Re: (ET) Curtis or any generic controller setup - Finally.

[Robert Adsett <subscriptions aeolusdevelopment com>]

On 8/3/2012 11:06 PM, Jim Coate wrote:
> To Robert, that center-off may just cover it.

That is a concern but it's actually the smaller one.  This is the 
standard configuration for a Curtis with a Series controller although 
you do want slow enough switches that off long enough for the controller 
to really turn off.  The controller goes into plugging mode when turned 
on if the controller supports plugging.  This will deal with the reverse 
situation in most cases so while it needs verifying it will likely work.

The real concern is the response to regen which happens not on reverse 
but anytime the motor is moving faster than the controller output.

A series controller never sees a negative current under normal operating 
conditions (plugging does not produce a negative current, i.e. a battery 
charging current). The controller could respond by faulting but I don't 
think many do.  It's more likely to respond in one of two fashions both 
risky.  It could clamp the current to zero since a negative current is 
impossible on a series motor or it could feed the negative into the 
circuit where it could play havoc with the circuits potentially latching 
them.  The Curtis is an analog control so this is a real possibility.  I 
think the most likely is that is will be treated as a zero current. 
Sounds like no problem but consider the consequences.  If negative 
currents are treated as zero then there is no current limit and thus no 
torque limit on regen.  The worst of this is you could get away with it 
for some time and then hit the right combination of circumstances and 
have things go very badly wrong.  This is one of those cases where just 
because it seems to work does not mean it's safe.

This is why an off the shelf series controller would not be something 
I'd pick.  I'd have no qualms about using PM controllers though.  Just 
need to make sure they are sized right.  Field weakening would not be 
difficult to add and with some cleverness field boosting might be possible.

There are some 1/2H style controllers that could handle it.  4QD does 
some that might but they are all specifically designed/adapted to deal 
with negative currents.  They are PM controllers.

I'm pretty sure I know what Curtises response would be if you asked if 
using a series controller on what's effectively a PM motor was a 
reasonable thing to do.

Robert


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