Resistance of small AC motor windings

[Tom_1000]

Hello. I have an old AO Smith 115 volt, 1/3 horsepower, two speed (1725/1140 rpm) motor from a belt driven fan. When trying to start, it was just humming, but would start and run fine if the fan was spun manually. I thought perhaps bad start capacitor or bad centrifugal switch. I realize I can replace motor pretty cheaply but wanted to try to fix this one. I removed the motor and found there is no start capacitor. It was extremely dirty so I cleaned it up. Centrifugal switch was dirty but contacts were not welded. Switch seems to manually operate ok but I suspect springs are weak because of how old it is.

Reassembled the motor and wired it to a plug and cord so I could bench test it before reinstalling. Hi speed = 115 volt, common = neutral, lo speed = no connection. The available outlet is GFCI which well may be part of the issue. When I powered the motor (no load), it started immediately but tripped the GFCI quickly, two seconds at most. Their was a visible spark internal to the motor when the GFCI tripped. I took everything apart again and didn't see any burn marks or other damage. Got lazy and decided to try again but same results.

I decided before trying again on a non-GFCI outlet to try to check the winding resistance, which is obviously something I should have done previously. I'm confused by what I found because I thought the start winding should have higher resistance than the run windings?

Motor off and centrifugal switch closed
Hi speed to common = 1.7 ohms
Lo speed to common = 1.7 ohms

Motor off and manually hold centrifugal switch open
Hi speed to common = 2.7 ohms
Lo speed to common = 6.4 ohms

Does that make sense? Looking for any input before testing again since I don't want to needlessly burn up the start winding if something is obviously wrong.

Thank you for any input,

Tom

 

[itsmoked]

start winding should have higher resistance than the run windings

Quite the opposite. The start winding draws considerably more current than the run winding. This is why it will smoke in more than about 2 seconds of operation. Ohms Law: Same voltage : less resistance : more current.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Tom_1000]

Thank you. That makes complete sense. I'm feeling dumb now.

 

[waross]

Hello Tom. Your question may be more appropriate on this site;
Link

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Tom_1000]

Thank you for reference to other forum. For what it is worth, just tested on non-GFCI outlet. Small spark (doesn't exit motor frame) when centrifugal switch opened but very obviously switched to run winding and ran unloaded for over a minute with no issues. I appreciate the replies.

Tom

 

[che12345]

Dear Mr. Tom
Do you have the present circuitry, if no; perhaps make a simple free-hand sketch.
It is hope that with the circuitry showing the connection of the capacitor, centrifugal switch including the windings would be helpful for the trouble-shooting.
Che Kuan Yau (Singapore)

 

[Tom_1000]

Hello Che Kuan Yau,

I do not have a schematic. As far as I can tell there are no capacitors anywhere. I appreciate your offer of help but no longer necessary. After cleaning centrifugal switch, everything appears to operate correctly.

Tom

 

[che12345]

Dear Mr. Tom_1000
I am glad to hear that it is back to operational.
It appears to me it is a [single-phase motor with a centrifugal switch] as you are aware of. What I wish to point out is it is [very unlikely] that a [singe-phase motor with (centrifugal switch) but without a capacitor]. This is the [starting capacitor] (usually in a metallic or in a plastic tank), which is a (big item) that you can not miss it.
Anyway, since it is back to operational, I bless you good luck. Good bye.
Che Kuan Yau (Singapore)

 

[waross]

Mr. Che
This type of motor was very common on air circulating fans on residential furnaces.
The starting winding has a different X:R ratio than the run winding.
That is achieved with a relatively high resistance winding.
As you know, the phase angle of the current is determined by the X:R ratio of the winding.
This gives the phase angle difference that develops the rotating magnetic field needed for starting.
Starting torque is low, however that is acceptable for fan use, which is the primary market for this type of motor.
These motors are economical to produce and serve their intended purpose well.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[che12345]

Dear Mr.waross

1. With due respect, I am doubtful it is a motor per your description "... X:R ratio of the winding...." which includes a [centrifugal switch] as mentioned by Mr. Tom. The (cost and the complicated design of the centrifugal switch) would be far far [too expensive] for a motor of the type you had mentioned.

2. I understand there are ["shaded pole" ] motors with only (one single winding), but (NOT with two windings of different X:R ratio). I also understand that the only one winding " X:R ratio" differs from that of the one turn [shorted] shaded-ring. [Take note that a shaded pole motor does NOT need any centrifugal switch], see following. The splitting of the phase is by the (one turn) shorted ring over the magnetic pole. The (one turn) shorted ring is unlike a "winding" of (many turns) as you usually see. A shaded pole motor DOES NOT need a centrifugal switch. The one turn shading-ring is there [permanently], no switching is required. The working principle of a shaded pole motor with only ONE winding is NOT by [splitting the X:R windings ratio but the magnetic flux].
Che Kuan Yau (Singapore)

 

[waross]

This is a fan and motor removed from a residential heating furnace.
These are common in North America.
There are millions of them in use across the continent.
They are called split phase motors.
It has a centrifugal switch but no capacitor.
They are also popular for small single phase centrifugal pumps and small turbine pumps.

Split Phase Induction Motor

The Split Phase Motor is also known as a Resistance Start Motor. It has a single cage rotor, and its stator has two windings known as main winding and starting winding. Both the windings are displaced 90 degrees in space. The main winding has very low resistance and a high inductive reactance (High X:R ratio)* whereas the starting winding has high resistance and low inductive reactance. (Lower X:R ratio)* The Connection Diagram of the motor is shown below.

* Notes added.
Link

Note:There is an error in the text following in the link.
The winding itself comprises the resistance.
There is NO external resistance connected, ever.
By the way that was an interesting tutorial on shaded pole motors.
Have you seen a reversing shaded pole motor?
There has been at least one type made.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[che12345]

Dear Mr. waross
1. " This is a fan and motor removed from a residential heating furnace.They are called split phase motors.
It has a centrifugal switch but no capacitor ". Split phase motor are commonly used in this region too, but usually with a [capacitor for phase splitting], not by [winding R/X ratio]. Thank you for your kind advice.
2. " Have you seen a reversing shaded pole motor? There has been at least one type made".
I have NOT seen any. I would appreciate it to receive your learned advice.
Che Kuan Yau (Singapore)

 

[waross]

I forget the actual device but there was a need for a small motor that could be filled with a shaded pole motor.
The problem was that the motor for that application must be easily reversible.
A motor was constructed with two adjacent stators and two rotors on one shaft.
One or the other stators would be energized depending on the rotation needed.
Control was simple.
A single pole, double throw switch was all that was needed for basic control.
If you are ever faced with a problem that may be best filled by a shaded pole motor but need reverse, consider using two shaded pole motors coupled together.
This may often be the cheapest, simplest and easiest to control solution.
Here a distinction is made between a "Split Phase" motor with no capacitor and a "Capacitor Start Motor" which of course does have a capacitor.

Bill
--------------------
"Why not the best?"
Jimmy Carter