Our trolley can pull around 27KW but that is only under heavy load. We are also only running two motors and 300v instead of 600v and have no compressor.

Correct, these values for setting the ABR (GE) or Limit Relay (WH) are per motor group.

The motor groups are the 1-2 and the 3-4, in each case, two 300V motors hard-wired in series on the truck and coming back to the control group with 4 motor leads, so effectively the same as two 600-volt motors. In acceleration, PCC motors are connected in parallel (there is no series-parallel transition) therefore the line amperage is double the motor amperage.

The ABR/LR series coil is in series with one of the motor groups on GE all-electric cars and WH cars. Therefore the current given in the spec for setting the ABR/LR is for the per-motor current. Btw, the GE cars put the ABR between the armature and field of the 3/4 motor group, with the resistances downstream, so those two wires running from the ammeter shunt terminals to the meter have quite a bit of potential to ground!


However, on the GE air cars, at least the early ones that I have worked on, the ABR series coil is on the ground end of the circuit, in series with the KM brush arm, so it is measuring the total current. I don't have a scanner handy at the moment, but here is a quote from the 17KM3A1 GE documentation (e.g. brooklyn cars 1000 and 1001):

Accelerating range line amps: 225-500 (240-535 for "Chicago car")
Braking load amps: 160-360 (chicago: 170-385)
Coasting load amps: 27-28

Note that these numbers are approximately double the per-motor currents given in the postwar GE documents you cited.

Also, I can tell you anecdotally from being in the substation while PCC cars are running that they are drawing north of 200A in "switching". A typical streetcar with 4, 40hp motors is about 100-140A in the first notch.

One other thing, real "back of the envelope" stuff: It is easy to make an estimate for how much current any car will draw off the line in "first notch" or "min power". At standstill, there is no counter-EMF. Simply divide the line voltage by the total grid resistance in the circuit. Generally the motor armature, interpole and field resistances are so small relative to the starting grid resistance that those numbers can be ignored with minimal error. Tack on a few percent for the resistance of the wiring itself.

So for example, the GE PCC cars. Pre-war air-electric: 4.0 ohms in "switching" or 150 amps, min accelerating the C1 contactor closes and we have 2.0 ohms (brush arm still at "A") or about 300 amps on a 600V line. More accurately: allow .12 ohms for the motor resistance (total of the series/parallel combination) and about .15 for all the wiring and miscellaneous contact resistances, gives about 265 line amps.

Likewise, postwar all-electric cars, start with about 3 ohms resistance in switching, but C1 picks up almost instantly cutting about about a half ohm, so 240 amps in switching (with fields shunted for reduced torque), then when the pedal hits the accelerating range, C2 closes and cuts another approx .5 ohms.

We recently got our North Shore running again (flood remediation program) and I measured its first-notch standstill current (brakes appleid): it was 380A, but that is on our 560V line, and probably a good 20-30 volts of drop too.