paul dove wrote:

> That made no sense to me but a DC motor controller takes the input voltage
> to power mosfets or igbts which switch the power on and off with a pwm
> signal to get the desired output to drive a motor. It has no knowledge of
> the battery impedance.

The controller does not have, or need, knowledge the battery impedance; this is 
simply Ohms Law at work.

The controller PWMs the power from the battery, and the duty cycle of the PWM 
can be varied to limit whatever parameter the controller logic cares about at 
any given time: input voltage, input current, output voltage, or output current.

Since the battery has a finite, non-zero internal impedance, its terminal 
voltage will sag as current is drawn from it.  If the motor controller enforces 
a minimum input (battery) voltage (as does the controller in question), then 
its logic will vary/limit the PWM duty cycle to prevent the voltage at the 
input of the controller from falling below the target level.

Since the controller logic will limit/vary the input current to prevent the 
input voltage from falling below the threshold, then the maximum voltage 
difference across the battery internal resistance is the open circuit battery 
voltage minus the minimum input voltage limit of the controller: 96V - 72V = 
24V, in this example.

The controller does not know what the internal impedance of the battery is, but 
Ohms Law still applies to limit the maximum current from the battery to 
*whatever* value results in 24V drop across the internal resistance: 24V / 0.08 
ohms = 300A in this example.

*IF* the sophisticated controller were replaced with a simple contactor 
controller, then if the contactor controller allowed the entire 96V battery to 
be connected directly to a stalled motor (~0 ohms), the voltage drop across the 
battery internal resistance would ~approach~ 96V - 0V = 96V, and the peak 
current into the motor would approach 96V / 0.08 ohms = 1200A.

Hope this helps,


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