Roof-Mounted Equipment On Catenary-Powered EMUs.

Hi, if anyone is willing to entertain the answer to a question, please. I'd like to know, what is the difference between catenary-powered EMUs (in the following respect): Why is it that some cat-powered EMUs have roof-mounted equipment other than pantographs and others just have pans? The MN M2/4/6s have that roof mounted gear, some of which I know are transformers. This might be beceuse the motors are designed to run on 3rd rail voltage and the transformers bring it down to that voltage, but SEPTA's Silverliners also have roof mounted "boxes" similar in appearance to the M2/4/6's. However, NJT's Arrows do not have any such apparent gear but just have the pantographs. One guess of mine would be that the NJT MU's traction motors run at higher voltages and don't need the transformer equipment. Another guess is that the transformer gear is at some other location on the car.

The roof mounted equipment is braking resistors and acelleration resistors. All traction motors operate at more or less the same voltages, and all AC powered EMUs have transformers, traditionally they're undercar. Putting the resistors up on top is useful for cooling - they get really hot.

Silverliners don't have resistors because they don't have dynamic brakes, except for the SL IVs (which have roof bumps). None have accelleration resistors since they're phase angle control, and thus don't need them. I think the M cars are actually phase angle on AC, but since they run on DC too, they need accelleration resistors, since DC choppers and AC inverters didn't exist back then.

The resistors on the Arrow IIIs are braking only, since AC inverter systems don't use acceleration resistors, but do need resistors for braking - the brake unit clamps the DC link by rapidly connecting and disconnecting the resistor bank accross the DC link's + and - terminals. That's why Arrows make that buzzing noise as they slow down - the brake unit sensed the rise in DC link voltage and is clamping it. You can do other things with that power, like run the HEP units, and if those are enough of a load, the brake unit won't activate, and the converter feeding the DC link won't be feeding power and the train will literally be powering the HEP via its regenerative braking. Some dual mode and straight electric locomotives do this too - they'll use the inertia of the train to generate HEP through power gaps.

You can also bounce the power back out over the power lines - the new NYC subway cars do this - if the third rail can accept the power, a slowing train can send it there, and a running or accelerating train on the same substation will use the power to accelerate. If there's no train to absorb the power, the brake unit and HEP stuff can. This is yet another great thing about AC inverter drives that you can't do with the classic DC systems that nobody buys anymore...

Thanx for the info, it's appreciated!