How to stop high dc currents...

[Keiv]

Hi to all:

Here is the deal: I'm building an electric kart using a Lynch DC motor rated at 48V, and I can't use an electronic controller for the speed because I'm building it for a one time race (2 miles, flat terrain) with very limited budget.

What i really want is to directly connect the 4 12V batteries (100A/Hr, very cheap lead/acid batteries) in series with the motor and start the race. But i need to reduce speed in some points of the track, and i can't figure out how to stop the 150A DC current the motor will consume... spending less than 20 or 30 USD

I was thinking in some relays in parallel, or using simply a high current switch to stop the current, but the switch capable to handle this kind of current is very expensive around here... So, i'm open to any advice or suggestion.

Thanks a lot (in advance ). Please don't call me crazy or stupid... I have a REALLY limited budget...

Pd: Can i use AC switches rated at the same current?

 

[BJC]

Look at starter relays in trucks. Lots of them are rated at 24 Volts and several hundred amps. They might work. I have read that newer trucks are being built with 48 volt DC systems although I havent seen one. It may not be cheap to get one now.

 

[BillJ]

Here is a link on re-rating AC switches:

http://us.switchzone.com/electrical.asp

 

[BillJ]

Sorry, the link didn't go all the way thru. Select the United States (map), Supplemental (top menue), Electrical Ratings (pull down menue).

 

[dydt]

What I have in mind is using an SCR or Silicon Controlled Rectifier. Its a 3 terminal component and acts like diode except that there is a third terminal which you called the gate that when triggered will let a DC current pass through. You have to check some books on basic thyristor circuits when you want to try this. Connect the positive side of the battery to the anode side of the SCR (forward biased ). Cathode to the DC motor and the other leg of the motor back to the minus of the battery. You might still need a high amp breaker in series to switch off when you want to disconnect the battery when you reach that part of the track. After passing that point switch back the breaker on but your battery is still not connected unless you fire the gate of the SCR. You can try the following to give you an idea. Get a low amp SCR, a 12 volt lamp and a 12 dc battery. Connect lamp in series with the battery. Minus of the battery to cathode and the anode in series with the lamp then back to the plus side of the battery. Using piece of wire, jump or momentarily short the gate to cathode and lamp should come on....You can only turn the lamp off when you break the circuit....Try this first to give you an idea...

I hope this helps you...

Best Regards
dydt (toronto)

 

[NEWFIEBOY]

Hi Keiv
Do you need variable control of the motor current? Would it work for you if you were to interupt the motor current and coast in the places where you need to slow down? If the latter would work then I would recommend a simple knife switch. You could operate the switch manually and use it to connect or disconnect the motor as needed. These switches and readily available and inexpensive. You could even make your own if you wanted to. There would be some arcing at the contacts each time you open and close the switch considering the 150 amp line current. Since the race is only two miles long, the life expectancy of the switch may not be an issue. If you wanted to reduce the amount of arcing and extend the switch life you could experiment with capacitors across the switch contacts.