We have a 1/2 HP 120/240V motor for a exhaust fan on the roof. There saying the CFM should be more on there tester. Right now we have the 120V wired on the Low Voltage side of the motor. Anything we can do to change the speed to get more CFM? thank you for your help.
can you change the speed of a motor?
- Thread starter zappy
- Start date
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- Occupation
- Licensed Electrician
Adjust the pulley width. You can't change speeds on that motor.
rambojoe
Senior Member
- Location
- phoenix az
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- Wireman
Wether or not the system is working (vents,makeup air).. Change the pulley ratios.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Increasing the speed of the fan blades will result in an crease in current drawn. How loaded is the motor?
rambojoe
Senior Member
- Location
- phoenix az
- Occupation
- Wireman
It looks like the output is already maxed...
rambojoe
Senior Member
- Location
- phoenix az
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- Wireman
Thats why id look at efficiency of it all which aint our job...
ptonsparky
Tom
- Occupation
- EC - retired
Rarely did I tell a customer it wasn't my job.
rambojoe
Senior Member
- Location
- phoenix az
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- Wireman
If you are a tin knocker then have at it. Especially if you keep air flow testers in your truck.. Im not sure what your comment means... But i understand the implication.
Or is there some other problem - restricted duct work from being too long, to many angles, too small a diameter, obstruction, clogged filter, ... Obstructions that reduce air flow should reduce load on the motor, so that would be one clue that ducting is a problem. More air flow needs more power.
If the motor current is near its rated current, then the fan is probably moving all the air it can. If you need more flow, you need a larger fan with a larger motor.
If the motor current is near its rated current, then the fan is probably moving all the air it can. If you need more flow, you need a larger fan with a larger motor.
hillbilly1
Senior Member
- Location
- North Georgia mountains
- Occupation
- Owner/electrical contractor
I have changed out to a larger motor and pulley increasing the speed but the customers engineer specified the size and pulley.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Exactly!
If current were low, it could be a duct restriction, and changing fan speed would not necessarily help and would draw more power.
Only if the motor is underloaded would I suggest a ratio change.
Is there a design document for the system that leads them to expect more airflow, or are they just saying that the downstream operation needs more air?
Note that a serious obstruction, that cut the airflow, but, say, a factor of 3, would also dramatically reduce the power drawn by the motor.
Positive displacement pumps are a whole different story.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Be sure the fan is turning in the correct direction; it makes a difference
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
For a bladed fan, as opposed to a squirrel cage impeller, the difference is enormous!
Not enough info in the pictures to tell for sure which it is.
ptonsparky
Tom
- Occupation
- EC - retired
Being able to tell the customer what is wrong and how it needs to be addressed is a plus. Even if you need to pass on the project yourself.
I got a lot of electrical calls that were anything but. Most of us do.
One of my help fixed an elderly customers oven door because he noticed she had to hold it shut with a chair. That got us more points than what he was there for.
hillbilly1
Senior Member
- Location
- North Georgia mountains
- Occupation
- Owner/electrical contractor
I think you may be on to something, It does look like the motor has been changed recently, they may have ordered and installed the wrong motor, or if it can be field changed, was not noticed.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
To the OP's original question, most (all?) single speed AC motors' spin rate is determined by the frequency of the AC power supply, so unless you can change that you cannot change the speed of the motor.
Aside: Many years ago a friend of mine ran a small recording studio, and he wanted to vary the tape speed through his TEAC four track machine. He got a large solid state bass amplifier and an oscillator, and he wired the AC input of the tape machine to the speaker output of the amp. He turned up the volume on the amp until the voltage was 120V; he monitored the frequency of the oscillator with a Conn Strobotuner and the AC voltage on the speaker output with a DVM.
It worked.
Aside: Many years ago a friend of mine ran a small recording studio, and he wanted to vary the tape speed through his TEAC four track machine. He got a large solid state bass amplifier and an oscillator, and he wired the AC input of the tape machine to the speaker output of the amp. He turned up the volume on the amp until the voltage was 120V; he monitored the frequency of the oscillator with a Conn Strobotuner and the AC voltage on the speaker output with a DVM.
It worked.
hillbilly1
Senior Member
- Location
- North Georgia mountains
- Occupation
- Owner/electrical contractor
Will the op return? Or just leave us hanging? Like so many others! LOL!
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