[PRCo] Re: voltage and frequency

[Boris Cefer]

On older equipment the motors are controlled rheostatically. I am an expert 
on this in regard to PCC cars, but don't we have something on this subject 
in the archive?
Later, in the electronic age, rheostats started to be replaced by chopper 
control (thyristors) and more recently transistors. These systems have far 
higher efficiency because no energy is dissipated in rheostats (resistors). 
And again, more recently DC motors on rail vehicles are being replaced with 
AC motors fed by DC/AC converters. The AC motors have no brushes and the 
control system contains only few contacts and other moving parts which means 
considerably reduced inspection and maintenance requirements.

But to reply immediately some of the PCC questions.... The metallic objects 
(4 on car) between wheels are electromagnetic track brakes. They are used 
only in emergencies when the motorman pushes the brake pedal more than half 
way down or when he releases so called "dead man" or safety pedal. Normal 
service braking is of electrodynamic type, energy generated by traction 
motors being circulated through braking resistors. At a low speed when the 
dynamic brake fades out, it is automatically followed by a mechanic brake, 
wheel tread brakes operated by air on earlier PCCs or motor drum brakes 
controlled electrically on most postwar cars. Both starting and dynamic 
braking current are controlled by a so called limit relay (or 
accelerating-braking relay) which is controlled by power and brake pedals. 
The limit relay controls all movements of "pilot motor" which is a low 
voltage motor operating the main controller called "accelerator" 
(Westinghouse) or "commutator controller" (General Electric). Controllers 
from these two manufacturers are of different design, but both consist of a 
large number of resistor elements providing small resistance steps.

My time is very limited now, but if you have specific questions, you can ask 
them.

Boris

----- Original Message ----- 
From: "robert simpson" <bobs at pacbell.net>
To: <pittsburgh-railways at dementia.org>
Sent: Saturday, September 01, 2007 4:54 PM
Subject: [PRCo] Re: voltage and frequency


> Thanks Boris and Bob;
>
>  A rectifier and battery system would make an excellent system.  It could 
> also provide emergency lighting for a short period of time should there be 
> an electrical interruption - and there would be no flickering/twinkling of 
> lights.
>
>  A previous responder mentioned they use variable transformers on European 
> trains to control/regulate speed.  How did they control speed on a Direct 
> Current system?  I remember watching the operators on PRCo pressing the 
> "accelerator" (sorry, I don't know the appropriate lingo) to the floor 
> when starting and the wheels didn't spin.  Was there some kind of 
> regulator which limited the amount of power going to the motors?  A simple 
> rheostat would seem to be very inefficient.
>
>  Did the Direct Current cars use resistive braking on long downhill 
> grades?
>
>  I have recollections of a rather large, flat metallic object hanging 
> between the wheels suspended only about an inch above the rails.  Was this 
> part of the braking system?
>
>  Bob Simpson
>  from Krazy California