DC motor problems

D

DaveC

Guest
A 1/2(?) horsepower 2-brush DC motor was having bearing issues so I
disassembled it and replaced the ball bearings, turned down the commutator on
a lathe, and installed new original equipment brushes. A good clean-up was
done as there was much grease and carbon dust inside.

Now when I apple power it just growls. If I turn it by hand with power
applied it will turn 1 or 2 revolutions then stop. Before disassembly the
motor ran as expected.

The field and armature are separately terminated at the outside of the motor
and wired to a motor speed control PCB.

The commutator segments are well-separated. I put an ohm meter on the brush
terminals and turned the motor slowly. I see 10 ohms across the armature with
each commutator position (each brush contacts 3 segments). The field measures
1000 ohms. I thought that a bit high, so I found where the 2 separate field
windings are connected (in series) and measured each winding separately: each
is approximately 500 ohms.

There is no mechanical reason the motor should not spin at speed. By hand, it
turns freely and there is no interference between the armature and field
laminations(?).

The motor is rated at 180 volts DC, 1.5 amps. There are 2 separate field
windings (wired in series) and the commutator has 36 segments.

I've had this motor apart several times but still can't find a reason it's
not operating like it should.

Any suggestions would be greatly appreciated (and the sooner the better ;-)
).

Thanks,
Dave
 
DaveC wrote:
A 1/2(?) horsepower 2-brush DC motor was having bearing issues so I
disassembled it and replaced the ball bearings, turned down the commutator on
a lathe, and installed new original equipment brushes. A good clean-up was
done as there was much grease and carbon dust inside.

Now when I apple power it just growls. If I turn it by hand with power
applied it will turn 1 or 2 revolutions then stop. Before disassembly the
motor ran as expected.

The field and armature are separately terminated at the outside of the motor
and wired to a motor speed control PCB.

The commutator segments are well-separated. I put an ohm meter on the brush
terminals and turned the motor slowly. I see 10 ohms across the armature with
each commutator position (each brush contacts 3 segments). The field measures
1000 ohms. I thought that a bit high, so I found where the 2 separate field
windings are connected (in series) and measured each winding separately: each
is approximately 500 ohms.

There is no mechanical reason the motor should not spin at speed. By hand, it
turns freely and there is no interference between the armature and field
laminations(?).

The motor is rated at 180 volts DC, 1.5 amps. There are 2 separate field
windings (wired in series) and the commutator has 36 segments.

I've had this motor apart several times but still can't find a reason it's
not operating like it should.

Any suggestions would be greatly appreciated (and the sooner the better ;-)
).
Click to expand...

Wave you measured the current in each circuit? Are the field
windings wired properly? If you reverse one it's not going to run.

--
You can't have a sense of humor, if you have no sense.
 
Wave you measured the current in each circuit? Are the field
windings wired properly? If you reverse one it's not going to run.
Click to expand...
No current measurements made.

I tried reversing all connections. No joy.

Thanks.
 
On Thu, 9 Feb 2012 19:57:15 -0800, DaveC <invalid@invalid.net> wrote:

A 1/2(?) horsepower 2-brush DC motor was having bearing issues so I
disassembled it and replaced the ball bearings, turned down the commutator on
a lathe, and installed new original equipment brushes. A good clean-up was
done as there was much grease and carbon dust inside.

Now when I apple power it just growls. If I turn it by hand with power
applied it will turn 1 or 2 revolutions then stop. Before disassembly the
motor ran as expected.

The field and armature are separately terminated at the outside of the motor
and wired to a motor speed control PCB.

The commutator segments are well-separated. I put an ohm meter on the brush
terminals and turned the motor slowly. I see 10 ohms across the armature with
each commutator position (each brush contacts 3 segments). The field measures
1000 ohms. I thought that a bit high, so I found where the 2 separate field
windings are connected (in series) and measured each winding separately: each
is approximately 500 ohms.

There is no mechanical reason the motor should not spin at speed. By hand, it
turns freely and there is no interference between the armature and field
laminations(?).

The motor is rated at 180 volts DC, 1.5 amps. There are 2 separate field
windings (wired in series) and the commutator has 36 segments.

I've had this motor apart several times but still can't find a reason it's
not operating like it should.

Any suggestions would be greatly appreciated (and the sooner the better ;-)
).

Thanks,
Dave
Click to expand...
Any chance that the angle between the brushes and the field got
changed on reassembly?



--

John Larkin, President Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
 
Any chance that the angle between the brushes and the field got
changed on reassembly?
[John Larkin]
Click to expand...
Actually, yes. The rear casing of the motor that contains the brush holder
can be mounted to the main motor frame (containing the field) at 90 degree
increments. So 4 possibilities.

I reassembled the motor as I thought it was originally configured but the
possibility that it's incorrect is large.

I'll try other orientations tomorrow.

Thanks John!

Dave
 
Any chance that the angle between the brushes and the field got
changed on reassembly?
[John Larkin]
Click to expand...
Is there only one correct orientation? Or 2?

Thanks,
Dave
 
DaveC wrote:
Any chance that the angle between the brushes and the field got
changed on reassembly?
[John Larkin]

Is there only one correct orientation? Or 2?

Two--one for each direction! :) Whenever I disassemble a motor, I
Click to expand...
scratch a mark across the "parting line(s)". Then I know how to
re-assemble it.

--
Virg Wall
 
"DaveC" <invalid@invalid.net> wrote in message
news:0001HW.CB59D59B00710F2AB01029BF@news.eternal-september.org...
A 1/2(?) horsepower 2-brush DC motor was having bearing issues so I
disassembled it and replaced the ball bearings, turned down the commutator
on
a lathe, and installed new original equipment brushes. A good clean-up was
done as there was much grease and carbon dust inside.

Now when I apple power it just growls. If I turn it by hand with power
applied it will turn 1 or 2 revolutions then stop. Before disassembly the
motor ran as expected.

The field and armature are separately terminated at the outside of the
motor
and wired to a motor speed control PCB.

The commutator segments are well-separated. I put an ohm meter on the
brush
terminals and turned the motor slowly. I see 10 ohms across the armature
with
each commutator position (each brush contacts 3 segments). The field
measures
1000 ohms. I thought that a bit high, so I found where the 2 separate
field
windings are connected (in series) and measured each winding separately:
each
is approximately 500 ohms.

There is no mechanical reason the motor should not spin at speed. By hand,
it
turns freely and there is no interference between the armature and field
laminations(?).

The motor is rated at 180 volts DC, 1.5 amps. There are 2 separate field
windings (wired in series) and the commutator has 36 segments.

I've had this motor apart several times but still can't find a reason it's
not operating like it should.

Any suggestions would be greatly appreciated (and the sooner the better
;-)
).

Thanks,
Dave

Use a battery and momentarily connect the field winding. This "magnetizes"
Click to expand...
the armature (do this with the motor un powered). You only have to do this
once.
 
After you turned the commutator, did you undercut the mica between the
commutator segments? It is critical that the insulation be below the
surface of the copper to ensure good contact between the brushes and
the copper. If there is still sufficient undercutting [you only
skimmed the surface of the comm] then be sure that there is no copper
shorting between commutator bars. The copper often 'smears' a bit when
being turned and will bridge the gaps here and there. A "pole growler"
will show up any such shorts.

Neil S.
Click to expand...
Thanks for that info.

I did not undercut mica. I presumed that since, before turning on the lathe,
the motor was functioning well enough (sans bearing noise) with not-undercut
mica that its purpose was not important.

What's the best way to undercut? Using an exacto or utility knife in my hands
gets quickly messy...

I looked closely and eliminated any segment shorts on the commutator.

Thanks,
Dave
 
"DaveC" <invalid@invalid.net> wrote in message
news:0001HW.CB5A90CE009CEF57B01029BF@news.eternal-september.org...
After you turned the commutator, did you undercut the mica between the
commutator segments? It is critical that the insulation be below the
surface of the copper to ensure good contact between the brushes and
the copper. If there is still sufficient undercutting [you only
skimmed the surface of the comm] then be sure that there is no copper
shorting between commutator bars. The copper often 'smears' a bit when
being turned and will bridge the gaps here and there. A "pole growler"
will show up any such shorts.

Neil S.

Thanks for that info.

I did not undercut mica. I presumed that since, before turning on the
lathe,
the motor was functioning well enough (sans bearing noise) with
not-undercut
mica that its purpose was not important.

What's the best way to undercut? Using an exacto or utility knife in my
hands
gets quickly messy...

I looked closely and eliminated any segment shorts on the commutator.

Thanks,
Dave
Click to expand...
Grind a tool from a hacksaw blade, mount it in the tool post, and use the
carriage
 
On Feb 9, 7:57 pm, DaveC <inva...@invalid.net> wrote:
A 1/2(?) horsepower 2-brush DC motor was having bearing issues so I
disassembled it and replaced the ball bearings, turned down the commutator on
a lathe, and installed new original equipment brushes. A good clean-up was
done as there was much grease and carbon dust inside.

Now when I apple power it just growls. If I turn it by hand with power
applied it will turn 1 or 2 revolutions then stop. Before disassembly the
motor ran as expected.

The field and armature are separately terminated at the outside of the motor
and wired to a motor speed control PCB.

The commutator segments are well-separated. I put an ohm meter on the brush
terminals and turned the motor slowly. I see 10 ohms across the armature with
each commutator position (each brush contacts 3 segments). The field measures
1000 ohms. I thought that a bit high, so I found where the 2 separate field
windings are connected (in series) and measured each winding separately: each
is approximately 500 ohms.

There is no mechanical reason the motor should not spin at speed. By hand, it
turns freely and there is no interference between the armature and field
laminations(?).

The motor is rated at 180 volts DC, 1.5 amps. There are 2 separate field
windings (wired in series) and the commutator has 36 segments.

I've had this motor apart several times but still can't find a reason it's
not operating like it should.

Any suggestions would be greatly appreciated (and the sooner the better ;-)
).

Thanks,
Dave
Click to expand...
After you turned the commutator, did you undercut the mica between the
commutator segments? It is critical that the insulation be below the
surface of the copper to ensure good contact between the brushes and
the copper. If there is still sufficient undercutting [you only
skimmed the surface of the comm] then be sure that there is no copper
shorting between commutator bars. The copper often 'smears' a bit when
being turned and will bridge the gaps here and there. A "pole growler"
will show up any such shorts.

Neil S.
 
Grind a tool from a hacksaw blade, mount it in the tool post, and use the
carriage
[Rick]
Click to expand...
So, just a sharp knife point thing?

Thanks,
Dave
 
DaveC wrote:

A 1/2(?) horsepower 2-brush DC motor was having bearing issues so I
disassembled it and replaced the ball bearings, turned down the commutator on
a lathe, and installed new original equipment brushes. A good clean-up was
done as there was much grease and carbon dust inside.

Now when I apple power it just growls. If I turn it by hand with power
applied it will turn 1 or 2 revolutions then stop. Before disassembly the
motor ran as expected.

The field and armature are separately terminated at the outside of the motor
and wired to a motor speed control PCB.

The commutator segments are well-separated. I put an ohm meter on the brush
terminals and turned the motor slowly. I see 10 ohms across the armature with
each commutator position (each brush contacts 3 segments). The field measures
1000 ohms. I thought that a bit high, so I found where the 2 separate field
windings are connected (in series) and measured each winding separately: each
is approximately 500 ohms.

There is no mechanical reason the motor should not spin at speed. By hand, it
turns freely and there is no interference between the armature and field
laminations(?).

The motor is rated at 180 volts DC, 1.5 amps. There are 2 separate field
windings (wired in series) and the commutator has 36 segments.

I've had this motor apart several times but still can't find a reason it's
not operating like it should.

Any suggestions would be greatly appreciated (and the sooner the better ;-)
).

Thanks,
Dave

You didn't by any chance unwire the coupling in the field windings? If
Click to expand...
so, it sounds like you have one winding reversed. This will give you the
effect you are seeing. But you did say that you had to hunt for them and
the F1 and F2 legs are on the outside?

Other possible problem is brush alignment.. if you don't have the
brushes orientated so that one crosses over to the next winding while
the other is lagging behind, it'll sit there and hum and some times a
spin of the rotor will start it but most likely will come to a stop.

Are you sure you have field voltage? And also, have you ohmed out
the armature to make sure it isn't shorting to the rotor? It is possible
you could of damaged it while it was in the lathe.

But in any case, what I've done in the past was to use a induction
meter to measure the cross over point on the armature to help align the
brushes.

P.S.
May want to check to make sure you didn't over turn the commutator
and also, it is very possible you had a shorted motor to start with.

Also check the field for ground shorts. YOu need to use a megga meter
for this. Or if you can get your hands on a IR bridge that will work too.

Jamie.
 
YOU SHOULD HAVE LEFT IT ALONE....NEXT TIME JUST FLUSH IT OUT WITH
SILICONE LUBRICANT.
YOU DUMMY !
TGITM
 
"DaveC" <invalid@invalid.net> wrote in message
news:0001HW.CB5AA7FE00A25E80B01029BF@news.eternal-september.org...
Grind a tool from a hacksaw blade, mount it in the tool post, and use
the
carriage
[Rick]

So, just a sharp knife point thing?

Thanks,
Dave
Click to expand...
I was going to mention cleaning between the communicators. Use a round
cutter, to scoop all the surface crap out.
Some also bevel the edges of the communicators so they don't tear up the
brushes. For small motors a ball point pen is usually used ( that should
give you an idea)

Cheers
 
VWWall wrote:
DaveC wrote:
Any chance that the angle between the brushes and the field got
changed on reassembly?
[John Larkin

Is there only one correct orientation? Or 2?
Two--one for each direction! :) Whenever I disassemble a motor, I
scratch a mark across the "parting line(s)". Then I know how to
re-assemble it.
Click to expand...
The idea is good, but there's a better way. Use a punch and put matching
dimples across the 'parting lines'. Put one set on one end and two sets
on the other.

mike
 
On Feb 10, 6:40 pm, m II <C...@in.the.hat> wrote:
VWWall wrote:
DaveC wrote:
Any chance that the angle between the brushes and the field got
changed on reassembly?
[John Larkin

Is there only one correct orientation? Or 2?
Two--one for each direction! :) Whenever I disassemble a motor, I
scratch a mark across the "parting line(s)". Then I know how to
re-assemble it.

The idea is good, but there's a better way. Use a punch and put matching
dimples across the 'parting lines'. Put one set on one end and two sets
on the other.

mike
Click to expand...
He now knows the importance of some sort of a mark
 
On 2/11/2012 1:15 AM, DaveC wrote:
After you turned the commutator, did you undercut the mica between the
commutator segments? It is critical that the insulation be below the
surface of the copper to ensure good contact between the brushes and
the copper. If there is still sufficient undercutting [you only
skimmed the surface of the comm] then be sure that there is no copper
shorting between commutator bars. The copper often 'smears' a bit when
being turned and will bridge the gaps here and there. A "pole growler"
will show up any such shorts.

Neil S.

Thanks for that info.

I did not undercut mica. I presumed that since, before turning on the lathe,
the motor was functioning well enough (sans bearing noise) with not-undercut
mica that its purpose was not important.

What's the best way to undercut? Using an exacto or utility knife in my hands
gets quickly messy...

I looked closely and eliminated any segment shorts on the commutator.

Thanks,
Dave
Click to expand...
Using a hacksaw blade the idea is to grind off the 'set' on the teeth to
a thickness that will be smaller than the commutator bar gaps.
Then it is used by having the teeth point backwards towards you, the
action is then pulling rather than pushing which IMHO gives more control.
Whilst doing this try to hold the blade at an angle so that it cuts
against the sides of the bars, usually you can see the material flake
off when done correctly. This is important as it removes any insulation
that might be on the side of the gap you are creating and then can
interfere with brush contact.
While grinding the blade snap off the end and make it about 45deg away
from the teeth to make it easy to get to the insulation at the
connection end.
However I doubt the commy is your problem :)

Rheilly P
 
As expected, orienting the brushes 90 degrees and the motor runs!

Thanks for all the helpful suggestions.

Dave
 
On Feb 11, 12:49 am, DaveC <inva...@invalid.net> wrote:
As expected, orienting the brushes 90 degrees and the motor runs!

Thanks for all the helpful suggestions.

Dave
Click to expand...
Glad you closed the loop here!
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top