Two speed furnace motor

[bigrockk]

Hello all
I have a question about motor connections.
I would like to convert a furnace with a single speed blower to a two-speed blower.
The furnace is old and there are no existing controls for two speeds.

The new motor is a 1/3-1/10H.P.
RPM 1725/1140
VOLTS - 115
AMPS - 6.7 / 3.3

The new motor has terminals for Common, Hi, and low speed

I was thinking about mounting a motor rated switch on the furnace to energize the low terminal on the motor (so the fan runs continuously on low when the furnace is not calling for heat) and using the N.C. contacts on a 24v relay (coil would be energized by the thermostat when it is calling for heat) to open the circuit to the "low" switch when the furnace calls for heat. I thought this would be the best way as there would be a bit of a time delay (while the plenum warms up) before the furnace "kicks" into high speed. This way the high and low terminals on the motor would never be energized at the same time.

Now what I need to know is do I really need the relay or can the Hi and low terminals on this type of motor be energized simultaneously without damaging the motor or
must a relay be used to de-energize the low terminal when the high speed terminal is energized.
Appreciate all reply's!

[CRM]

Bigrockk, I believe you should use the relay to separate the low and high windings. I was once on a service call to check out a burned out motor on a furnace with a 2 speed motor, I replaced the burned out motor with a new one and when I checked out the current draw on new motor, it was way to high, the problem was the low and high windings were energised at the same time, so a relay had to be installed, the original motor only lasted a couple of months, the new one has been working for years since then.

[bigrockk]

Thanks for the quick reply CRM
I kind of suspected I would have to use the relay but I just wasn't sure how to tell
what type of motor I had.
I remember reading something about two types of two speed motors where one has two distinct separate sets of windings and they can not be energized at the same time and another type which has a winding I think was called a "booster winding and that type could have both terminals energized at the same time. I am not sure how to tell the difference in the two motors or maybe I'm just way off base??
Any way I think I will go with the relay,
Thanks again.

[bigrockk]

I was thinking about this last night and realized that wiring the relay across the thermostat is a really bad idea. When the thermostat is satisfied for heat it would drop out the relay and both the high and low terminals on the motor would be energized until the plenum cooled enough to satisfy the stack switch. Also I am not sure if the thermostat could handle the load of the relay.
So I guess I will use a 120v relay energized from the stack switch so whenever the furnace is calling for the fan to run, the low terminal on the motor will be de-energized.I don't have a lot of experience with furnaces so....
Appreciate any comments.

[bigrockk]

In this particular case they don't want to be able to control the fan from the thermostat as the existing wiring is only two wire and they don't want to upgrade.
All they want is a switch on the side of the furnace so the fan will run on low (when the thermostat is not calling for heat) continuously all winter.
They have installed a wood stove and want the furnace to circulate the air to distribute the heat thru-out the house.
Thanks for the reply TWH

[twh]

You need two relays - one controlled by furnace temp and the other by the thermostat 'G'. Two are necessary because when the thermostat fan switch is in the auto position, 'Y' and 'G' are connected.

Connect the relays so that when both are de-energized, the low speed winding is powered; and, when either is energized, the low speed is disconnected and the high speed winding is powered.