Neo-magnets and Current draw

[81F]

Although meant mostly for a DCC application, I am posting it here as I have a DC loco with the same issue.

Briefly would replacing a loco's conventional magnet with a Neo-magnet reduce the amount of current a locomotive requires?

One reason for asking is that when I run a loco and Tomix track cleaner on the 009 part of my layout, the Gaugemaster E controller cuts out fairly quickly and has to wait a few moments before continuing?

The second is that I have a Wrenn N2 locos converted to DCC (with an R1 currently waiting) and I would like to reduce their current draw since the N2 cuts out after two or three laps of the layout and has to "rest" before continuing.

[What About The Bee]

Hello 81F

Here is my understanding.

The resistance of a motor is a function of the windings. The current a motor draws is a function of that resistance and the voltage.

Now the interesting bit is the back emf. This is the voltage generated by the motor as the windings pass through the magnetic field. There is a relationship between the back emf, the velocity of rotation and the strength of the magnetic field.

As the motor generates back emf, it works in opposition to the applied voltage. As such, it changes the voltage to the motor itself.

With the change in voltage, comes a change in current. This is because resistance remains constant, but the change in voltage means change in current.

You asked "Will this reduce my current?"

Answer: as the back emf is a function of magnetic field strength, the correct answer is "I dunno". How did you change the magnetic field? Just changing to a different magnet type does not tell us if the field increases or decreases.

The geometry of the rotor and stator is inverse square related. So you may change to magnet that has a higher field strength, in general, but the specific geometry of the magnet coil relationship may be that the magnetic field is actually reduced. Or increased. It is the gap between the coils (and laminations) to the magnets which most directly affect the magnetic field.

If I was in this position, I would simply get a more powerful controller. Determine your stall current. Size your controller from that.

Bee

[Fishmanoz]

A simpler answer - yes it will, although these magnets are known on occasion to cause maintenance issues due to their pull on the rotor.

[P-Henny]

It is a known and well documented fact that magnets that become age related weak increase the current draw.

Therefore it is logical to deduce that increasing the magnet strength will decrease the current draw.

But as Fishman says, Neo magnets are so strong that they can potentially put more stress and thus more wear on the rotor shaft and bearings.

Could your current magnets have become weakened with age and (if feasible) be rejuvenated by using 'Scalespeeds' motor remagnetising service.

A 'stall test' The test demonstrated in the YT video below could provide an indication of your current magnets condition.

See this YT video.

[Seacommander]

I am not sure that a stall test with a stationary armature will reveal any information about the strength of the magnet. My understanding is that a magnetic field needs cutting with a rotating armature for any influence of the condition of the magnet to show up in current consumption. The results of the stall test (ie current consumption) will just be a function of the resistance of the armature and applied voltage.

[96RAF]

Neos do not work in all situations. I installed a D-shaped neo to a motor and it worked fine, the motor runs as smooth as silk, but a pair of arch-shaped neos did not work. The motor needed half throttle to run and was lumpy.

[ntpntpntp]

The Gaugemaster E series controller may be underpowered for higher loads - it's only rated at 0.4 amps whereas others are rated 1 amp (and more for the large scale variants). Use a more suitably rated controller if you're going to run multiple locos together or run old models.

[P-Henny]

I am not sure that a stall test with a stationary armature will reveal any information about the strength of the magnet.

 

 

Watch the linked YT video that explains all. Quite correct. I was working from memory and incorrectly associated a 'stall test' with the video demonstrated process. It was a 'running' current test not a 'stall' current being measured in the vid.

Will edit my original reply to correct, and appreciate the correction.

[Simon-372339]

From my schoolboy physics:

When a coil is energised a magnetic field is produced and when the power is cut the magnetic field created collapses producing back emf. In a std dc motor a capacitor is used to capture the back emf to reduce radio interference and sparking. An added bonus is the energy stored by the capacitor is released giving a small kick to the next winding.

If we use a simplistic equation based on Coulombs law, m1 = armature field strength, m2= magnet field strength.

attraction is based on m1 x m2

so if m2 is swapped from a weak magnet to a neo the attraction increases for the same current applied to the armature.

[What About The Bee]

Since there appears to be some confusion about my remarks, I will assume I communicated poorly.

When I said "If I was in this position, I would simply get a more powerful controller. Determine your stall current. Size your controller from that." I was attempting to say..."the controller might need more grunt" and that the new controller might need a few more amps.

I did not indicate that the stall current would be indicative of a change in magnetic field. Yet it would!

When the magnetic field of a motor is changed, the Kt or torque constant of the motor is changed. That is, there is a direct relationship between the current input and the torque output. You may find the Kt of most commercial rotary motors. For linear motors, the Kt is linear thrust per unit of current.

Now let us examine how stall current is determined. We measure the resistance of the motor and the current when the motor just begins to move and calculate the stall current. For motor manufacturers, this is followed by a destructive test to remove uncertainty, but this is unnecessary for our purposes.

The motor will just begin to move when the stiction and friction are overcome by the torque. The torque produced is a function of the Kt. Voila! The stall current will be indicative of a changed magnetic field.

(Old stall current) / (new stall current) not = 1

I do hope this clarifies my remarks.

Bee

÷÷

Postscript for Simon. A motor spun by external means is a generator. It generates electricity as a function of its spin. The polarity of the voltage generated is inverse to the voltage required to make the motor spin. That is, as the motor spins under voltage, the motor generates an opposing voltage which must be overcome by the amplifier in your controller.

[81F]

Many thanks for all the help.

Sadly with my DC 009 layout, I think I will just have to clean it in small sections as it is only the track cleaner that causes the problem and I have just found that a KATO steeple cab will take it much further than my Farrish pannier did.

Regarding the Wrenn loco, the magnet could well be weak so will try a Neo on it. Given that the bearings on the old Wrenn motors look significantly more robust than even the X04s I will chance the extra wear as this loco is only used for the occasional "Heritage" train of tinplate stock only very occasionally.