Nostalgia & History > "Balancing speed"?

In reading about interurban and street railroads I have run across the phrase "balancing speed". I have no clue what this means. Clearly the cars are balanced, lest they tumble from the rails! I suspect that the phrase has something to do with achieving the intended operating speed, but I'll await the true answer from those more knowledgeable than myself.

I used to know this. . . I think it's the car's top speed, where it can't go any faster due to wind resistance. Unfortunately, Mr. Google couldn't come up with a definition.

wlankenau Wrote:
-------------------------------------------------------
> I used to know this. . . I think it's the car's
> top speed, where it can't go any faster due to
> wind resistance. Unfortunately, Mr. Google
> couldn't come up with a definition.

Walt, I burrowed further into Mr, Google while trying to push the morning fuzzies out of my head with a cup of coffe and came up with this explanation from a Canadian freight train derailment report.

"the speed at which the weight of a car would be equally distributed on both rails and no lateral force"

The complete report in PDF form can be located here www.tsb.gc.ca/en/reports/rail/2004/r04m0032/r04m0032.pdf

Cheers,
Africansteam

Just speculating, but based on AfricanSteams' find it sounds like a reference to the speed on a superelevated curve at which the car is neither pushing against the lower (inside) rail, as would happen at low speed, or the higher (outside) rail, as would happen at high speed.

Dave

DavidP Wrote:
-------------------------------------------------------
> Just speculating, but based on AfricanSteams' find
> it sounds like a reference to the speed on a
> superelevated curve at which the car is neither
> pushing against the lower (inside) rail, as would
> happen at low speed, or the higher (outside) rail,
> as would happen at high speed.
>
Dave, you are correct. The accident in question happened on such a curve.

Africansteam

I believe balancing speed as used on an electric car is the speed at which the back EMF of the car in full parallel (or as parallel as the switch gear will permit for cars with motors permanently wired in series) is balanced against the line voltage and further speed is not attainable on level track, without shunting.

I have some old traction manuals at home and will try to verify this tonite.

MTMEngineer Wrote:
-------------------------------------------------------
> I believe balancing speed as used on an electric
> car is the speed at which the back EMF of the car
> in full parallel (or as parallel as the switch
> gear will permit for cars with motors permanently
> wired in series) is balanced against the line
> voltage and further speed is not attainable on
> level track, without shunting.
>
MTM, I think you nailed it. see below.

"When the car is starting, the starting resistances are in the circuit to limit the current and the tractive effort. At a speed of 3.9 mph, the available tractive effort from the motors equals the adhesion. At a lower speed, the wheels would simply slip. If full power were applied at this point, the acceleration would be 4.4 mph/sec, the same as in the emergency stop. Since this is uncomfortably rapid acceleration, some lower value, such as 2 mph/sec, would be used (by keeping some of the starting resistance in the circuit). This value could be maintained up to about 5 mph, when full power would give 2 mph/sec. Acceleration drops rapidly as the speed increases, both because the tractive effort is inversely proportional to the speed, and because the resistance begins to increase. At 10 mph, the acceleration is 1.6 mph/sec; at 20 mph, 0.73 mph/sec; at 30 mph, 0.42 mph/sec; at 40 mph, 0.23 mph/sec, and at 50 mph, only 0.09 mph/sec. The balancing speed, when maximum tractive effort equals the resistance, is about 56 mph. On a 1% grade, a speed of about 40 mph can be sustained, and on a 2% grade, 30 mph."

From: http://www.du.edu/~jcalvert/railway/trolley.htm

Cheers,
Africansteam

MTMEngineer is correct...I've always heard that 'balancing speed' referred to electric traction...even GG-l's

AH, many thanks

Righto re "balance (or balancing) speed" for any piece of propelled rolling stock powered with electric motors.

Balance speed means the fastest the car or locomotive can move without adding horsepower and/or reconfiguring the traction motor circuits. That's why a unit, a GP9 for example, will not be able to reach full maximum speed if it cannot make transition from series-parallel to parallel circuits.

"Balancing speed" is the maximum rate of rotation in rpms that the traction motors will turn under load at a given voltage, taking into account bearing friction, train resistance, the rate of dissipation of heat caused by the electric currents in the motor, and most importantly the ability of the armature to keep its windings from birdcaging (flying out of the slots in the armature). In other words, at "balancing speed" the energy input from the trolley is equal to (or "balances") the mechanical output of the motor.

With fixed voltage, variable current systems, like electric railroads, the balancing speed is expressed in terms of the highest practical speed the car or locomotive can go when operating at full line potential on straight, level track. For example, the SP red electrics were designed for something like a 43.5 mph balancing speed, the Key System transbay cars, 1/2 mph less. This doesn't mean they never ran faster (as in "downhill"), or that a radar gun would show no variation from these values, just that with the controllers on the brass, these are the highest typical speeds either set of equipment would go on level track.

With variable voltage, variable current systems (i.e. diesel-electrics) the balancing speed changes with each throttle position because each position changes the output voltage of the generator (and the current).

Any traction motor-driven locomotive will "balance" at a speed lower than its maximum permissible speed depending on train weight and resistance, track gradient, curve friction, and throttle position (which is dependent on the maximum authorized speed for the track it's running on).

"Balancing speed" has been the way electrical engineers express the maximum normal operating speed for an electric car or locomotive for a very long time.