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Converting belt drive oil furnace blower to variable speed?

larboc

Limestone cowboy
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Da yoop
Been kicking this idea around for awhile and never did anything but now yet another replacement furnace motor has given up the ghost and it's time to do something different.
90's oil fired downdraft furnace feeding an "add on" forced air wood furnace. I heat with wood except on vacation so the blower motor does a lot of start and stopping which is hard on it as well as the wood stove from the heat cycling. Starting contacts in the motors seem to last about 5 years based on the pile of replacements that came with the house. I'd guess I could also gain some energy savings from going to a more modern blower.

I'm considering a couple options:
1) 1/2hp 3-phase belt drive motor and 1/2hp single phase fed VFD with PID set to control plenum temp. I'd prefer to find a direct drive 3-phase blower assembly but I'm not finding anything off the shelf.

2) Buying a direct drive ECM blower assembly and figuring out how to control it based on temperature, or maybe using a VFD? I can't find anything very helpful regarding people going from PSC to ECM blowers on teh internets.

Since this is a pretty important appliance I don't want to half butt it so I'd be buying new parts and name brand stuff regardless of route I'm taking.

Anyone do something like this?
 
Can you do a direct drive single phase one?
I've pulled them out of junked furnaces that have multiple speeds in the windings themselves.

Make great "box fans" although you do need to restrict the inlet of the fan, as otherwise they burn up the motor real quick. They size the fan to the motor assuming that you'll be running inlet ducting and a filter. Running the fan in the slight vacuum unloads the motor. Running them wide open makes the motor draw over the rated FLA

ETA: I'm just leery of VFDs from the bad luck I've had with them dying where a dinosaur-tech motor would have probably survived.
 
I replaced my psc blower motor with a Genteq evergreen IM 3 years ago. It's an ecm but the only time the speed varies is starting and stopping, otherwise it operates at fixed speeds depending on what tap has 24v applied.

Probably not ideal for what you want to do but they have a line of motors and controls worth looking at anyway.
 
I replaced my psc blower motor with a Genteq evergreen IM 3 years ago. It's an ecm but the only time the speed varies is starting and stopping, otherwise it operates at fixed speeds depending on what tap has 24v applied.

Probably not ideal for what you want to do but they have a line of motors and controls worth looking at anyway.

that's interesting just because you could easily control a few 24v inputs with an arduino or something to make it fully temperature controlled.
 
I like the idea. Why it isn’t done more in residential is probably the cost.
I believe your energy cost would be lower but the equipment cost probably offsets that,

If you go this way I see you having two control loops.
One controlling the heat source and one controlling the fan speed.
Temperature indicators would be your control points.

If you are the controller for stuffing wood in the furnace then you just one control point.

The VFDs , Allen Bradley, that I work with have protection and limit parameters in them but we use a separate PLC to run the PIDs from the monitoring and set points.

I have never researched if you can buy a VFD that you can input a analog signal and then process that signal for a output to motor. I am sure you can with a discrete, on/off.

With a normal fire in the furnace you might start watching how much your fan runs per hour and day.
I know it depends on outside ambient temperature but this info might be useful when deciding how much your willing to spend on infinite fan speed control.
 
I replaced my psc blower motor with a Genteq evergreen IM 3 years ago. It's an ecm but the only time the speed varies is starting and stopping, otherwise it operates at fixed speeds depending on what tap has 24v applied.

Probably not ideal for what you want to do but they have a line of motors and controls worth looking at anyway.

Where did you get it from and was it easy to figure out the controls? It seems like there isn't a ton of support for doing this locally. All the local hvac guys I've called only replace parts for parts, no interest in what I'm wanting to do.
My power utility has a $100 rebate for an EC blower conversion which would sweeten the deal.

I did some reading and what I read is that a VFD+3p motor isn't a good way to go, not very efficient at lower speeds. An ECM blower is a PM motor commutated by the control board, I think kind of like a brushless rc car or drill motor.

I found a cheap replacement motor for what I have now so I'm going to swap that in while I figure out my plan.
 
I like the idea. Why it isn’t done more in residential is probably the cost.
I believe your energy cost would be lower but the equipment cost probably offsets that,

If you go this way I see you having two control loops.
One controlling the heat source and one controlling the fan speed.
Temperature indicators would be your control points.

If you are the controller for stuffing wood in the furnace then you just one control point.

The VFDs , Allen Bradley, that I work with have protection and limit parameters in them but we use a separate PLC to run the PIDs from the monitoring and set points.

I have never researched if you can buy a VFD that you can input a analog signal and then process that signal for a output to motor. I am sure you can with a discrete, on/off.

With a normal fire in the furnace you might start watching how much your fan runs per hour and day.
I know it depends on outside ambient temperature but this info might be useful when deciding how much your willing to spend on infinite fan speed control.

It's not just a power savings goal, but I get cracks in the wood stove that I have to weld up every few years and I'd guess the cycling of the fan isn't good for it. Unlike a normal furnace where the heat output is on or off based on call, the wood fire is constantly fluxing heat into the stove walls. At very low firings the furnace ducting will natural draft enough to not need the blower to kick on. Most of the heating season though the blower is cycling on and off at a 10% to 50% duty cycle every 2-20 mins. Seems like an efficient variable speed blower would be the answer.

I'm not longer looking at VFD's but yes, most if not all VFD's I've been looking for have a PID function built in and a lot have two analog in's so you can setup a thermocouple with an amplifier in the stove plenum and use the PID to try to maintain that at a certain temperature. I'd probably use the existing thermodisc to enable or disable the blower so that it's not trying to spin below 10hz.
I'd guess if I go the ECM route I'd need a separate DIN controller or arduinio to control it. I've had pretty good luck with the 4-20ma cheap chinese din controllers on ebay if I wanted to avoid making a robust arduino setup.
 
I'm not longer looking at VFD's but yes, most if not all VFD's I've been looking for have a PID function built in and a lot have two analog in's so you can setup a thermocouple with an amplifier in the stove plenum and use the PID to try to maintain that at a certain temperature. I'd probably use the existing thermodisc to enable or disable the blower so that it's not trying to spin below 10hz.
I'd guess if I go the ECM route I'd need a separate DIN controller or arduinio to control it. I've had pretty good luck with the 4-20ma cheap chinese din controllers on ebay if I wanted to avoid making a robust arduino setup.

Does ECM=PLC?
 
Where did you get it from and was it easy to figure out the controls? It seems like there isn't a ton of support for doing this locally. All the local hvac guys I've called only replace parts for parts, no interest in what I'm wanting to do.
My power utility has a $100 rebate for an EC blower conversion which would sweeten the deal.

I did some reading and what I read is that a VFD+3p motor isn't a good way to go, not very efficient at lower speeds. An ECM blower is a PM motor commutated by the control board, I think kind of like a brushless rc car or drill motor.

I found a cheap replacement motor for what I have now so I'm going to swap that in while I figure out my plan.

I'm an hvac tech by trade so I picked it up at Johnstone locally. The IM works kind of funny because of it's intended application. It has 120/240 all the time but also uses the old psc motor outputs as a way to time blower on/off delays, as well as 24v applied on 5 different taps to choose speed. Hopefully that makes some sense.

I mainly used it because I like the slow ramping up/down of an ecm motor, and the low fan speed for continuous fan operation.
 
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