Split phase motor question 1

[wptski1]

Working on a starting problem with a split phase induction start motor in a dryer. Couldn't find any info on how much the difference in resistance should be between the start and run winding. From what I read the start winding uses smaller wire therefore higher resistance causing a phase shift to get the motor running and the centrifugal switch drops the start winding out at 75% of max RPM.

Old motor start winding is .6 ohm higher than run winding but doesn't start all the time and draws 35A. Worse when motor gets warm.

Ordered new motor but turns out to be the wrong one because shaft is too long. I did check the resistance which was about one ohm higher but start winding was higher.

Returned that motor and received another with the correct shaft lenght. Checked the resistance and I have less on the start winding than the run winding! At first I though the it was connected wrong but the wire to the start contact is smaller wire. I did email the vendor that I got the motor from but this was before I noticed the wire size.

Not sure I made a mistake in my notes about the first new motor or not!

Is it possible that for some reason they "now" use smaller wire but less of it for the start winding and it still operates?

BTW- The old motor once it did run only draws 5.2A. I compared to another dryer and it had about the same starting/running current.

All reading were taken with a Fluke 1507 insulation tester and the old was tested at 1000V. Of course, that won't catch turn to turn shorts!

Bill

 

[ccjersey]

Sounds like the switch in the original motor was bad more than the windings. Or the bearings were shot. Have you tried the new motor in the dryer? It may be a different design for use with a capacitor, maybe a PSC type. There should be some literature with it showing the connections.

 

[wptski1]

ccjersey:

Nope, the switch is good, possible bearings as it does require effort to turn by hand after sitting but after sitting is usually when it starts. No capacitor start here. It gets plenty warm too, 180-200F but I read that they can get that hot.

I was reading something today that mentioned any more endplay than 1/64" can cause the centrifugal not to work and it's got about 5/32" endplay!!

I haven't installed it yet. I may connect and run it somehow first.

Bill

 

[ccjersey]

switch may be good, but as you say too much endplay and it won't operate.

Still it sounds like a temperature related short or dry bearings. I finally gave up on a PSC fan motor in a window unit AC. Kept hoping it just needed oil in the bearings, but no. It would actually run for a while and gradually slow down and stall.

 

[fsmyth]

I'll not profess to being a motor expert, but I have checked
a few split-phase out of curiosity, more than anything else.
From what I have seen, the only thing that you can depend on
is that every one will be different. <g>
It looks like some vendors use the same size wire throughout
and some of those even measure the same resistance. Most of
the small ( <1/2 HP ) motors I have autopsied have a smaller
start winding, but most of the larger had roughly the same
appearance and measurements. Add start and/or run caps in
the mix, and it seems to be a toss-up.
I know that this does not answer your question, but if I was
trying to determine the ratings, I believe I would take the
start current on the run winding as a reference, and look
suspiciously at a start current on the run winding that
differed by more than a factor of two. Maybe someone else
with more experience in the Real World (tm) will pop up
with better answers.
<als>

 

[wptski1]

Checked the switch while moving the rotor shaft, no problem there!

From what I've read only the split-phase induction motors have smaller wire for the start winding. The capacitor start motors have the same size wire for both windings.

One of the drawbacks to this type of motor is the high start up current but it's no different than another dryer that I checked. There's no way to just check the start winding alone for current as both windings are used for starting.

A capacitor start motor's startup current is supposed to be 2/3 less than a split-phase.

Bill

 

[fsmyth]

Quote:
"no way to just check the start winding alone"

Can you not get to one of the terminals on the centrifugal
switch? Simply add a loop to bring it out the end bell and
measure current there. Or better yet, bring both out and
use a contactor or switch to control the start application.
If the run windings are difficult to identify or break,
just subtract the start winding's current from the total to
the motor. Use an analog meter if you are trying to catch
the start current in normal operation; it will be easier to
get an indication of the peak. Too fast to read, generally,
but unless you have a peak hold function on a digital, it is
a better indicator. The magnetizing current peak is almost
impossible to measure otherwise.
Don't leave the start winding energized for more than a
second or two.
<als>

<als>

 

[wptski1]

Are you refering to running with the start winding only? What I meant was, when the motor is first started or the centrifugal contact is closed both winding are in parallel, so any current measured is from both. I can disconnect each winding at the switch but the other is buried inside the motor.

I've measured the peak current drawn at startup with a Fluke i1010 clamp. I've used a Fluke 189 and 87V in fast//min/max. Both go down to 250 microseconds but the 87V is at 100ms where the 189 is at 50ms. In the faster settings on both shows 60A or so and in the slower settings it's at 40A or so. It draws 5.2A when it does start which is normal and a good all around check of a motor's condition which is confusing!!

It a Sears dryer and I check another newer Sears model which gave the same results.

I've been using the dryer right along. Once it starts, it's fine. Once it runs and the motor get hot, it won't start, just hums and you let it do that too long, the motor overload opens up which I did only once. Sometimes it starts right up, other times it won't, so I just wait a bit and try again. I finally got smart and disconnected the timer, so I can add some time to the 45 minute cycle without having to restart while hot.

Bill

 

[machmech]

Hello fsmyth

Thats a good Tip

Thanks! Chuck

 

[wptski1]

I forgot to add, on the new motor, the connections for the windings are external, although both are on one pin of the overload switch.

The new motor's centrifugal switch has three different sizes of quick disconnects, 1/8", 3/16" and 1/4" just to make it more confusing! They supply two different ones which you must change when installing the motor.

Bill

 

[wptski1]

Just did a load, started right up but wouldn't restart when hot which is the problem. It gets around 200F which I've read is normal, maybe not for this motor which has no case. The new motor has a added fan.

Bill

 

[ScottyUK]

...If the run windings are difficult to identify or break,
just subtract the start winding's current from the total to
the motor...

The two currents are phase shifted. Scalar addition and subtraction doesn't apply here - you would need the phase relationship too before this technique could be applied.


----------------------------------

Start each day with a smile. Get it over with.

 

[fsmyth]

Yeah, Scotty, I know. Didn't want to conflusticate the
issue. Shoulda' said "somewhat useful readings -
ought to get a readable bump on the start."
After so many years of using two clamp-on's or a scope,
it is hard to remember that not all folks have plenty of
test gear. My bad.
<als>

 

[wptski1]

fsmyth:

Read my post of (17 Dec 05 11:56).

Bill

 

[wptski1]

Several days ago I connected the new motor on the bench with no load of course. I get similar start up current and running it draws about 4.3A.

Whatever/however the start/run winding relationship, it appears to work just fine! I plan on playing with the old motor after it's replaced.

Bill