Motor poles confusion

[richrock]

I'm probably being dumb, but I'm confused about motors. I can't seem to find any reference to the difference between 3 and 5 pole motors, and if it makes any difference to performance (and when I say that, I mean slower speed performance). I have a bunch of OLD locos which I assume would be three pole, and are terrible at slower speeds - I can't seem to get much slower than a scale 15-20mph. They're n-gauge BTW.

 

Can someone point out the differences, and is a 5-pole really beneficial? Or is it just marketing lingo?

 

Thanks

[bertiedog]

The minimum number of poles is three, the more the better. There is less drag and cogging with more poles. But in N gauge in particular there is a practical limit, and five is the most usually fitted.

However 3 pole does not affect the minimum speed that drastically, the problem that you have is basically the controller. It is probably either not giving a true zero voltage at O, or the design is bad at lower settings, if a plain DC type.

 

To be able to control locos at lower speeds pure DC is not used, but a series of pulses is sent to the track, this can be as simple as half wave, through pulses added only at low speed, or PWM control, which is the most sophisticated type.(Pulse width control). Makers may add feedback as well, to electronically adjust the PWM to match the demands of the motors as it runs.

 

If DCC is used then PWM is used anyway, but with DC then you should replace the controller with a more advanced type. Some types are switchable, as some motors do not respond well to PWM, making them a bit noisy, some makers use part PWM to over come this with a mixed system.

 

Pick a price level and see what is made, these days decent controllers are not cheap, There are types than can take the power from existing units and are added as panel units.

 

With a decent PWM controller then the locos will respond from creep speeds upwards, if it still will not respond it will be a badly designed mechanism. Most current Ngauage is quite satisfactory, but earlier N gauge was very patchy as to minimum speeds.

 

Check with the makers as to the best type for you and mention slow running, Gaugemaster are a make that covers most types, there are others and I have no connection with any maker nor make a recommendation based on makes.

 

Hope this helps,

Stephen.

[richrock]

Thanks for the excellent reply, very helpful. Most of the loco's I've 'inherited' seem to start after a good boost of voltage, but can then be turned back down to creep speeds. They have been serviced and everything runs smoothly, a little lube in the right places, etc.

 

I have one of these : Bachmann Controller

 

I checked the output with my multimeter and it read 0.03v forwards, 0.02v backwards, not a true zero reading.

 

I will look at alternatives, Gaugemaster was one of the possibles as I am looking to make a control panel, with two controllers. I'm an old-school DC boy, although I've tried DCC, I started modelling with DC and the size/scope of my layout doesn't really justify DCC, IMO.

[hollywoodfoundry]

I agree with what Stephen has said previously, however I do question whether the drive mechanisms are really as free as they should be. A method you can use to test this is (if possible) to disconnect the motor from its drive gears so it can run freely. If the motor starts rotating at a low voltage, say 2 volts or so, then it might be that there is too much friction in the drive system or something is binding and robbing the motor of power. When this occurs, the motor can suddenly jump into life at a higher speed, then can be backed off to run slower.

[bertiedog]

Right, we have more information, and can offer better advice, the controller on the base of the figures seems OK, but a bit more of this later.

 

The mechanisms have been serviced OK, but as Geoff mentioned the pickups can drag a lot in small mechanisms and should be adjusted after examination very carefully, as long as the whole mechanism runs freely at medium speeds then all should be OK at lower speeds.

 

Now back to controllers,, the fact you have a multimeter means a more meaningful test can be done, and that is to check the controller under load.

 

• A non load test of a controller should show in best terms 0 volts at both forward and reverse positions at Zero setting.
• Simple controllers with rheostat control or variable transformer taps, will show slight voltage at Zero in ether direction.
• Poor design controllers may show slight voltages both forward and reverse and should read the same forward and reverse.

None of the above are a serious issue with a normal motor, as the load of the motor means the start voltage is higher before the current flows to make the motor actually turn.

 

Unlike applying voltage to a resistor, and getting a fixed current, motors have a resistance and an inductance, and have to "power up" the magnetic field as it starts up. There is a lag between applying the voltage and the current flowing, and it can only flow if the source can deliver it. .Many controllers do not do this.at the lowest settings.

 

• The best test is to apply the controller to the motor with the meter measuring the voltage, and see what voltage the motor actually starts turning at, both off the track,and then under load on the track

I suspect that the voltage remains low then suddenly rises , this cause a lurch to speed, and it often due to a controller that cannot deliver the current required for the motor to turn, then as the voltage increases the current will flow at a higher than required voltage and the motor catches up.

 

This can occur with variable transformer, (multiple taps), type of controller where the lowest steps may delver the voltage, but the current cannot be delivered, due to transformer losses etc.. As the setting increases the transformer will deliver the expected current as the speed increases.

 

The rheostat types also suffer from the same issue, they use a resistance pad or a special metal rheostat that varies the voltage, but a the lowest steps the current is restricted by the motor load, and the steps are too far apart to really get low speed control. Again a slight lurch may occur as the motor starts.

 

Simple electronic controller types overcome most of the issues, there are no steps and simply increasing the voltage should get control of the motor, all other things being equal.

 

• So if the load tests reveal a sudden lurch and the controller in known to be stepless then the mechanism of the loco is too blame.

But if the controller is a "stepped type" it may cause a good mechanism to run worse than usual, so some analysis may be required to sort out what is going on.

 

I have no idea what type of control circuit the Bachmann uses, but suspect a simple approach, and it may be a variable or tapped transformer type, the residual voltage at zero seems to indicate this.

This does not make it a bad controller, just one that operates better with motors that start turning at higher start voltages.

 

A lot of earlier N gauge motors are actually quite good, the makers used fine windings and the current drain is low, but this aggravates the problems with low speed control on simple controllers. Later motors are actually better due to better magnets and designs, and have higher starting voltage generally.

 

Sorry this all sounds complex ,it is! ..the answer , as long as the mechanisms are serviced and run fine, is the controller, it is not easy to put right the problems with commercial designs, alterations can be made, diodes in series or resistors added to lower the output, and force the control region to higher point up the scale for low speed. I do not advise altering commercial units unless you are into electronics and know what your doing..

 

So back to the initial advice, a good controller especially with feedback, or a PWM design will cure the slow speed problems , given all other issues with the locos motors and mechanism are addressed.

 

You may not get the entire answer by just changing the controller , all mechanisms differ, but at least you can eliminate the controller completely and it will work correctly with decent mechanisms.

 

If you are into electronics you can build your own controller, (and for pence rather than pounds!), and tailor to the locos requirements, a PWM type will force control at the expense of nosier operation, but bear in mind nothing cures a bad mechanism, they do exist!!

 

Hope this helps,

 

Stephen.

[richrock]

Phew!! That's a comprehensive reply. But very very useful. I'll check the mechanisms tonight on my favourite two (that run better than the others).

 

I'll also see what voltages they start at without loads and post it up. The idea of making my own controllers seems nifty, I'll dig out my practical electronic projects book for model railways. It's old, but worth a look.

 

Thanks for the superb help!!

[bertiedog]

On controllers I did have a post last week on a simple controller on.

 

My link

 

This is about a simple as you can get, it could even be powered from the Bachmann set to max output! But it suffers from the low voltage step problem, although cured by adding the two diodes. most motors will behave themselves perfectly on this type of controller.

 

It does not suffer from load issues as soon as the voltage increases though, it will deliver the current requiered for the setting, as long as the input power can,and we are talking only about a few milliamps with modern motors, not amps as older OO motors required.

 

It is not a PWM type, there are designs on net or in electronic books, usually based on a multi vibrator circuit with two 555 timers to source the variable length pulses the system uses. I have circuits should you want to try them,easy to build and cheap.

 

Stephen.

[Dr Al]

Specifically which mechanisms and locos are these?

 

As a long term N gauge modeller I've found usually older N gauge models need a good strip down type overhaul to really get the best out of them, including cleaning of commutators and removing brush carbon from their slots, as well as careful lubrication of bearings and axle slots.

 

Of the 3 early N gauge manufacturers Lima was probably the poorest in terms of mechanisms, but they still can run reasonably. Both Farish (anything made before 1984 was 3 pole, after this time it was all 5 pole) and Minitrix models should be able to run very well at slow speed, even the 3 pole locos.

 

Only the very earliest out of the Farish is poor - the J69, early 94xx, Hall and Merchant Navy all had a plastic chassis with an Arnold 3 pole motor, and the chassis was fragile and very prone to split gears.

 

To return to the original point though - I'd suspect the mechanisms need more than simple lubrication if they've been around for some time.

 

Best Regards,

Alan