controls the time (several times per second) the power is applied to the motor. If you want to lower the volume of air you can throttle the suction side of the blower. In example use a piece of sheet metal and inserted behing the suction grid. Another solution would be a thermostat that would switch the fan on and off based on the temperature in the room.

You are refering to a pulse width modulator. Mind you.. a PWM circuit that is variable probably does use a VR/POT or rheostat ( it's a nomenclature/use thing )..

I wouldn't replace / ad din another resistor unless I saw the circuit to see what they were doing... I'd find it quite odd if the motor speed control was something crude like serial resistance and a motor.. vs a pwm.. or similar variable speed control..e tc. perhaps a triac circuit similar to a dimmer.. etc..

soundguy

 

[Redneck in training]

Probably the best solution would be ceiling fan speed regulator, but if your motor is much smaller (by power) than typical ceiling fan you might be better off using a light dimmer but you have to use one that is designed for low voltage halogen light with transformer. Those are designed to work with inductive load and have DC current protection (They produce symetrical waveform) othervise DC might heat up and burn your motor.

 

[kenstrac]

The OP never said if this is a gas,wood or pellet insert,going under that assumption I gave what I thought was a reasonable answer.To start adding light bulbs to the circuit could damage a PC board, to change the CFM of the unit without contacting the MFG.IMO is asking for trouble.
In contacting the MFG. he will be able to determine if the rheostat is functioning properly if he can lower the speed of the fan or any updates they might have to quiet the fan down.
We are talking about a heating appliance and safety is the #1 concern here.