Diagnosing series wound DC EV motor. 3

[Adam_c.]

Hi everyone,

Has anyone seen behavior like this from a similar motor?

I have a series wound DC motor commonly found in older conversion electric cars.
Its the FB1-4001-A from Advanced D.C. motors in Syracuse NY. This motor has advanced brush timing.

I'm experiencing a very strange problem while testing it. It is pertinent to mention that I'm using an old "Kelly" brand DC motor controller, which works fine with a slightly smaller series wound brushed DC motor I have, so I'm pretty sure all of the symptoms can be attributed to the FB1 motor.

The problem: When I apply constant power power, the motor accelerates and then decelerates hard, then accelerates again. This results in a jerking kind of rotation, which comes to a near stop every 90 degrees.

Facts about the motor: It has four pairs of brushes, offset at right angles to each other.

​

The resistance of any commutator pair is very very low, because the motor is so big. Nothing I own has the ability to measure resistance values that low.

​

The brush timing is advanced, but the problem seems identical to me either way I run the motor. Including the exact four spots where the motor slows rotation.

Thanks for any and all thoughts!
Best,
Adam

 

[waross]

Check for grounds and or shorts in the armature. Try your controller at a lower setting.

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

 

[waross]

Nothing I own has the ability to measure resistance values that low.

Your car battery, a couple of headlights in parallel or other resistive load and a multimeter.
Connect the battery in series with your load and connect two opposite commutator bars in series with that. Now use your multi meter to take a comparison reading across each pair of adjacent bars. You should easily identify shorts and open circuits.

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

 

[electricpete]

I think the bar to bar resistance test is a good one and Bill gave you a way to do it that seems pretty reasonable.

EASA DC motors document indicates that this test is reliable for finding high resistance connections including in the commutator. They say it may not be reliable for finding shorted windings… and that more reliable tests for shorted windings are growler test, high-frequency bar to bar test and surge test. All three of these tests involve some specialized equipment, so given that you don’t have a DLRO for the bar to bar resistance test, I’m guessing these tests are also beyond your capabilities. You can revisit these tests later if your bar to bar test doesn’t show anything.

I imagine you already inspected the commutator.


=====================================
(2B)+(2B)' ?

 

[Adam_c.]

Thanks for the responses, guys.
Bill, I would like to try this tactic, but can you break it down for me in more depth?

I get the basic idea of hooking it up like this:

[Battery]---[Load]----[Motor]--|
\_________________________|

Where I lost you was in the measuring part. You're suggesting I power the commutator posts and since it's a series wound motor, it won't spin, right? Then I'll stick my multimeter probes in there and measure resistance? to the adjacent commutator bar? Or were you suggesting I measure voltage drop across the posts and calculate the resistance?

Thanks again

 

[edison123]

A DC series motor should always be run loaded. An unloaded series motor tends to overspeed due to very low flux. Are you running the motor with load?

Muthu

http://www.edison.co.in

 

[waross]

Connect the lead from the battery to one brush.
Connect the other lead from the battery to the load.
From the other side of the load connect to a brush adjacent to the first brush.
Now measure the voltage drop between each pair of commutator bars in turn, between the powered brushes.
The voltage drop between any two adjacent commutator bars will be equal in a good rotor.
This limits you to 1/4 of the total number of the bars to test.
You may complete a number of tests and then turn the rotor to test more bars.
You may also measure the other 3/4s of the bars on the back side but the readings will be lower.
You are looking for one reading of a set that is noticeably different fro the others in the same set.
After turning the rotor the readings may be slightly different due to the brush contact.
Not to worry, after turning you are starting a new series of tests and all readings in the new test should be equal.

No growler? Not to worry. You probably have what you need and don't realize it.
I hope that you like this one, Pete.
Are you familiar with the poem?
"The Cremation of Sam McGee."
.."The Northern Lights have seen queer sights,
But the queerest they ever did see
Was that night on the marge of Lake Lebarge
I cremated Sam McGee."
Link
I was sitting with a very sick generator a mile or so from the "marge of Lake Lebarge".
The exciter would fail when it came up to operating temperature.
We had it growler tested in a shop in Whitehorse, Yukon Territory, and it tested good.
The shop hesitated to open up a rotor that tested good.
They heated it in their oven but the differential expansion was not the same as when it was heated in service and it tested good.
Took it back to the site and it failed again.
How to test it when it was hot and failed when the nearest growler was almost an hours drive away.
Back to basics.
How does a growler work?
It induces a flux in the rotor and any shorts divert the flux into leakage flux which can be detected by a hacksaw blade.
First, how can I induce a flux and second how can I detect any irregularities?
Then I started to think about the brush neutral test.
The field is excited with AC and the induced AC voltage at the brushes is measured.
The brushes are shifted until the voltage is the lowest, ideally zero volts.
Let's see what happens;
I ran the set until the exciter failed.
I energized the field with AC and connected a voltmeter to the brushes as if doing a brush neutral position adjustment.
Then I turned the rotor. (Not part of the brush neutral test, but not a bad idea,).
As I turned the field the voltage varied as the shorted coils changed position in relation to the flux.
Back to the shop where I explained my test.
They knew the basics and ageed that there was a shorted coil.
"Now that we are sure of the problem we can fix it."
A few days later we were up and running.

Now Adam,
What do you have on hand to replicate this test with a series motor.
Do you have an extension cord that you are willing to cut?
Do you have some appliance that draws about 10 or 12 Amps.
Most shunt motors or generators will easily withstand 120 VAC across the field windings.
Not so a series motor.
You need to limit the current.
If this is a motor from any kind of an electric vehicle it should withstand 10 or 12 Amps through the field.
If you are not familiar with safety precautions working with live 120 Volts AC find someone who is to help you.
You need an extension cord and an electric appliance. An electric kettle (with water in it. grin), a toaster or some appliance that draws about 10 or 12 Amps.
Disconnect the series field.
Wire the appliance in series with the field and connect the extension cord so that the current flows trough the appliance and the field.
Now you have a flux established through the rotor. Connect your meter across two adjacent brush holders and check for AC voltage.
Turn the rotor while watching the voltmeter. It should stay at or near zero.
Any variation of voltage indicates a shorted winding.
There may be minor variations as the brushes cross from one bar to the next.
One shorted turn will show a voltage rising to a peak and dropping back to zero over 1/4 turn of the rotor.
Verification.
When zero is a good test result, how are we sure that the test was performed correctly?
Easy.
With the current flowing through the test setup, put one voltmeter lead on one of the brushes.
Touch the other voltmeter probe to an armature bar next to the brush.
Drag the probe across the commutator bars to the other brush holder.
You should see the voltage rise to a peak at the halfway point and then drop back don as you get near the other brush.
Good luck.


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

 

[Adam_c.]

Hi Bill,
You seem to be a brilliant mad scientist! Do you have any reading recommendations that could help explain what you described and what I'm seeing here with the AC test? I still remember my college physics electricity and magnetism, so using the AC current in the stator to create a magnetic flux to induce a current in the armature windings makes sense, but visualizing why a shorted commutator winding affects that in this way is really fascinating me. I will try to diagram it and make sense of it.
I had a cut extension cord in my shop, so I wired it up as you said to the dehumidifier that was there already, set that to run constantly.
As I measured AC voltage across adjacent brushes I indeed saw the voltage approach zero and rise to around 0.35v on a quarter turn basis. This corresponds to the spots the armature seizes when given power.
Tomorrow morning I will measure the voltage drop with a dc current going through the armature as well.
Thanks again for all your help,
Now I guess the question is, who wants to fix an armature made of 12gauge copper bus bar?
Adam

 

[waross]

Not brilliant any more.
Remember that line from Top Gun;?
"Your ego is writing checks that your body can't cash."
Well as I get older I find that my memory is writing checks that neither my body nor my mind can cash.
But thanks for the comment.
You are lucky that you had a question that I could still answer from long term memory. grin
For simplicity consider a motor with two brushes. As the flux passes through the rotor it induces a voltage in all the winding segments except for two. Those are the two segments that are momentarily moving parallel to the flux. As the rotor turns and the brushes momentarily bridge across two commutator bars they bridge the bars connected to the segments momentarily producing zero volts.
There are two paths for the current, from one brush clockwise to the other brush and from the same brush counterclockwise to the second brush.
These voltages are equal. If a coil is shorted, the induced voltage in that current path will be less than in the other path and the point of zero volts will be shifted away from the brush.
When the field is excited with AC, an AC voltage is induced in each path. If there is a shorted coil, the two current paths will no longer balance. The null or zero point will no longer be at the brush location and you will see a voltage across the brushes.
The voltage induced in any one coil depends on the angle of the coil in relation to the lines of force.
When a coil is at right angles to the lines of force the maximum voltage will be induced.
When a coil a parallel to the lines of force no voltage will be induced.
That is why you see the voltage vary as the angle of the shorted coil changes in relation to the lines of force.
All that I can think of for reading just now is Robert Service;
The Cremation Of Sam McGee "The northern Lights Have Seen Strange Sights"
The Call of the Yukon "I wanted the gold and I sought it"
The Shooting of Dan McGrew "Out of the night that was 50 below and into the din and the glare"
Sorry if mad has just overcome brilliant. grin

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

 

[waross]

What would be a half turn on a two brush motor will be a quarter turn on a four brush motor.

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

 

[itsmoked]

Damn Bill.. A star for that demonstration of long-term memory.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Adam_c.]

Thank you for the description of what's happening, that makes a lot of sense now. Your help is much appreciated! I'll check out Dan McGrew too

 

[waross]

Anecdote Alert
Back in what Louis L'Amour may have called my "Yondering" years I wandered into the Yukon Territory.
By a string of coincidences that seemed reasonable at the time I found myself as the bar tender in the pub in a small northern villiage. population maybe 500.
In some pubs there is a copy of the Guinness World Records behind the bar to settle disputes.
In the Yukon Territory the Guinness book was optional but a copy of Robert Service poetry was mandatory.
A bunch of the boys would be whooping it up and then someone would start to misquote Service and the argument was on.
Service poetry was good reading on slow days particularly a few hundred feet from the Yukon River.
ps; This post has been copied to Pats' Pub.
Bill
--------------------
"Why not the best?"
Jimmy Carter