Most of the variable speed motors I've seen are actually 3 phase AC. The control board is an inverter that generates the 3 phase (much easier to control a 3 phase motor). Pretty hard for a do-it-yourselfer to fabricate a home-brew board. The motor your talking about sounds like a basic 3 speed single phase AC motor (common lead and high/med/low taps). If the air handler specs don't say variable speed, it's most likely not...only 3 speed selectable since variable speed is something they can charge you more for. Also the 14 degree temp seems quite low for an air-coupled heat pump. Even here in Florida my old house's heat pump would shift to the backup heat below 25 degrees.

If you were able to find a new control board, motor, module, wiring harnesses, etc., the price would exceed having a new unit installed.

Thanks. Three phase? I hadn't considered that. Is "inverter" the correct term? Certainly a possibility, my point was, if it's just a question of DC, I can probably manage that. I'm not sure what demands would be expected of a variable speed motor, as you say mine is probably 3 speed, I've never heard the speed vary, so it's probably not variable in that sense.

My thought was, if only one speed is required at a time, but it has to be a dc motor, it's a pretty simple job to construct a separate board that rectifies the current to dc, (with or without filter cap) and use an ac-capable relay to switch the dc motor on when there's a call for a fan. If one must supply different voltages to the motor for different speeds (and it's a manually selected thing) again that can be done, with the right transformer, all dependent on the kind of current required. I have a lot of components on hand.

I'm told my electric bills would be huge if my blower motor was a/c and I ran it all the time, I do and the bills aren't bad...but electricity here in BC is relatively cheap.

I'll have to see if there's more specs available, or talk to Bryant. Obviously the thing CAN work with a heat pump as the literature discusses it, but the govt wants us to have that variable speed dc motor.

Gary, would that A/C motor be low voltage, or 120vac? The quick look I gave to the wiring seems to suggest:

Two lines supply the control board from a transformer. There doesn't seem to be any other supply to the motor as its harness runs from the same board, so the motor is either dc or an ac voltage lower than the line current. I haven't measured what the ac supply to the board is. I didn't notice any large-scale voltage regulators on the board so I assume most of it runs from whatever that primary supply is. I'm guessing 24v? That's kind of the furnace standard, isn't it?

I realise the backup heat kicks in when it gets colder, -10C/14F is USUALLY as cold as it gets, there may be a week or so of -24C/-11F (didn't happen at all this winter) but for *most* of the time, the daytime temps are quite close to 0C/32F, so I believe the heat pump would run most of the time. Parts of Jan and most of Feb would be the only significantly cold times, going on the last couple of winters.

Since having A/C would be a big drawcard on our house (hot albeit short summers here and almost everyone has it) for resale, I'd like to add it, I don't think there's any subsidy for regular a/c but a generous one for heat pumps, seems like a no-brainer to me.

PJB

No doubt your furnace blower motor is 120V ac.

The transformer is used for the control circuit, safeties, T-stat and gas valve.

If you had a variable speed blower there would be more like 15 wires going to it. In addition variable speed blower furnaces will have 2, 3 or variable stage gas valves, matched to a multi-stage stat, communicating stat or a time delay before firing 2nd or 3rd stage.

With regards to speed, it would be difficult to distinguish between two speeds next to each other. White is neutral, Black is high speed, blue is medium, and red is low. On a standard 9-10" wheel, these speed selection changes typically vary the airflow by 100-150 CFM per speed selection.

Normally the AC speed is one step higher than the heat.
Heat is set up using recommended temperature rise across the heat exchanger. Cooling speed is typically set at 20 degrees drop across the coil.

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zl700....All the variable speed motors I've seen so far only have 3 wires for 3 phase operation. You've seen a 15 wire motor??? The control boards I've examined seem to have a simple rectifier circuit to provide DC from the incoming single phase AC then a 3 phase inverter to provide a variable frequency 3 phase output to allow blower motor soft start and stop as well as speeds that are usually selected by DIP swithches on the board.
pjb...My old house had a variable speed air handler which given the smaller size of my house was good in that it minimized noise when the system started up. My new house has the air handler in the garage and noise isn't an issue. I really don't think there's a measurable savings realized with the variable speed. As ZL states, the important thing is the temp differential across the heat/cool device and you can achieve that with a simple multi-tap motor.

Thanks Gary and zl700, I am perfectly happy with the motor as it runs, although I understand dc blowers are cheaper to run, however, in order to keep the existing furnace as an air handler and back up heat to a new heat pump, and avail myself of the BC government/CDN fed govt rebates, the current furnace/air handler needs to have a dc variable speed motor.

Of course, I don't know much about HVAC but am learning, but wondered myself what a difference it made.

I haven't looked exhaustively and the circuit diagram I've seen is rather oversimplified but I think it's got to be lower voltage ac at the least (or some form of pseudo 3 phase) because there are only *two* lines running to the control board, and they run from the transformer, and the blower motor leads affix to same control board, therefore as far as I can see, it's not 120v. That being said, the supply wires are quite thin, and the wires running to the blower are pretty heavy. There could be something I've missed, but I didn't see it.

Sorry if I confused- it would depend on whether the VFD control module is integral with the motor or not, depending on the manufacturer. And in fact one diagram I checked it was 12 wires.

PJB, follow the line voltage wires feeding the unit. You will see the nuetrals (white) all on one block on the board and the power going through the door safety switch going to the board that powers the TX and blower via a relay on the board. I bet your blower leads are connected to HT, CL and one marked park? Park is the unused blower tap speed. A volt meter to nuetral or ground and one of these terminals will read 120VAC while operating.

I'm not sure how Canada works but, in the US when we had Energy Star tax credits available, the furnace model must be listed on the official listing and install date within a certain time frame was required to get the VS blower credit. Merely converting, especially individually wouldnt count.

FYI, there is one manufacturer, Nordyne that labels Maytag, Frigidaire, Westinghouse, Tappan, Kelvinator and a few others that had available and sold variable speed, complete slide in blower assemblies for field conversion. Nice concept for energy savings but did nothing for the heat inputs.

My experience has been using both Trane and Carrier equipment energy comparison programs has shown that a VS blower operates at less than 25% of a standard comparably sized PSC blower motor.

There was a time where every model A/H or furnace required a factory programmed and matched motor and module. What a headache for stocking parts distributors, which resulted in parts acquisition problems for techs in the past. That has gone away with simpler field programmable units.

Below is a sequence of operation so, a proper ECM setup senses what the blower compartment is doing along with AC/Ht input/output. Merely driving the motor is not just the only thing that happens. VS units are capable of adjusting to airflow changes to maintain proper AC airflow and temp rise during heat.

Here is a simple explanation on how the work from GE:

The G.E. Electronically Commutated Motor (ECM) shown in figure 3 is different than previous generations of variable
speed blower motors. This motor has all of the capabilities of previous ECM blower motors but it does not have to be
pre-programmed at the factory. Instead the variable speed furnace control programs the ECM blower motor upon
application of power to the ECM blower motor via the serial communication link between the variable speed furnace
control and the ECM blower motor. This eliminates the need for a multitude of different ECM blower motors where each
one is programmed for one specific furnace model. The ECM blower motor is energized with 115-vac whenever power is
available at the variable speed furnace control, but operates only when the variable speed furnace control communicates
the proper serial commands to the ECM blower motor at PL13.
The ECM blower motor is first fed 115-vac power through the 5-pin connector PL14. The 115-vac power is then rectified
to DC by a diode module. After rectification, the DC signal is electronically commutated and fed in sequential order to 3
stator windings. The frequency of commutation pulses determines motor speed.
Setting up the desired airflow CFM for an ECM blower motor is done by setting the A/C or CF selections on the variable
speed furnace control shown in figure 1. The ECM blower motor then delivers requested airflow CFM as defined by serial
commands received from the variable speed furnace control.
The ECM blower motor is configured via the variable speed furnace control to react to changes in system static pressures
to maintain constant airflow CFM. The ECM blower motor delivers requested airflow CFM up to about 1.0 in.w.c. of static
pressure for most airflow settings. The variable speed furnace control is pre-programmed and contains all the
information relative to each furnace model for all modes of operation. Blower characteristics for each model (airflow
CFM, torque, and speed versus static pressure) are known from laboratory testing. If any 3 characteristics are known,
the fourth can be defined. The airflow CFM is known because of the A/C and CF selections, model plug, and the
thermostat input signals on the variable speed furnace control. The variable speed furnace control then communicates
the airflow CFM to the ECM blower motor. Torque is known by the ECM blower motor because it is directly related to
armature current which is measured by the ECM blower motor control. Speed is measured from the generated back
EMF by the ECM motor control. This information (airflow CFM, torque and speed) are entered into an expression which
calculates torque from speed and airflow CFM numbers. If the calculation does not match stored blower characteristics,
torque is adjusted every 0.8 seconds until agreement is reached. The ECM blower motor does not directly measure
static pressure, but does react to changes in static pressure to maintain constant airflow CFM.