Safe Delay between switching polarity

546 Views 4 Replies 2 Participants Last post by galderdi, Jun 21, 2017

galderdi

Discussion Starter · Jun 21, 2017

Hi everyone.

I have an Advanced DC FB1-4001 9 inch Series wound DC motor.

Currently I achieve reverse by physically selecting it with a gearbox. But I am using the car for motorsport and some of the time trials require reversing. So physically selecting the gear is slow. Plus sometimes the gear does not select properly so I am left motionless with a spinning motor and have to wait before selecting reverse again.

I am going to install a reversing contactor. I understand how it works.
I believe care must be taken to pause between switching from one direction to the other. One reason for the pause is to ensure the throttle is not active when the switching occurs and the other is to ensure no residual current is present in the motor.
I also understand that most people using reversing contactors on road cars would carefully pause between switching from one direction to another.
But in the heat of the moment, trying to save time, I could easily switch too quickly resulting in significant issue.

I plan to put some safe guards in place to enforce a sequence and include pauses at the appropriate steps in the sequence.

1. Cut the throttle first
2. Introduce a delay
3. Cut the active contactor
4. Introduce another delay
5. Connect the opposite contactor
6. Introduce another delay
7. Reconnect the throttle

What is the appropriate delay to ensure now residual power is in the motor and protect my system?

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Duncan

· #2 · Jun 21, 2017

Hi Galderdi

I'm using a reversing contactor
What you actually have is two contactors
If neither is active - no drive
If both are active - no drive

If "A" is ON and "B" is OFF - Forwards
If "A" is OFF and "B" is ON - reverse

So I have simply got two switches - one of them has a "gun lock" on it

The other thing I do have is another brake light switch on the brakes this shorts across the throttle - so when the brakes are applied I am at zero throttle

I don't do the rapid reversing autokhana stages
Two reasons -
My motor is 8 degrees advanced - which will mean that it is wrong for reversing
My rearwards visibility with my full harness seat belts is totally crap!

Thinking about it I think I will try some of those BUT I will only do the reversing with the switches in the 30% power position - and with my seat belts slack
I wonder if I can do those events without the shoulder straps?

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galderdi

Discussion Starter · #3 · Jun 21, 2017

Thanks Duncan.

Here the motorkhanas are the events with reversing and they do not require the shoulder straps.

I too have advanced timing on the motor. But I figure reversing is always slower anyway, because it is more dificult. But I will rig up a way of reducing the throttle while in reverse as an added precaution.

I am aware the reversing contactors are actually 2 contactors. But I am still concerned the time between them switching over may not be sufficient to ensure the dissapation of any residual power in the motor.

How much time does the motor need?

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Duncan

· #4 · Jun 21, 2017

Hi
I'm not sure that the motor needs any signifigant time
In my car I will be hard on the brakes - which will mean that I am feeding the motor zero amps - for a long time in electronic terms - 14,000 cycles per second for the controller - so 1/2 a second is a long time

Then throw the switches - which switch the contactor that had been Off on and the other one that was On Off
The time required for this is just the time for the contactors to do their thing

If both of the contactors are ON together - then nothing happens!

Then there is the time for me to move my foot from the brake to the throttle

As far as I can see the failure mode would be if I switched with some throttle on - and the part that could be damaged would be the reversing contactor

If I was brave I could think about doing the change-over while still moving forwards - I think that the motor would be OK with that - but I'm not sure about the rest of your transmission

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galderdi

Discussion Starter · #5 · Jun 21, 2017

OK sounds good.

I am going to automate the killing of the throttle as the first step as there is a risk that I could be touching the throttle as I slam the brakes.

It sounds like I can apply the slightest of delays between each step of the sequence.

There is a fair chance I will flick the switch with some forward momentum. But I will be aiming for 0 momentum and if there is any it would be by mistake rather than intentional. Hopefully if I do make that mistake it will be so close to 0 it won't brake anything.

I will have to implement yet another dial to control the throttle limit while in reverse. I would set a permanent limit but I don't yet know how much throttle I can use in reverse without causing arching.

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