Hi! Was brought to this forums from trying to research a bit about motors. The reason I'm researching motors has to do with my furnaces blower motor going bad (Was smoking and humming in the furnace). I figure I'd research it a bit and a lot of people say the capacitors go bad. Didn't think nothing of it as my tests on the motor show that it is bad. However when I called the HVAC supply company they said they the motor was a "Something" pole motor and they now use "PCS?" or maybe it was "PSC". He said it was (twice as much as my old motor) and required a capacitor to run it. I didn't know if I could easily wire up a capacitor. I've wired up electrical in a home and some things that are fairly self explanatory but I didn't want to change the way the circuit is handled in my furnace without experience. So I found a replacement motor that was almost exactly the same here: https://www.grainger.com/product/CE...Blower-4KA41?searchBar=true&searchQuery=4ka41

The actual motor is a discontinued item but grainger said that this is a direct replacement. Opening it up and everything looks good. But it really has me wondering why the capacitors? How does mine not require it, but then others do? Why don't HVAC companies have non capacitor motors anymore? It is just an efficiency thing? Should I of paid the price to get a motor with a capacitor? And would I need both a start and run capacitor if I went that way or just a start capacitor?

 

This may help...
https://www.allaboutcircuits.com/textbook/alternating-current/chpt-13/single-phase-induction-motors/

You may have had a Shaded Pole motor. That is an induction motor with a couple of short circuits on diagonal sides of the stator pole pieces. Simply put, this delays the magnetic field on that edge because of the induced current flowing in the short circuit link and so produces the rotating magnetic field needed to spin the armature.
A capacitor start motor has a couple of windings, one with a series capacitor, and the phase difference of the currents in the 2 windings produces the rotating field for starting.
The capacitor start motor will have more torque I think, and be more efficient too.
The above is just my simple understanding. There will be better info no doubt.
The circuit should not change as the motors will be a 2 wire device externally.

 

Broadly, capacitors are used in two ways with AC motors.

Capacitor-start motors use a capacitor in series with a winding and a centrifugal switch. At start-up, the cap is in the circuit to produce a phase shift, relative to the main winding, in the start-up winding. When the motor reaches some speed, the centrifugal switch will open, disconnecting the start-up winding. The overall effect is high torque at start-up, so the motor starts the load quickly. The current during startup will typically be several times the running current. The switch is a reliability concern, causing grief if it either fails to close or fails to open.

Permanent Split Capacitor motors have two windings, one directly across the line and one that always operates with a capacitor in series. Again, the capacitor produces a phase shift so that there is a "rotating" magnetic field to make things spin. PSC motors don't have great startup torque, but a blower isn't a difficult load to start and it doesn't matter if it takes a few seconds to come up to speed. The startup current isn't much higher than the running current. Reliability is improved because the mechanical switch is not needed. For a motor that starts up frequently, like a furnace blower, this is a big plus.

Capacitors are used for phase shifting for motors to run on single phase AC power. If three phase power is available (very rare for residential in North America), a motor that doesn't require caps can be used.

 

The motor used for HVAC systems mainly now are ECM (Electronically Commutated) motors, these are 3ph with P.M. field, they have a circuit board internal that does the commutation and converts to 3ph out.
The older style was PSC Permanent start cap motors, often multi-speed.
If this is a furnace I would have though it would need something a little better than a shaded pole like the one in the link, most I have come across are the older PSC versions.
Max.

 

Thanks for all the answers!

I just bought the closest thing I could find to the old motor. I guess it makes sense since spinning the fan in the blower is extremely easy being so light. So upgrading to a capacitor motor would be unnecessary? How would I go about switching it over to a cap start motor? My furnace has Med and High speeds. The red wire (low speed) doesn't connect to anything but a dummy wire. There are no jumpers or other wires connected to the pigtail connector that the circuit board connects to. No sure if that is normal or not but it hasn't had any issues in the few years I've owned the home.

This could've been my fault as well since I didn't keep it really clean. I did filter changes but I never did a full maintenance on it in the last couple years. So when I installed the motor today I gave it a nice cleaning by vacuuming all the dust I could find and cleaning off the thermocoupler.

 

Your furnace unit may have originally been designed for a two speed PSC motor and has either not included a motor with this feature, or as often happens, someone may have changed/replaced the motor for a single speed for cheapness.
Max.

 

The motor itself has 3 speeds. A black (high), Blue (Med), and finally the Red (Low) and the neutral (White). The wires coming out of the computer only has White, Red and Blue. I wired it back up the same way before. Maybe the furnace is only a two speed? Would explain why the motor only has Med and High hooked up. But the pigtail connector itself has 2 more spots for it. The red wire from the motor is attached to it but the computer isn't attach so red does nothing. Think when they installed it before they were just making sure there wasn't a lose wire hanging around from the motor.

 

What do you mean by 'Computer' generally older furnaces were non-electronic, recent ones have a control board on them and many of these use the ECM motor.
Max.

 

It has a circuit board that controls the gas valve, safety features, and the motor.

 

So probably simple logic control circuit.
If using the motor shown in the OP, this is a single speed and rather low rpm, what is the details of the one you removed?
Max.

 

The one I posted has 3 speeds. All the specs are pretty much the same except for a few things like the body length and such. Both the same HP, RPM, etc.

This is the one I pulled: https://www.grainger.com/product/DA...archBar=true&suggestConfigId=8&searchBar=true

I looked around seeing if going with a stronger motor was a wise idea but I saw a forum post about someone wanting to upgrade to a 1600 RPM motor and that it was a bad idea because the blower fan would likely not handle the RPMs. I don't need anything to blow any hard anyways. Just wasn't sure if a capacitor motor was the way to go or not. HVAC companies seem to think so, but from what I'm gathering its more reliable to stick with the type of motor I have now if it works.

 

Hmm.. 3 speeds and only 100rpm difference (10%) in total?
I would not expect to tell that much difference between speeds?
If it works pretty much the same as the original then why not go with it.
Max.

 

I did end up buying it and installing it. All worked well. The HVAC company raised a lot of questions when they told me that they don't make the type of motor I needed anymore and that I should be using one with a capacitor. They are making me second guess if I should've went with a capacitor type motor or not.

 

Probably depends alot on fan air capacity, but as per post #4, most I have come across are PSC motors.
Quite hard to change rpm effectively on a shaded pole motor non electronically, hence the small range.
Max.

 

I guess it makes sense since spinning the fan in the blower is extremely easy being so ligh

The weight of the actual fan/squirrel cage doesn't have anything to do with it. It's the force that it takes to move the air that takes the power.

 

Heating only requires less HP. Heat and cool generally use 1/2 HP. Simple furnaces may use the low speed for heat and the high speed for cool.

My furnace uses and ECM motor (variable speed). The motor can actually measure static pressure and thus can change the RPM based on that.
It also does a filter check (high speed, all zones open) once a day. RPM, CFM and static pressure are available on the thermostat.

 

My older furnace has 3 speed PSC fan and increase to full speed when the thermostat cuts in.
Max.