Blower motor fusing

redhawk

Is it a good idea to put a fuse in series with a blower motor in an air-handler to prevent a catastrophic development in case of a locked-rotor or capacitor failure...?

If so, what amp value would be apporopriate to provide sufficient run and start-up current...?  The motor pulls measured run current of 1.5 amps, 230V.  Capacitor is 5 mfd/370V.

Steve Whitbeck

fuse

The air handler should already have a fused disconnect.

That fuse should be sized to protect the motor.

redhawk

Blower motor fusing

Thanks, Steve.  My apologies for not explaining in more detail.  The air handler is actually a Lennox electric furnace.  12 years ago I converted my heating system to gas by installing a water coil above the Lennox, and gas water heater and Taco 005 recirc pump which now supplies the heat.  The electric heat strips are still there, but never used, except for about 1/2 hour every summer to burn off any dust that may collect on them.  (I've built a simple controller that allows switching between the "electric furnace" or "water furnace" mode.)

I just replaced the original blower motor which has been runnning wonderfully since 1977 when the house was built.  It was still running smooth and quiet, but I had bought a new one for a spare when I did the conversion, and decided to install it now instead of waiting for the inconvenience a failure might cause.  This got me to thinking about possible system failures and consequences. 

I understand the motors have "thermal protection", which a technician explained to me would open the circuit if the motor overheated, but then would re-connect when it cooled.  Then he said it may do that several times before pernanently shutting it down.  He also said he had seem them fail to shut down permanently and ultimately fry the motor.  At that point I began to imagine coming home from vacation one day and finding my house full of toxic and pungent smoke from a fried blower motor.

From the schematic of the Lennox, it appears the only fuses in the motor circuit are the 60-amp breakers on the incoming line power.  Obviously, these will not protect the motor from frying.  

The failure conditions I am concerned with are the following:

1)  Failure of the recirc pump or water heater, which would remove the heat input so the thermostat would cause the blower motor to run continuously, possilbly leading to bearing failure and locked-rotor condition.

2)  Failure of the capacitor, which might cause the same if the motor fails to start or run normally, and so overheat.  (I'm not sure what the function of the capacitor is, or what it's failure might imply.  I am a retired EE, but never worked with AC motors.)

I thought I might add a fuse in the motor circuit, but not sure how to size it to ensure sufficient run and start-up current, while ensuring it will blow if one of these failures occurs.  Perhaps it's not even possible to achieve that....?

The motor is 1/4 HP, 3-speed, 230V.  It is an official Lennox replacement for the original (but made in China).  Measured run current is 1.5 amps in this application which only uses the lowest speed.  It is rated 2.2 amps on the nameplate, which presumeably would be at it's highest speed.

Thanks for any assistance or comments...!

icesailor

Fused thoughts:

I think that you are over thinking this. Chicken Little was convinced that the sky was or would fall. It hasn't. I have never seen a motor fail in the manner you are worried about. But I have seen far more things fail that were a lot simpler.

If it really is a concern to you, get some of those line fuses and splice then in the lines to the motor There are multiple motor feeds for the different speeds of the motor. You would need to protect every one.

Some boards have a fuse on them to protect them from an overload. I recently had a WA furnace that wasn't running. The board fuse was blown. Because mice had chewed the insulation in the 24 volt thermostat/control wire going out to the compressor where an installer had drilled a 1 1/2" hole through the side of the building to run the power and control wire. The mice found it useful as a highway and chewed the wire causing the short. I see far more rodent damage in homes that failing motors.

Mark Eatherton

If memory serves me correctly...

The electrical code requires a means of disconnect in the same room as the appliance. Fusing is a function of size, and whether or not the motor has its own internal thermal overload protection.



Best bet would be to run it by a licensed electrician in your area. There are numerous electrical codes in these United States, and there is enough subtlety to get yourself into trouble...



When fusing is required, I think it is based on the rating plate, and the fuse is supposed to be 150% of listed amperage to compensate for inrush start up amperage.



But I am not an electrician... Best ask a Sparky, or maybe one of the Sparkies who frequent this board will comment.



ME