DC motor problems

On 2/9/2012 22:57 PM, DaveC wrote:


<snip>
A growling sound without turning can be a couple of things.
First, check that commutator bars aren't shorted. If you turned it on a lathe or something, you should have then cleaned/undercut the slots between the bars. This cleans out any copper that would short them and it lowers the mica between the bars so it doesn't interfere with the brushes sliding across the tops of the bars. If you didn't, take the rotor out and clean the slots with a thin saw blade or tool that will just fit in the slot. Don't use a triangular file, that will bevel the edges of the bars and cause other problems (more sparking/burning at the brush edge). But a lot of shorting between bars usually just keeps it from turning at all, it doesn't 'growl' much, the shorts just trip the supply breaker.
Next, if it has only two sets of brushes, this thing probably has just two field poles in the stator. If you disconnected the wiring between each pole, it is critical that you reconnect them together correctly. If they end up so that both are creating a north pole towards the center of the machine (or south pole), then the torque created from current in the rotor windings will just cancel out and it will sit and buzz/growl. With the rotor removed, connect a small battery (D flashlight cell would work) to the winding and slowly move a compass near each pole, noting which end of the compass needle points to the pole. The two poles should be opposite polarity.
If the brushes are re-installed 90 degrees from where they should be, that too will cause the symptoms you describe. If you didn't match mark these before dis-assembly, well experience is a great teacher, isn't it :-). Move them 90 degrees and try again. If it rotates the wrong way, swap either the field wires, the armature wires.
Lastly, a damaged winding that shorts across several coils will do this, but since it worked before you took it apart, you would have to have bashed the windings with a hammer or something to damage them like this.
This doesn't sound like it's really big enough of a DC motor to have commutating poles or a series field winding, so I won't bother with those issues (suffice to say, getting those straightened out takes more)
Good Luck,
daestrom
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sat, 25 Feb 2012 13:08:33 -0500, daestrom wrote:

Dave posted that he'd solved the problem, 15 days ago.
Brushgear wrongly aligned on reassembly.
--
"For a successful technology, reality must take precedence
over public relations, for nature cannot be fooled."
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I would suspect that the replacement brushes are a little short or that the spring force on the brushes is too weak. I used to go as far as slightly contouring the brushes to the commutator to make max contact (and reduce arcing).
David_J

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Thu, 01 Mar 2012 19:36:35 -0500, David J wrote:

Wossgoinon?
This was satisfactorily resolved ages ago.
Read the thread.
--
"For a successful technology, reality must take precedence
over public relations, for nature cannot be fooled."
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.