8 replies to this topic

[freudent]

When adding refrigerated AC to my furnace, I replaced the worn & slow belt-drive blower with an exact-fit 3-speed direct drive blower from grainger. Unfortunately, this blower does not have a slow enough speed to prevent a "wooshing" in some of the return registers near to the furnace (I find this remarkable since the blower is an _exact_ fit for the old mounting) and the duct system is constant area all the way back to the 22x24 furnace opening). Is it reasonable to use an electronic speed control with this motor (can you recommend one) or do you recommend an alternate approach? Must I add more returns?

Specs of relevant parts are at the end of this note.

Also: I left the existing foam seal where the blower output extends through the rectangular hole to the heating chamber. Can this be replaced? Where can it be obtained? (also checking: Since the new blower has rubber isolators in its motor mount & the old blower's rubber isolators were decayed, I mounted the new blower upon non-elastic spacers -- a stack of washers -- I hear no hum so assume this is ok)

Blower=grainger 5c97d (lookup on grainger.com)
wheel=12 5/8 x 9 1/2,
opening=13 7/16 x 12 1/4
Motor: 3 speed, rated current=14A @ 120V
1075 RPM, 1HP
Refrigeration capacity: 3T
Furnace: an old and servicable york cimaster 120k BTU

Thanks, Eric

[marke]

Hello freudent
Welcome to the forum.
I assume from the information posted, that this is a single phase motor.
Unfortunately, it is not easy to control the speed of a single phase motor. If it were possible, I would recommend that you replace the motor with a three phase motor and use a single phase input three phase output inverter. These are very common and relatively cheap. You would then have total control over the fan speed.
Some fan motors can be controlled by variable voltage controllers. This will not directly control the speed, but will control the torque available. Reducing the torque on a fan, will cause the fan to slow down.
You must not apply a variable voltage controller to a standard induction motor.
Best regards,

[marke]

"You must not apply a variable voltage controller to a standard induction motor""

A standard induction motor is not designed to slip very much, infact it is designed to have a low slip. If you reduce the voltage only, you will reduce the torque to a point where the motor will stall. Under these conditions, the motor may run at redduced speed, but the current will be hihg and the slip losses will be very high, you will damage the motor.
If you use a high slip motor, you can vary the speed with voltage control.
To alter the speed of a standard induction motor, you must vary the voltage and the frequency.
Best regards,

[jraef]

No you cannot do that.
1) No Variable Frequency Drive will allow you to attach only 1 phase to the output. It will trip off on Output Phase Loss immediately, and no drive on the market that I know of allows you to defeat that protection.
2) 120V single phase motors are almost always going to be capacitor start because the capacitors are needed to make the motor start tunring (3 phase motors turn on their own because there is a rotating magnetic field from phase to phase). They have a centrifugal speed switch in them that takes out the capacitors withing miliseconds as the motor speed increases. If a VFD is put on them, the speed does not increase fast enough and the capacitors are left in the circuit too long, damaging the motor, start switch, capacitors and the VFD! Other than that, no problem...

Most likely the reason for the "whooshing" on your intake registers is that this motor is starting at a higher speed than the original. Are you sure you are starting at the lowest speed?

[jraef]

OK, I just tried looking for that blower. Grainger.com indicated that was not a good number, but I looked at several of their other direct drive blowers such as the 5C197 and 5C198. They are both PSC (Permanent Split Capacitor) type motors, which means you may be in luck (sort of). PSC is a type of motor that can have a VFD on it because the capacitors are imbedded in the motor and have the inductance of the windings in series at all times. Not to bore you with technical details, but it means the caps will not cause harm to the VFD and vice-versa. There is a 1 phase VFD on the market that can be used with PSC motors (only) made by Anacon Drives. I do not know how you can buy them retail, but you can find them on the web and contact them for that. You can also use a 1 phase soft starter on PSC motors. Baldor is one of the few remaining manufacturers of 1 phase soft starters that I know of.
http://www.baldor.co.....ols_SoftStart

PS: That link is too long for the format of this form, so you need to copy and paste it into your browser.

[jraef]

Yes, that is the one I mentioned for 1 phase motors. Mind you, it MUST be a PSC motor, any other type will cause a failure. It can also technically be used on a "shaded pole" motor, but then again that can also be controlled by a dimmer switch, so the VFD becomes a little spendy for that.

I just remembered that someone else showed me another option. This product also controls 1 phase PSC motors. I don't know anything about them, just that they exist as an alternatie to Anacon.

http://www.hoffmanco...ducts.php?ID=23

The Anacon drive can also be purchased through another comany as a brand-label. http://bardac.com/pa...optidrive7.html