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Unread 12-12-2011, 10:30
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Re: 2 motor drive VS 4 motor drive

An important consideration here that everyone seems to be ignoring is the loss that occurs due to internal resistance. The internal resistance that the manufacturer reports for the battery is .011 ohms. Based on my experience with these batteries, this number is very low. As a battery ages, the internal resistance gets higher. In addition, there are other resistive losses such as the battery connection terminals, the Anderson connector, the PD connections, the PD itself, the breaker connections, the breaker itself, the wires, the motor controller the.... But you get the point. All this can add up up to a significant number. What this means is that when you go from a two motor drive to a four motor drive, you do NOT double the available torque. The current draw goes up, but doesn't double because the voltage goes down. In 2010, we had a three motor per side drive (2 CIMs and one FP). After the season, we did some testing and found that the current draw caused the voltage to drop so far that we had more available torque without the FP motor than with it. We also tried adding another battery (two in parallel) to reduce the drop in voltage. The difference was incredible. I almost think two batteries with a two motor drivetrain would out-perform one battery and a four motor drivetrain. (so far, not legal in competition)

We know for certain that we can't draw full current because even with our three per side drivetrain, we never tripped a 120 amp breaker. With all those motors stalled, we should been pulling ~750 amps. We definitely stalled the motors on more than one occasion, but never tripped a breaker (the battery voltage dropped down below 7 volts at times though)

If you want to run some numbers, decide on a number for total resistance for the system, .035 to .045 ohms is not a bad guess. Multiply this number by the current draw to get the voltage drop. Subtract that number from the starting battery voltage (~12.6V for a charged battery) to get the voltage available to the motor.

Unfortunately, the motor curves are all shown at 12V so it is difficult to estimate the real torque available when the voltage drops to 8V or so.

Try some calculations, then run some experiments. Tell us what you found.
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