Tuesday, December 29, 2009

Re: [Electric Boats] Forward/ reverse potentiometer.

 

Matt's answer is probably the correct explanation for your hesitation but there is another possibility. 
 
Your controller knows the speed and direction of the motor shaft via the hall sensors.  It may not give you reverse current until it detects the shaft has stopped moving forward. 
 
I had a real problem with my Sevcon PMAC at first.  It took 15 seconds for the boat to stop coasting forward; that is a long time to wait for reverse.  Sevcon changed the controller's firmware to fix the problem.
 
Denny Wolfe
 
----- Original Message -----
Sent: Thursday, December 24, 2009 4:24 PM
Subject: Re: [Electric Boats] Forward/ reverse potentiometer.

 

ebdrives wrote:
> Dennis,
>
> I use a Merritt Joystick to run my electric 26'powerboat. It runs good to control forward/neutral/reverse, etc. But, I will tell you that there is a hesitation when I go from forward to reverse. My motor is a BLDC (brushless) motor so there is no coil to switch over into reverse.
>
This may not be the joystick causing the hesitation, it is probably the
motor controller.

Flipping from forward to reverse would put a lot of strain mechanically
and electrically on the system, so when you flip the control handle from
forward to reverse, the motor controller isn't instantly changing
direction, but ramping the motor speed down at some pre-determined rate
, stopping and then reversing the fields and ramping up to your throttle
position.
These ramp up/down rates are almost certainly programmable.

I'm running a brushed motor and an Alltrax AXE, so reversing is done by
contractors, but the forward/reverse switch is 3 way, so that 'off' has
to be passed on the way to reverse, this causes the controller to reset
and it then 'ramps up' at the programmed rate to the throttle setting it
reads off the speed pot.

In my case, flipping from forward to reverse with out shutting off
would result extremely high motor currents as it fought to change
direction against the high forward inertia of the boat. This would
almost certainly put my motor into overload and cause the controller to
go into current limit. It may even cause the reversing contactor to weld
shut as it closes onto a huge current spike.

So reprogramming the throttle ramp rates on your controller to get a
'snapper response' may not actually be a good idea.... (Assuming of
course, that your controller allows 'in the field' changes to the
throttle response ramp rates).

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