Traction motors: brushed vs. brushless?

[Convicted One]

A  few years ago I upgraded all my cordless power tools to brushless design.  I'm sold on the concept.

This combined with the rail industry's  recent affection for A/C locomotives got me to wondering.

Were most D/C traction motors brushed? if so what was the anticipated service life of the typical set of brushes? Was this something that was just scheduled for periodic replacement as other items on the machine required service, or were they especially prone to wear, requiring more frequent attention?

Was there any point where "D/C brushless" technology was in vogue for locomotive traction motors?

Are the newer A/C traction motors brushless?

Thanks in advance for any answers.

[jrbernier]

  Most DC traction motors I have seen are series wound, with 4 wire leads.  Maybe early trolley  car motors had brushes...

[Erik_Mag]

The term "brush" was derived from early dynamos and motors using brushes made of copper or brass wire. Staring in the mid 1880's, brushes were being made out of graphite blocks, with one end milled to fit the shape of the commutator. All DC traction motors and most AC traction motors up till ca 1960 had brushes. Recall that the motors used on the GG1 were AC series morirs and needed brushes for the commutation, the traction motors on the original GN 3 phase electrics, the N&W phase converters and first class of VGN electrics used wound rotor induction motors requiring brushes for the slip rings.

Brushless DC motors are some sort of AC motor (squirrel cage induction, hysteresis synchronous or permanent magnet synchronous fed with an inverter. The devleopment of power MOSFET's drastically reduced the cost of small inverters used on power tools. The development of Gate Turn Off (GTO) thyristors, IGBT's and SiC MOSFET's have made locomotive sized inverters practical.

[Convicted One]

Thanks for the wealth of info. So does that mean that contemporary locomotive traction motors are all brushless?

[Erik_Mag]

Modern AC traction motors are indeed brushless, DC traction motors still have brushes for the commutator.

AC traction motors have several advangates. One is generally lighter weight for a given power rating. The motors are generally more rugged. Brushes are a maintennance headache. AC induction motors have a steep torque curve, where torque falls fast when wheel slips.

Disadvantage with AC traction motors is needing an inverter along with associated control system.

[Convicted One]

Okay, so "brushless" might well be one advantage that makes A/C  power  attractive to the railroads?

I've done a bit of work with smaller electric motors, and just the thought of the power needed to start a huge train rolling, seems like it would be a nightmare on the brushes.  I guess if Randy Stahl is still around he could give some idea of the service life of brushes in freight railroad traction motors.

A few years ago I rebuilt an antique 1905 era GE  variable speed A/C repulsion  motor that had the brushes mounted in a movable yolk which moved around the commutator similar to the way the  old spark advances moved in an automotive distributor. By moving the brushes it shifted the polarity in the rotor windings relative to the charge in the stator  windings and would vary the speed from full forward  to stop to full reverse with very fine control. Had 4 sets of brushes that were a pain to access for changeout. Scaling all that up to a size  sufficient to start a heavy train from a dead stop seems overwelming

Here is a picture, the actual size of the motor was about the size of a 20 gallon drum.... the control arm that shifted the  brush yolk is front and center

[Erik_Mag]

An impressive looking beast!

The brush shifting was an ingeneous way of geting variable speed without the use of rheostats. I've seen brush shifting on a 400kW generator of similar vintage to the motor, with the need for brush shifting eliminated by the development of commutating poles.

[Overmod]

Convicted One

Okay, so "brushless" might well be one advantage that makes A/C power attractive to the railroads?

It's certainly one, but other advantages far outweigh it.  (Keep in mind that some AC motors still require 'brushes', but they run on slip rings for clean contact, not an interrupted commutator surface with high effective inrush currents and large inductive capacity for nifty fat sparks at 'break')

There were some frankly ingenious designs for 'sparkless' motor commutators; the Sprague papers in New York have some discussion of them.  It's also relatively easy to wire up 'pony brushes' that make contact slightly ahead of the main ones, and soft-start the current (or bleed off induced current, etc.)

 

[oltmannd]

DC traction motors are and have been a huge pain in the neck for railroads.

Yes, you have to replace the brushes periodically.  An electrician would check them and wipe down the commutator every 92 day inspection, typically.  

The carbon brushes had to be the right grade for the service.  The current going through them softened them and allowed them to lay down a lubricating film on the commutator.  High speed, low current service needed a softer brush than low speed drag.  Too soft a brush, and you get short life. Too hard, and you get scoring.

Brushes were held against the commutator with springs that attempted to maintain constant pressure even as the brushes wore down.

If a DC traction motor overspeeds, it will "birdsnest" as the commutator comes apart from centripital force.  If you go over a frog or rough spot on the rail, the brushes may bounce and cause a flashover.  It's quite the light show at night, but it burns the heck out of the motor.

The locomotive builders tried like crazy to keep upping the capacity of the DC traction motors - thinner insulation, better cooling, but still, the DC motors were the weak link in locomotive design.  Over heat them dragging up a hill, and the motor was toast - almost literally. 

Depending on the road, traction motor maintenance was between 25 and 50% of the total cost of maintaining a locomotive (including overhauls).

The squirrel cage AC traction motors on AC units can take all the traction power supply can give the all the time.  They rarely fail. Its part of the attraction of AC power.

[Shadow the Cats owner]

I remember reading something in my hubby's old Trains that an early SD70MAC had broken a traction motor shaft off at the gearset or something like that.  It was not discovered until the next scheduled service interval for the engine.  The railroad said if it had been a DC engine the motor would have been a chunk of scrap within a couple of minutes.  

[Erik_Mag]

I remember seeing that same story. the birdnesting problem is with series motors with no theoretical limit to unloaded motor speed (small motors may be windage limited, large motors birdnest). A few locomotives, such as the original AEM-7's, used separately excited field windings and the unloaded speed would be a bit higher than when normally loaded (very much like induction motors).

Another reason that the problem wasn't noted before was that the early EMD traction inverters used a single inverter per truck. A GE locomotive with an inverter per axle would have spotted that one axle wasn't loading.

[Erik_Mag]

Overmod

 

Convicted One

Okay, so "brushless" might well be one advantage that makes A/C power attractive to the railroads?

 

It's certainly one, but other advantages far outweigh it.  (Keep in mind that some AC motors still require 'brushes', but they run on slip rings for clean contact, not an interrupted commutator surface with high effective inrush currents and large inductive capacity for nifty fat sparks at 'break')

AC motors with slip rings could be either a wound rotor induction motor or an externally excited synchronous motor. Vacuum cleaners typically have AC series motors as the torque characteristics of series motors are a good fit with the torque versus speed requirements of the impeller.

There were some frankly ingenious designs for 'sparkless' motor commutators; the Sprague papers in New York have some discussion of them.  It's also relatively easy to wire up 'pony brushes' that make contact slightly ahead of the main ones, and soft-start the current (or bleed off induced current, etc.)

I am assuming the sparking was due to "armature reaction", where the magnetic fields induced by the current in the armature windings was distorting the magnetic field applied by the field coils through the pole faces. First real fix was use of the interpole and for some applications interpoles combined with pole face winding (as used on the DC side of the Milwaukee M-G sets).

Besides drastically reducing commutator wear and tear, the interpoles also allowed for a higher voltage difference between commutator bars. This made it practical to make DC motors that could run off of 1200 to 1500VDC directly (3,000VDC required two motors in series).

[zardoz]

oltmannd

If you go over a frog or rough spot on the rail, the brushes may bounce and cause a flashover.  It's quite the light show at night, but it burns the heck out of the motor.

I used to (when possible) reduce the throttle to the 5th notch when the power was going over those aforementioned 'bumps in the road'. Unless I was in a mood, then I'd leave it in the 8th notch and wait for the show.

However, flashovers are not cool when one has the high-voltage cabinet open while trying to fix some electrical problem enroute, which was quite common with the F7 and E8 suburban units.

First rule of electrical repairs: If one hand is in the electronics, the other hand (and arm) is not to be in contact with anything (thereby denying electricity a complete path). Rather difficult sometimes due to the locomotive bouncing and swaying.

[BaltACD]

zardoz

 

oltmannd

If you go over a frog or rough spot on the rail, the brushes may bounce and cause a flashover.  It's quite the light show at night, but it burns the heck out of the motor. 

I used to (when possible) reduce the throttle to the 5th notch when the power was going over those aforementioned 'bumps in the road'. Unless I was in a mood, then I'd leave it in the 8th notch and wait for the show. 

However, flashovers are not cool when one has the high-voltage cabinet open while trying to fix some electrical problem enroute, which was quite common with the F7 and E8 suburban units.

First rule of electrical repairs: If one hand is in the electronics, the other hand (and arm) is not to be in contact with anything (thereby denying electricity a complete path). Rather difficult sometimes due to the locomotive bouncing and swaying.

I seem to recall that the B&O had a rule for the locomotives to be in no higher than the 4th notch when going over railroad crossings at grade.  I am not away of any restrictions when going over the frog of a regular switch or crossover.

Worked a number of towers where there were railroad crossings at grade - never saw 'the light show'.

[Overmod]

BaltACD

I seem to recall that the B&O had a rule for the locomotives to be in no higher than the 4th notch when going over railroad crossings at grade.

It's also in the EMD manuals; someone with a copy should look up the exact language and post it here.  (For some reason I remember the F3 manual saying you closed the throttle entirely going over road crossings, so particularly interesting if it varies by year or model.)

Part of the fun with flashovers is not just brush bounce; some of the carbon from brush wear can remain in "finely-divided powder" form below the commutator, and a good bang to the motor case levitates it (much like the coal-dust in certain older ocean-liner bunkers, like those on Titanic or Lusitania) forming a nice transient conductive path between commutator segments around what could be a substantial arc ...