Need more torque

[JohnnieB]

Originally Posted by jfw432

Sorry for the slow response as I've been out of town lately. Thanks for all the good information. I wanted to follow up to make sure I'm taking away the right information. It sounds like I might be able to pull a few more amps by upgrading my cables but not very much.

Ultimately, it sounds like the motor is what is holding me back since it can't accept enough juice. Is that just all stock motors can take or is there something wrong with this one? I would've thought that by upgrading the stock motor to the Bandit motor would've fixed any issues it may have had in power delivery.

I did some following up to make sure the information I'm providing is worthy of being taking away.
The motor windings, brushes, F/R contacts, solenoid contacts, high current cables, controller and all the connections that connect everything in series with the battery pack (which is eighteen 2V cells connected in series), form a series circuit effectively made up of of both fixed and variable resistors as well as a valve that limits current flow.

The controller is the valve, but the battery amps and motor amps have the same values on the data log, as well as the battery and motor voltage, when the max amps are being drawn by the motor, so the valve is wide open and not limiting current flow through the motor.

The internal resistances of the cells in the battery pack is one of the variable resistors in the circuit. The internal resistance of the cells increase proportionally with the amount of amps being drawn from the battery pack, so the battery pack's voltage decreases proportionally with the amps being drawn from it. The battery voltage on the data log is measured between the controller's B- and B+ terminals, so it includes the voltage drop across the fixed resistance of the five cables connecting the six batteries together, plus the cable connecting B- from the battery to B- on the controller, plus the cable connecting B+ from the battery to the solenoid, the solenoid contacts and the short cable from solenoid to B+ on controller. Plus the sixteen connections made by those eight cables.

The data log shows a battery voltage of 38.4V when the solenoid contacts first close. (Lower battery voltage readings prior to solenoid contacts closing are what is passing through the pre-charge resistor) The battery volts logged when max amps were being drawn is 31.0V, or a voltage drop of 7.4V when 283.7A were being drawn from battery. The typical voltage drop for a healthy 36V battery pack is 1.0V/100A or about 2.8V, so 4.6V is being dropped across those eight cables, solenoid contacts and sixteen ring terminals on the ends of those cables, the studs and nuts they are connected with. Also the crimp or soldered connection between ring terminal and 4Ga copper conductors. I estimate the total length of those eight cables to be about 10', which only accounts for about 0.7V for 10" of 4Ga conductor at 283.7A. Therefore, over half of the 7.4V voltage drop (3.9V) is being dropped by connections and contacts, so the is room for improvement.

The motor voltage logged is measured between the M- and B+ terminals of the controller and includes the fixed resistances of the other five high current cables and F/R contacts as well as the variable resistance of the motor windings.

Using Ohm's law, the resistance of the output circuit of the controller is about 0.1093Ω. Subtracting the 0.0032Ω of about 13' of 4Ga conductor, you get about 0.1061Ω for the motor's resistance (That is high because the remainder includes connections and contacts). If you could eliminate the 3.9V drop from the connections and contacts in the controller input circuit, the motor would have 34.9V applied to it and draw 319.4A instead of 283.7A

I'm not sure how much the torque output of that motor will increase by drawing 12.6% more amps, but it ought to be a noticeable gain.

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There may not be anything wrong with the motor. It is more likely to be the wrong motor for your application.

I may be wrong, but I understand that the bandit is basically a stock motor rewound for higher than stock speed and rewinding for higher speed tends to reduce the lower end of the torque curve while raising the upper end of the torque curve. You lose 10% of the stock torque by using 20" tall tires, so you need a motor that produces at least 10% more torque to break even with stock performance climbing the 20° hill you have to climb.

Bottom line is that upgrading to 2Ga cables (all 13 of them) with soldered & crimped ends will help. Going to a high torque motor will help. Going to 42V or 48V will help. Doing all three will help a lot.

[Raystar57]

Wow JohnnieB, that was great!
I think you need a new Sherlock Holmes Deerstalker hat for that one!
If I get and EZGO sometime and need some torque, I will look you up!