# controller design



## muffildy (Oct 11, 2011)

So i had an odd thought...
a controller is basically a microprocessor that tells some capacitors when to discharge and keeps the caps from overdischarging or overcharging right?

What if someone built a controller with a bank of supercapacitors with enough capacity to run the motor for 20 seconds at peak power?


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## Ziggythewiz (May 16, 2010)

Or you could buy a Tesla Roadster for the same price & weight.


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## muffildy (Oct 11, 2011)

it would really take that many caps just for 20 seconds?


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## EVfun (Mar 14, 2010)

muffildy said:


> So i had an odd thought...
> a controller is basically a microprocessor that tells some capacitors when to discharge and keeps the caps from overdischarging or overcharging right?
> 
> What if someone built a controller with a bank of supercapacitors with enough capacity to run the motor for 20 seconds at peak power?


I don't think you understand what a controller is doing. A controller is rapidly switching (somewhere around 15,000 times a second) the battery pack. The motor "sees" a voltage that is the controller's on time, times the pack voltage (as a percentage expressed in decimal form.) The current is a function of the voltage on a motor and its rpm. Your right foot dictates on time, with modification. At low rpm (hight current at low on time) the on time lags behind (less than) the throttle position to promote drivability. If the motor current wants to exceed what the controller (or motor, or battery pack) can handle the controller again reduces the on time below what is being asked for by your right foot. 

The capacitors keep the battery pack and wiring inductance from blasting the switches (generally IGBTs or MOSFETs) with a voltage spike when they turn off. The freewheel diodes keep the motor inductance from pulling the other side of the switch low which could present across it a voltage difference greater than its rated to handle. If either of those fail the IGBTs or MOSFETs will fail. 

Smart controllers also watch the switch temperature, freewheel diode temperature, and capacitor temperature so nothing gets overheated when the controller is working hard. The controller takes some action if the heat reaches unsafe levels. Often this is a further reduction in the current limit or slowing of the switching speed. Modern computerized motor controllers also check other parts of the system and shut down if unsafe conditions are measured.


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## muffildy (Oct 11, 2011)

oh ok, so the capacitors used in the controller are there just for protection and not for energy storage.


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## Ziggythewiz (May 16, 2010)

Correct. Capacitors are not really used to store energy, but to buffer it. 

Running some quick numbers I think I was off by an order of magnitude with my earlier statement, so you'd only waste $10K, and 300 lbs. So, wanna drive 20 seconds on supercaps, or 80 miles on lithium?


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## domosher (Jul 10, 2011)

The capacitors in a controler supply power to the motor during the initial pulse time less than 0.00001 seconds each time a new puls is created the rest of the pulse is supplied by the battery the voltage drop is usually less than 5% of the initial charge on the capacitors if the batteries with the series inductance had to supply this high frequency current the voltage to the motor would be less than half of the battery voltage. The voltage drop through the IGBT typically about 2 volts they have some lower 1.6 volts weather running low or high current need more cooling than MOSFET's which have a more linear voltage drop based on voltage drop through on resistance. The MOSFET's work best below 300 volts and the IGBT above 300 volts. The comutating diodes conduct the motor current between the power pulses a standard diodes diode has between 1 and 2 volts drop across it while conducting this is motor amps 200 to 1000 where a Shotkey diode has 0.4 to 1 volt drop by comparison Shotkey diodes can be found that work with up to 200 volt rating.


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