1980 VW Rabit no throttle responce

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evdriver2008

1980 VW Rabit no throttle responce

Post by evdriver2008 »

I checked the clutch Pedal
The switch has been removed
But 3 of us can not find the reed switch on the schematic?
Anyone have photos or an updated schematic?
Even where it might be or what it looks like or even a part number?
I now have no throttle response

Sharkey
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Post by Sharkey »

Some background:

This message is from a fellow SCT Rabbit owner whose car is experiencing problems with the accelerator. I directed him to inspect the "clutch slip" micro switch on the clutch pedal, and suggested that we continue the discussion here so that the information could be available to others who might use search engines and directories to attempt to find the solution to a similar problem in the future.

To begin with, the circuit in question is located in a PDF file on this site.

The motor on the SCT EV conversion is described in some detail in the SCT manual, chapter 8. Briefly, it is a shunt-wound design, also known as "separately excited". This means that during operation the armature is connected directly to the battery via a heavy-duty contactor, and the intensity of the field windings is varied through a controller transistor. Full field winding voltage (and consequently, current) equals an armature rotation speed of about 1800 RPM. Weakening the field causes the speed of the armature to increase.

It's important in this instance to note that full field equals an "idling" motor. This means that the field is fully energized, and evdriver's problem is that he is unable to weaken the field to increase the speed of the motor. This can be caused be several things, but we'll take the simplest, and most likely first.

In the schematic of the field circuit mentioned in the link above, the field is supplied voltage through relays K1 , sections A, B and C. This is actually two relays connected with the coils in parallel and the three contact sections in series. The series connection is to increase the amount of air gap so that when the relay(s) opens, the arc of the DC current is quickly extinguished.

The field voltage then goes through a 20 ampere fuse, and then to the field windings in the motor (F1 in the diagram, the positive side of the field).

The negative side of the field (F2) is connected to the battery negative through a 120 ohm "safety resistor", which is there to prevent the motor from over speeding in the event of a failure in the controller circuitry. It provides a minimum amount of field current. Without it, the motor would be destroyed if the field was completely removed.

The negative side of the field also returns to the battery negative through Q2, an open-collector NPN pass transistor. This transistor is in turn driven by Q1, an NPN transistor that has been biased to provide a positive voltage to the base of Q2 that is proportional to the PWM pulse duration. Transistor Q1 is driven by a PWM (pulse width modulation) signal from the controller mother board. The PWM is proportional to the position of the accelerator pedal, sensed by an optical pickup in the controller.

The field control circuit is constructed on an angled piece of aluminum inside the controller cabinet. It has only four external connections, and is mounted by four machine screws. My recommendation is to remove the field controller (after disconnecting both the traction battery pack and the auxiliary battery and labeling the wires to the field controller) and check the components individually for proper value and operation. This will mean unsoldering the leads from the transistors and checking them (at a minimum) with a multimeter.

The most common failure I've found with the field controller is that Q2 shorts collector-to-emitter. This effectively connects the field negative to battery negative, resulting in full field voltage (and current), and the resulting inability to speed up the motor. Also check each and every connection for integrity, look for corrosion, pinched wiring, terminals that are improperly crimped, cold solder joints, etc. Separate any Molex connectors between the mother board and the rest of the controller cabinet and look for bad connections. Spray any and every connection with electronic contact cleaner, and "work" them together a few times to remove any surface oxidation.

While we are on the subject, I hope your inspection of the "clutch slip" switch included checking to make sure that the wiring where the switch was removed was properly insulated to prevent accidental contact, yes?

If this doesn't return your car to operation, let me know, there are more diagnostics, but they involve going deeper into the controller mother board, which is uncharted territory for the most part. I have had to reverse-engineer some of that circuitry so I have an idea where to start. I also have (rather blurry) photos of the controller schematic, which will help some.

I hope you have access to an oscilloscope...

deltasonics
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PDF schematic

Post by deltasonics »

Hello Sharkey, can you divulge what program you used to generate the schematic in the PDF for the EV motor controller board? Is it expensive?

Very nice work.

Bill
Things are more like they are now, than they ever have been.

Sharkey
Original Founder
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Post by Sharkey »

The schematics were drawn using Microsoft Publisher, which came bundled with Office 2003 Pro, so I have no idea what the cost is. Does ~anyone~ pay for MS products??? :roll:

Publisher includes vector drawing tools, which means that anything you put together is scalable, can be rotated, stretched freehand or constrained, includes text boxes, clip art, symbols etc. There are a lot of templates, artwork, etc on the net for Publisher.

Once you have a basic set of components drawn (switches, transistors, resistors, coils, etc) it's pretty easy to copy-paste them into new schematic diagrams and size them to fit. Being vector maps, you can gang parts of the drawing together and drag them around with the mouse, etc.

There are probably much better programs for CAD drawing, but this was already installed on my computer, and it worked, so WTH...

deltasonics
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Location: Gloucester, NC
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Post by deltasonics »

Thanks for the info Sharkey.

I looked around and found an interesting open source software SVG program called Inkscape.
I've been playing around with it getting a feel for it. Pretty cool.
I also stumbled on this http://fritzing.org/.
Also open source, still in development. Its an electronics prototyping program that allows you to breadboard virtual circuits with life size parts and dimensions. You can switch between breadboard, schematic and PCB. It is apparently to facilitate prototyping for production. There is a limited parts bin, switches, diodes, inductors and the like but it allows you to design and add your own parts. Could be an interesting pass time during those long winters days.

At any rate, thanks for your kindness and energies invested in this wonderful site.
Things are more like they are now, than they ever have been.

evdriver

Still NO throttle responce

Post by evdriver »

Since last posting still can not get any throttle response
Had a friend clean the board and check the connections
Have re soldiered all the wire connections
Found schematics but they are faded and can not be read

Stealth Camper
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Post by Stealth Camper »

Just a quick 'off topic' here. There are a few PCB places around that give you schematic and layout software. ExpressPCB is one I use from time to time.

Eagle also has a free version that is fairly decent. Plus bigger, better for money.

Sharkey
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Post by Sharkey »

The postscript to this topic is that Evdriver shipped his controller to me in late February, and after replacing a $4 relay, the controller is repaired. I tested it in my car and shipped it back in early March. After replacing a few bum batteries in his car, Evdriver is now enjoying what electric car enthusiasts call the "EV Grin".

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