GP38 Excitation and the PCP

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GP38 Excitation and the PCP

Postby WP Mechanic » Mon Feb 10, 2014 5:18 pm

I was going through my GP38 (non -2) service manual reviewing some information on the excitation system. For the equipment labelled the performance control panel, it is listed as "(When Equipped)" which got me to thinking about a few things concering a project I am involved with: 1) is the PCP necessary for operation of the locomotive? 2) If not, how would the removal of the PCP affect low speed performance (shortline and switching) 3) What railroads ordered the GP38 without the PCP, and are there any schematics available showing the changes.

The project I'm working on is rewiring a GP38 with a stripped electrical system. The intended use is only for shortline haul and switching. The plan was to not include transition, but rather just basic motor connections through the S contactors and make for an extremely basic electrical system.

If anyone could shed some light on these questions, I'd be much appreciative.
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Re: GP38 Excitation and the PCP

Postby WVU » Wed Mar 19, 2014 2:48 pm

The Straight GP38 came from the factory configured in several different ways. I am wondering if the GP38 you have is a Battery Field Excitation Unit with a DC Main Generator and no PCP Panel. What I am saying is that the Battery Field Load Regulator is controlling the Load through some Contactors and Resistors for each Throttle Notch. The FTR & BTR relays will control the Transition Circuit. The Battery Field Current can be between 60 to 80 Amps in in Notch 8. The COLR should be set to pick up at 2450 to 2500 MGA. There was also Straight GP38 units with AR10 Alternator with a PCP panel and these units will have Static Excitation. When you have a unit with Static Excitation, you will find a TRP (Throttle Response Panel) which is made up of a block of 4 relays. You can also adjust the stall current in notch 1 as well at the top of the TRP. You will also have a RCP (Rate Control Panel) and with a throttle sweep, you should be getting 47.5 to 52 VDC out of the RCP within 10 to 18 seconds. With Static Excitation, you will find a Sensor and a Sensor Bypass Panel also. You will also have an SCR on these units also. These units did not make Transition, they made what was called Recalibration There was Straight GP38 units built with a DC Main Generator and a PCP Panel as well and they too had Static Excitation and they made Transition. Your Straight GP38 units with DC Main Generators also made some Steps of Field Shunting as well. Field Shunt 1 would happen around 18 mph and Field Shunt 2 would take place around 22 mph and Transition would happen at 25 mph. On Battery Field Excitation units the Load Regulator will start out in Minimum Field. These Load Regulators are capable of carrying a lot of current. On units with Static Excitation, these Load Regulators will start out in Full Maximum Field. These Load Regulators should have a ohm value of 1500 ohms from connections 1 to 2. Connection 3 is the Brusharm.
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Re: GP38 Excitation and the PCP

Postby RCman2626 » Tue Nov 17, 2015 12:55 am

Resurrecting an old thread. I'm working on diagnosing a load issue. The engine I'm working on is is a straight 38, not a dash-2. Some components it has are an IDAC wheelslip control panel, Performance Control Panel (PCP), excitation control panel (SENSOR, SENSOR Bypass panel, and Rate Control Panel). Unit has a D32E1 main generator.

When engine is pulling, at around 18-20 mph we start getting a surging condition on the load meter. The engine pulls very nice and hard at lower speeds in all throttle positions. The load regulator is set up for maximum field start, which is the default setting on these engines. As the speed increases the load regular starts to drop off toward minimum field as the train builds momentum. But right around that 18mph mark, the load regulator abruptly falls toward minimum field and power drops off. It then smoothly comes back on, only to immediately drop again. Engine speed remains fairly constant for that speed setting, only varying slightly due to the change in load. The condition continues even if you notch back on the throttle and then try to come back.

See this video I made of the condition. I was in the 6th notch and the conditions were dry, straight track on a slight upgrade. https://www.youtube.com/watch?v=SLCgkBEUJDQ

I'm wondering if this is an issue with the transition sequence. I know Field Shunting happens around that speed. Reading my service manual for this engine, to does say that around 1020 volts when the unit begins Field Shunting the Over Riding Solenoid inside the governor is energized for "soft shunting" which would drop the load regulator to minimum field. I wonder if the ORS is being energized for shunting, but the actual shunting is not happening. I did not hear any relay action from the electric cabinet while the surging condition was happening.

Any pointers on where to start out? Thanks!
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