Field Shunting for dummies (HUH?)

Have you ever wondered what the term Field Shunting is I did, but for a long time i didn't bother to find out. Finally, when i did, i came up with this :

Field Weakening:

The DC motor can be made to run faster than the basic "balancing speed" achieved whilst in the full parallel configuration without any resistance in circuit. This is done by "field shunting". An additional circuit is provided in the motor field to weaken the current flowing through the field. The weakening is achieved by placing a resistance in parallel with the field. This has the effect of forcing the armature to speed up to restore the balance between its magnetic filed and that being produced in the field coils. It makes the train go faster.

 

Various stages of field weakening can be employed, according to the design of the motor and the intended purpose. Some locomotives used as many as six steps of field weakening.

 

Thought it was pretty interesting.

if the MTA ever does rebuild the R62/A and the R68/A, they should re-include Field Shunting cause those trains are mad slow.

Back in the late 70's through late nineties, Field shunting was gradually phased out from the fleet to improve the safety margin starting with a test train of R42's.

Link to article:

http://www.railway-technical.com/tract-01.shtml

this website has really detailed information of almost every aspect of what makes a train run and work.

Have you ever wondered what the term Field Shunting is I did, but for a long time i didn't bother to find out. Finally, when i did, i came up with this :

 

 

 

Thought it was pretty interesting.

if the MTA ever does rebuild the R62/A and the R68/A, they should re-include Field Shunting cause those trains are mad slow.

Back in the late 70's through late nineties, Field shunting was gradually phased out from the fleet to improve the safety margin starting with a test train of R42's.

 

Link to article:

http://www.railway-technical.com/tract-01.shtml

this website has really detailed information of almost every aspect of what makes a train run and work.

LOL when some of the trains were rebuilt the field shunts were removed during higher speed acceleration! (They still do their thing at lower speeds). See R-44, for one.

Also not mentioned is that when field shunting is done it actually reduces the power of the motor, so even though it will go faster, if it has to climb a hill it will lose speed much faster than if not shunted because the shunting reduces the overall torque of the motor.

LOL when some of the trains were rebuilt the field shunts were removed during higher speed acceleration! (They still do their thing at lower speeds). See R-44, for one.

 

 

Also not mentioned is that when field shunting is done it actually reduces the power of the motor, so even though it will go faster, if it has to climb a hill it will lose speed much faster than if not shunted because the shunting reduces the overall torque of the motor.

i guess if you include a "smart" computor system that can control when to apply shunting (for example on flat grades and acceleration from dead stop) and disengage it (uphill grades) it would be better...

but that's just me. plus, sources don't exactly mention whether or not field shunting was removed completely during overhaul, or if they still do infact function at low speed acceleration (0-20).

i guess if you include a "smart" computor system that can control when to apply shunting (for example on flat grades and acceleration from dead stop) and disengage it (uphill grades) it would be better...

but that's just me. plus, sources don't exactly mention whether or not field shunting was removed completely during overhaul, or if they still do infact function at low speed acceleration (0-20).

They do function at low speeds. Without them, when a train takes power, you'd get tossed on the floor unless you had a really good grip on something steady.

They do function at low speeds. Without them, when a train takes power, you'd get tossed on the floor unless you had a really good grip on something steady.

Correct. It reduces the tq output from stopped. If full tq was available and delivered from stop, no one would use the subway.

- A

ok.. quick propulsion lesson.

The system is operated by a mechanically driven cam switch unit. Its main purpose is to switch the traction motors from a series circuit (which gives each motor a maximum of 150volts to each) to a parallel circuit (1 per truck) which puts 300v to each motor. In the process as the cam progresses, it drops out or picks up resistor banks as needed. The resistor banks are the big white squares that you see underneath the cars on the sides.

These resistors work according to a set speed notch. i.e. When the train is at a complete stop and takes power... all resistors are in the circuit to prevent the wheels from spinning and/or snapping peoples necks off that are riding. As the car gains speed and hits a specific "speed notch". The cam moves up a position and drops a resistor. This will increase torque slightly to pick up more speed. Same as stepping on the gas in your car. This happens until all resistors are dropped out and full parallel voltage is being seen by the motors.

When power is removed and braking is applied.. the same theory replies only reverse. DC motors now become generators and the cam will run backwards to remove or add resistance accordingly until all the resistance is back in the circuit. After that.. mechanical brakes are applied for the final stop.

Field shunting is not used anymore because the cars were too fast and has been removed from EVERY car. The cars are limited to a max 50=55+ mph depending on downgrade. There is a switch in the T/O cab that says Energy Conservation that would enable field shunting. Its been disabled.

Quick explanation but there is tons more involved.

so i'm going to assume that AC Traction motors don't have this kind of feature? or do they:confused:

nope.. they tried something in the lines of it on the R110 but didnt work. The nice thing about AC traction motors is the smooth acceleration curve. Less torque than a DC motor but smoother.