Is there a rule of Thumb Here?

1parkpointer

I have a Dayton 3LU83G, .5-hp direct-drive blower motor that will be connected to a wood furnace blower housing. It is rated at 6.3A-125VAC no load 1075-rpm with 4-speeds. The manufacturer says the rpm's are: low at @625, med low at @775, med hi at @925, and hi at 1075-rpm's.
I would like to install a switch to control speeds and found a Grayhill rotary model #19101-7UL rated at 15A; would it be sufficient to operate this motor? The first one I found is only rated at 6A maximum, and I think it is too small, especially at start up.

Wildie

I think that you have answered your own question!
Unloaded the motor draws more than 6 amps. Obviously, the 6 amp switch won't cut it!

nap

the switch has to be HP rated. It will state right on the switch the HP rating it is designed to handle.

1parkpointer

nap,
I see your point. The switch that I got was too small. After looking around at various switches on the web, I found one at a manufacturer site, only to learn that they don't sell to individuals. After talking to a reseller, the price jumped from $30 to $48 for one rotary switch.
I've gone back to my original plan. Four toggle switches, one for each speed. Makes more sense, they were certainly less expensive, and they are motor rated. Thanks for the input

Yoyizit

Try a small rotary switch and four relays. That way no one can ruin the motor and the switch will be easier to find. The rotary switch should be break-before-make.
http://www.google.com/search?hl=en&q=rotary+switch+4+position&aq=4&oq=%2 2rotary+switch%22

You'll also need a 24vac xformer that can deliver enough current to drive one relay coil at a time.
http://www.hosfelt.com/
xformer, pn 56-915, $1.95
relay, pn 45-640, $1.75 ea. You'll need four.

In general, pick your rotary switch first.
Then find a relay with contacts that can handle the motor load.
Then the relay coil should pull current small enough that the rotary switch can handle. The relay coil can be AC or DC, with any of a number of voltages.
Then pick a wall xformer that can drive the relay coil. Hosfelt calls them "adaptors".
Iterate this process until you find the minimum cost.
Since you're not going to change motor speeds very often I don't think contact arcing is going to be a problem.

micromind

Be careful with toggles, if more than one is on at a time, it'll burn the motor up in just a few minutes.

If you're using this motor for a fan, and the fan loads it to its rated HP at full speed, then the lower speeds will work. If the motor isn't loaded much, the lower speeds will be about the same as the highest speed.

Rob

kbsparky

Simply use 3-way switches in series then. Switching any one of them on will disconnect the others preventing more than one winding from being energized at a time.

In this configuration, the higher speed overrides any lower speed.

EDIT TO ADD: A Single pole switch would be sufficient on the last switch.

Attached Thumbnails

 

Yoyizit

Yes, electrically simpler than my idea. But for four speeds you only need 3 switches (with no "off" position).

wirenut1110

My logic may not be right on this but, is it possible just to wire the high speed to a fan speed control switch?