Rheostat...

[Eduardo Maun]

Can I use a rheostat to a Centrifugal Exhaust Fan? I want to control the speed of the fan.What would be the effect of using a rheostat?

Thank you!

Eduardo

 

[chris kennedy]

The fan data plate my say "Adjustable Speed". Contact the fan manufacture, they may spec. and or supply the device you need.

 

[Eduardo Maun]

Rheostat...

In the specifications there is an options and accesories for speed controllers,
What I'm trying to do is add a rheostat in series with the supplied voltage is this possible? What would be the effect by adding a rheostat?

 

[Dennis Alwon]

Eduardo--- If the fan is rated for a rheostat then you must match the rheostat for that fan. If the fan draws 5 amps you would need a controller for that fan that is suitable for at least 5 amps. Also the rheostat should be rated for fan use. Do not use a standard light switch rheostat as they function differently.

I hope this helps. As said before the manufacturer of the fan would be the best bet to contact to see if they have a specific controller that they specify.

 

[hillbilly]

You will need a large rheostat since it will be carrying the full current of the motor.
A better application would be a "Variac" resistor.
This is a variable transformer.

If the motor load is constant, as you decrease the voltage to the motor, the current will rise, so the rheostat would have to be sized for the motor current at it's highest load at it's lowest speed.

A rheostat might work Ok on a small centrifugal fan or pump, but it would be very inefficient.
Plus, you need to consider motor cooling because a standard motor will run hotter at lower speed, especially if the load requires constant horsepower.

What is the motor type and what are you pulling with it?

Just my opinion
steve

 

[rattus]

First, let us not confuse a rheostat which is a variable resistor, usually wirewound, with a dimmer switch type control which is electronic. A rheostat will dissipate more heat than an electronic control.

Second, dropping the voltage will lower the speed which lowers the load, and the current will fall.

Third, reactances in series have long been used to control the speed of buzz fans and celing fans.

Fourth, blowers used in HVAC systems usually control speed by switching in and out different motor windings. Maybe you just need a speed control switch.

 

[Eduardo Maun]

Rheostat...

Are you saying using a rheostat is not advisable for this system? Rattus suggest to use speed control switch...Can you give me other type of speed contoller? What about using a pulse width modulation?I tried to e-mail the manufaturer unfortunately up to this moment no one answer my inquiry about the product.

What is the motor type and what are you pulling with it?

I'm not sure about this but I presume this is a shunt motor, I believe shunt motor usually use in fan motor.

I will try to call/e-mail again the manufacturer.

Thank you much!!!

Eduardo

 

[rattus]

Nameplate info please:

Nameplate info please:

Eduardo,

We need the nameplate data from the motor before we can provide an answer to your question.

 

[hillbilly]

Second, dropping the voltage will lower the speed which lowers the load, and the current will fall.

That is not always true.
It depends on what the work requires...Constant Horsepower or Constant Torque.

At Constant Horsepower, as the motor slows, the torque increases and the horsepower remains the same.
The motor current will remain the same or (more likely) increase.

At Constant Torque, as the motor slows, the Horsepower decreases and the torque remains the same.
The motor current will decrease.

Eduardo....When you use the word "shunt", I think of a Direct Current motor.
Is the motor AC or DC?
What is on the motor nameplate?

If it's AC (which my first post assumed), you would be better off with a Inverter drive to control speed.
This type of drive regulates the frequency (hertz) of the electrical supply to the motor to vary it's speed.
The reason that I mentioned the Variac was because you suggested using a Rheostat for speed control.

Just my 2 cents.
steve

 

[zbang]

Are you saying using a rheostat is not advisable for this system?

A real rheostat (variable resistor, see wikipedia) is never advisable for, well, pretty much anything at this point. It lowers the voltage by dissipating part of it as heat, and for even a 2a load it can get mighty warm. It sounds like you're using the word "rheostat' to mean any kind of speed controller, which is confusing.

You probably want some sort of electronic motor speed controller, the type will depend on the motor to be controlled.

 

[rattus]

That is not always true.
It depends on what the work requires...Constant Horsepower or Constant Torque.

At Constant Horsepower, as the motor slows, the torque increases and the horsepower remains the same.
The motor current will remain the same or (more likely) increase.

At Constant Torque, as the motor slows, the Horsepower decreases and the torque remains the same.
The motor current will decrease.

steve

Hillbilly,

It is neither constant horsepower nor constant torque. Eduardo wants to lower the speed of the blower which means less power and less torque.

 

[don_resqcapt19]

Doesn't a standard motor try to maintain its speed based on the frequency and not the voltage? As the voltage goes down it can't maintain speed and the "slip" increases and motor heat goes up.
Don

 

[rattus]

Doesn't a standard motor try to maintain its speed based on the frequency and not the voltage? As the voltage goes down it can't maintain speed and the "slip" increases and motor heat goes up.
Don

Yes, the slip would increase, but the load is lightened considerably as the speed falls.

My experience with 240V fan motors running at 120V has shown no undue heat rise. You are probably right though that efficiency is lowered by running at less than rated voltage.

Maybe I will run a little test to see if I am right.

 

[rattus]

Test Results:

Test Results:

Doesn't a standard motor try to maintain its speed based on the frequency and not the voltage? As the voltage goes down it can't maintain speed and the "slip" increases and motor heat goes up.
Don

From a quick and dirty experiment:

1/4 hp, 1100 RPM, AC condenser fan, 208-230V, 1.8A:

At 240V, I = 1.74A

At 120V, I = 1.91A

Iron loss is reduced by 75%, copper loss increased by about 20%. (based on equivalent circuits)

Motor got warm but not hot.

I would not recommend dropping the voltage on such a motor. A multispeed motor as used in HVAC blowers should work just fine.

 

[tshea]

I would seriously look at the a variable speed drive. AB makes some small low cost units. Contact your supplier.

 

[rattus]

Maybe it is a shunt wound motor?

Maybe it is a shunt wound motor?

If this really is a shunt-wound DC motor, then the speed is controlled with the field current.

 

[Eduardo Maun]

Rheostat...

I will use the rheostat to lower the speed,does rheostat differ from a speed contoller? I know that rheostat can limit the supplied voltage and if the supplied voltage is decrease then the speed of the motor will be slower. Right or Wrong?

Here are some of the specifications I got from the nameplate - shunt-wound DC motor,1/3 hp, 240 volts, 50 hz and 1451 rpm,I can't read all the spec. I hope this will help.

I know efficiency of the motor will drop.Base on this "current drawn by the motor increase as the load on the motor increase so if the current increase beyond it's rated current creates heat which in turns to destroy/burn the motor.I fear the motor will burn this is my concern Or it will have a minimal effect by using rheostat?


Again thank you to all...by answering my question(s) I'm able to learn more, enhance my knowledge and skills thru your valuable help and have the confidence to share it to my colleague(s).

God Bless to all of you!!!

Eduardo

 

[hillbilly]

Since you do have a DC motor, you will have to have a source of DC power to supply the motor.....it will not run on Alternating current.

The (armature) voltage on the motor is 240 volts DC, and the field is more than likely supplied by permanent magnets (you need to confirm this) built into the motor.
If not, you will also need a small (constant) DC power supply for the field windings.

If I were you, I would purchase a small (packaged) DC drive to power the motor.

One to power the motor that you describe is not expensive.
It will include everything that you need to run the motor.

You can't just power the motor with 240 volt AC power thru a rheostat....it will not work.

Hope this helps.
steve

 

[rattus]

There should be a resistor somewhere which limits the field current. This resistor might be replaced with a rheostat. Motor speed is an inverse function of field current though, and this may not yield the desired results.

You need some on-site help with this problem.