1/2 hp blower motor on rheostat

[jdou]

I have connected a 120v 1/2 hp 1350rpm 5.5a motor for a rooftop exhaust fan. I have a disconnecting means at the motor on the roof. I was going to put a 20a single pole switch as the controller in the lease space, but the ac installer discussed with the customer to put it on a rheostat to control the cfm's. I haven't really read the literature on the fan motor but looked at the nomenclature and did not see anything readily apparent about varying the speed of the motor. I recommended to the customer to install dampers in the duct as opposed to varying the speed of the motor as from what I see the motor is designed to operate at 1350rpm and there may be risk to the motor to do otherwise. Is there anything I can look for that would indicate using a rheostat safely on the motor? Thanks

 

[tom baker]

Speed control is almost always done with a VFD today and not a rheostat. Both dampers and rheostats are inefficent and waste power.
If you cut the speed of the motor in half the energy useage is cut by 1/8, as its a cube function.

 

[jdou]

Ok, the energy useage isn't really the issue to the customer, the fan is venting heat from a dryer used to cure ink on teeshirts in a screenprinting shop. The concern is venting too much heat too fast not allowing the ink to cure. But in the interest of the motor and safety............I almost think with the heat rising naturally no need for a fan but the customer doesn't see it that way. I don't know about a vfd, or cost involved. Thanks

 

[GeorgeB]

Speed control is almost always done with a VFD today and not a rheostat.

That sounds like a single phase motor ... has to reach a speed to drop out starting windings ... don't think speed control is practical.

Both dampers and rheostats are inefficient and waste power.

You might want to review the damper comment ... I have been told by HVAC guys that you are incorrect.

If you cut the speed of the motor in half the energy usage is cut by 1/8, as its a cube function.

That depends, I believe, on the type of blower. A few are linear, a few roughly square (1/4), some indeed are about cube.

 

[sgunsel]

It's not the motor, it's the load that determines how much HP is required. Some loads may be linear, but fans are not. A fan load is a function of pressure as well as flow rate. All other things remaining unchanged, halving the air flow will reduce power consumption by a factor of 8. Similarly, doubling the air flow takes 8 times as much horse power. Installing bigger pulleys on a fan can easily burn up a motor once the motor's rated HP is exceeded.

 

[Jraef]

Applying a "rheostat" to a single phase motor is all about the TYPE of single phase motor.

If it's a "Shaded Pole" motor, you can apply a dimmer to it and it will reduce the speed. Most interior ceiling "paddle fans" use Shaded Pole motors, as do most table top and consumer pedestal fans. I don't know about roof top exhaust fans though. By the way, most dimmers on the market now are no longer rheostats, they are triac based (high speed SCR switches). Makes no difference really, but rheostats used to get extremely how inside of the box, triac dimmers not so much.

If it's a "Universal" motor, (AC/DC, similar to what is in your hand drill), then it will work as well, but I have never seen an exhaust fan that uses that kind of motor.

If it's a Capacitor Start or Cap-Start / Cap-Run motor, varying the voltage will only result in smoking the motor. There is no way to vary the speed on those.

If it is a "Permanent Split Capacitor (PSC) motor, you can buy 1 phase VFDs for those, but they are usually not worth the money for what you get out of it. The 1 phase VFD will likely cost you more than a new motor, and if that's the case, buy a 3 phase motor and use a VFD that takes 1 phase input and gives you a 3 phase output. That motor/drive combo would be more reliable.