Varying speeds on 7 Pole X04 type motors

[Simon G]

Some time ago, I bought a small number of 7 pole motors that are similar in shape and size to X04s.  I have eventually got round to seeing what I am going to do wih them - either mod them to fit as an X04 or dismantle and replace a failed X04 armature with the seven pole one.

 

However before I do this, I have run them using an old Hammant and Morgan power supply, and found that the speeds tends to increase, then decrease, without altering the power setting.  These variations can be quite significant, but I dont get them with other motors, so I dont think it is the controller.  Has anyone else encountered such an issue?  Would a back EMF controller stand a better chance of the motors holding a steady speed?

 

[34theletterbetweenB&D]

Better to correct the cause.

 

Could be a tight bearing heating and binding from the description.

 

Could be the armature heating leading to increased resistance. If these are fairly old motors the magnets may be below ideal flux.

 

If there is an increase in noise and vibration associated with the slowing, check for one or more windings heating.

 

The last is something I recall from long ago experience of 7 pole motors. To get the number of turns required in the windings a thinner grade of wire is used as compared to a five pole job. For any given wire quality in terms of conformance to gauge, the effect on conductance is more marked as the wire becomes finer. I rather formed the opinion that to make a 7 pole motor work well for a model railway application required a much more closely toleranced wire spec. than the manufacturers were using.

[stewartingram]

Well I posted this earlier but it went off into the ether!

 

1 - Is the controller capable of producing enough current? (Probably ok, but if no.3 happens, current is increased anyway).

 

2 - Is the magnet ok (pm me if you need help here, I have a remagnetiser). (more likely to give a constant reduction in rpm).

 

3 - Check the slots in the commutator - are they clean, and in particular, not contaminated with oil. Does arcing occur when speed reduced? Also check the brush tension is sufficient.

 

Stewart

[Simon G]

Thanks for the ideas/thoughts!  I subsequently tried with a second controller and got the same results, and they are both suitably 'beefy' controllers.  The motors appear to be quite new, so the commutators are all OK.  The brush tension is fine and there is no arcing.  I did originally think that the magnets were OK, but then decided to remove one and replace it with a Neo magnet.  The results were much improved, but there was still some speed variation. 

 

Also after running one of the motors at a relatively slow speed on the bench (ie, without any load) for a few minutes, the armature was then too hot to hold comfortably.  When I have had hot X04s in the past, it is almost invariably as a result of a weak magnet, but as this occurred with a Neo in place, it must be a feature of the windings.  I seem to recall from my old physics lessons that resistance increases as conductors get hot, so the speed variations could be occurring due to the windings getting hotter.  I will investigate further and see what current the motors are taking, but I think that 34theletterbetweenB&D is probably spot on with his point about the quality of the windings.

 

Thanks again, Simon

[stewartingram]

Magnets should NEVER be removed, there should always be a keeper plate to complete the magnetic path between poles. Failure to do this results in a loss (drop) of intensity of the magnetism.

 

Stewart

[stewartingram]

I've had an occasional problem - X04 and Ringfields included - where the commutator has shifted, or even is loose, on its shaft. Thus the orientation of the switching is not right. Less critical on a 3 pole (X04) but if this happens on a 7 pole, it doesn't take much movement to put it out of sync.

 

Stewart

[Simon G]

I have done some tests with the current required to drive these motors.  The two that have the varying speed will start to run freely at a current of about 0.3 amps, which is pretty high, and explains why they get hot. I compared that with a good 5 pole and another 7 pole.  The 5 pole started to run freely at about 0.15 amps, while the other 7 pole was running well at 0.1 amp.  I can only assume that production quality is pretty variable when making the armatures and the windings.

 

Stewart - like you, I always use a suitable housing while removing a magnet - is this case, I slid it into an empty X04 frame.  Neo magnets dont suffer from the same loss of magnetism as the old ones, so I dont have a problem with them.  I hadnt thought about the commutator shifting, so I will disassemble the motor to check that and also take the opportunity to measure the resistance across each pole.