PWM for a 1.5 HP motor

Discussion in 'The Projects Forum' started by G8erB8, Aug 24, 2010.

  1. G8erB8

    Thread Starter New Member

    Aug 24, 2010
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    Hello All,

    I am looking to make a whole house fan. I have a 1.5hp 115V single phase motor. Nominal rpm is 3600. I have fan blades for it from a 220v 1/10hp that is 800 rpm. I would like to control the 1.5hp motor to a max of 1000rpm, but also be able to have a low speed (maybe 300rpm).

    I would like to do this using a PIC controlling a relay that would open and close as necessary for the PWM. I don't have any experience using any of this, so opinions on the setup would be appreciated.

    It seems as if a PIC would have no problem switching power on and off to the relay quick enough, but would the relay respond fast enough? I will have to test, but I think it will have to cycle maybe 5 to 10 times a second to control the speed properly. Is a solid state relay the best choice or is there another type that I'm not aware of that would work better?

    Other issues I have to work through is that this motor has start and run capacitors. Obviously I have to remove the run capacitor, but I worry about amp draw when the relay switches on. Shouldn't be too much with the shaft free spinning already. Since I'm slowing the motor down, I may have to have another relay disable the start capacitor because the centrifugal switch that disconnects the start capacitor doesn't engage till about 75% of rated speed (from what I've read anyway).

    Goals right now are to just have a low, medium, and high option. Future plans include a temp sensor to automatically start the fan. I'm open to other ways of controlling the speed, but PWM is the best I've found so far. And resistors would waste way to much energy with this motor. I'll worry about what PIC to use after I've settled on the speed control issue.

    Thanks in advance, Jerry
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    You can't efficiently control an AC motor like that via PWM. You'll burn it up running it on the start cap.

    If you want to have three speeds with that motor, you'll need some kind of transmission, like stepped pulleys and a V-belt (preferred) or a chain drive like a 5-speed bicycle. Of course, that means you will have to manually move the V-belt or the chain position.

    You can't alter the frequency of the AC signal, either - unless the motor is specifically designed for that kind of operation. Your motor was most likely designed for operation in the range of 57Hz - 63Hz. If you tried to increase the frequency to increase the speed, you would lose torque. If you tried to decrease the frequency to decrease the speed, you would gain torque, but current would be excessive and the motor would overheat.
     
  3. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Find a dead window air conditioner that the fan part still works on and was multi-speed.
    They show up on craigslist from time to time as free.
     
  4. G8erB8

    Thread Starter New Member

    Aug 24, 2010
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    I'm looking to move a large volume of air, so I don't think the air conditioner fan will cut it. I actually have one sitting in my basement.

    Sgt.
    I wouldn't think I'm changing the frequency by switching power on and off. It would still be 60hz. Do you mean it is the inrush amps when power is switched back on that would overheat the motor? What if I use a single pulley to slow the max rpm at 100% motor speed to 1000rpm max blade speed, then use the relay to slow the motor for the low speed. Motor rpm would vary between 3600 and maybe 2000. And use another mechanical relay to disable the start cap circuit after it starts so that the lower speed doesn't re-energize the circuit?

    Thanks very much for the responses so far!
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Your idea will result in a burned-up motor, after lots of surges on your service panel.

    You may very well blow the circuit breaker before the motor burns up. Either that, or the motor's internal thermal breaker will cause the motor to run in your "hobbled start" mode for perhaps 30 seconds, and then stop completely until it has cooled off.

    The motor was designed to operate with the start cap until the RPMs are high enough so that it can use the run cap; there is a centrifugal switch inside one of the motors' end bearing supports that swaps the caps in and out.

    If you want to run the fan at a lower speed than your existing motor can provide, you will need to use a speed reduction unit of some sort, or select another motor that is capable of variable speed drive.
     
  6. marshallf3

    Well-Known Member

    Jul 26, 2010
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    I was going on your mention of the fan blade you described, I guess this means you're wanting to drive multiple blades?

    Just a hint - if the blade was designed to operate at 800 RPM it's not going to double in output at 1,600, matter of fact it may end up putting out less.
     
  7. G8erB8

    Thread Starter New Member

    Aug 24, 2010
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    The blades are a 24" dual bade off a 208/230v 1/10th hp industrial fan. Probably works too, I just don't want to run 220 service to my attic as it already has 110. I wanted to reduce the motor speed to keep the blades a 800rpm max to maybe 200rpm min.

    Thank you sgt. I'm thinking now of keeping my eyes open at work for some 1/6th hp single phase motors and using a multiple blade setup. I'll save the 1.5hp unit for the gate opener I'm planning.
    Is there a cheap single phase to 3 phase vfd unit out there? All I've seen are $400 or more. I can get just about any hp 3 phase motor from work free, but we don't use single phase that often. And if we did, they'd be start cap run cap's too.
     
  8. windoze killa

    AAC Fanatic!

    Feb 23, 2006
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    Here is another thought. You could try using a variac to alter the voltage. You could control the variac a micro controlling a servo motor that adjusts the variac. You won't want to adjust it too far as once again you may burn out the motor.
     
  9. marshallf3

    Well-Known Member

    Jul 26, 2010
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    There's a seller on eBay that turns up with stuff like this from time to time. I got a brand new Hitachi 10 HP 480V one from him for about $125 but it was 3 phase in & out.
     
  10. Andrew Leigh

    Well-Known Member

    Sep 8, 2008
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    I would check the tip speed of the blades, at 800rpm it seems very high.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    That wouldn't work very well either.

    Once the motor fell below a certain RPM, the centrifugal switch would drop out, changing over to the start cap.

    Our O.P. really needs a different type of motor if they want to run it at different speeds.
     
  12. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Not to mention the increase in reliabilitiy. I'm not a big "fan" of those motors with the centrifugal switches.

    Hate to say it but eBay can be a source as can surplus houses, flea markets, garage sales and even thrift stores. I once scored a number of 1 HP Dayton motors from a thrift store for next to nothing as they had no clue what they were much less what they were worth.
     
  13. G8erB8

    Thread Starter New Member

    Aug 24, 2010
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    Yea, I'll get a 3 phase from work. We go through a ton. Pump's will come back for warranty issues and the perfectly good motor gets scrapped.

    Found a nice site for cheap VFD's!
    http://dealerselectric.com/
     
  14. marshallf3

    Well-Known Member

    Jul 26, 2010
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    I think that's the guy I got mine from on eBay, if so he's got used stuff too.
     
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