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Discussion Starter #1
Re: [EVDL] EV controllers- controlling both voltage and current

Roger -


I agree with Morgan that you can't control current and voltage independently
in a series motor. What I mean is that you can't arbitrarily apply a
desired voltage and a desired current at the same time.

But, you can certainly limit both, as you suggest. I don't think this is
what he meant, though.


As an example, if an ADC 8 inch motor ( 203-06-4001) is turning at 3000 RPM,
and you apply 75 volts to its terminals, there will be 270 amps flowing
through that motor. That's it. No other current value is
possible at that voltage and motor speed.

You can't apply 75 volts to the motor (at that RPM) and somehow control the
current to be, say, 400 amps. ( or, limit it to 100 amps)


This particular value is based on a published ADC set of performance curves
for this motor - I don't remember where I originally got it. The curve I
have is hard to read, so, maybe it's really 265 amps, but you get the idea.



Similarly, you can't apply 10 volts to a 10 ohm resistor and get 2 amps to
flow.



Phil Marino






>From: "Roger Stockton" <[email protected]>
>Reply-To: Electric Vehicle Discussion List <[email protected]>
>To: "Electric Vehicle Discussion List" <[email protected]>
>Subject: Re: [EVDL] EV controllers
>Date: Mon, 20 Aug 2007 15:12:09 -0700
>
>Morgan LaMoore wrote:
>
> > Yeah, it's not possible to control amps and volts
> > independently in a DC series-wound motor
>
>Sure you can. Think of the controller being just like a bench power supply
>with a voltage limit knob and a current limit knob; you can independently
>set a voltage limit and a current limit. At low motor speed the controller
>is going to bump up against the current limit first, so the controller will
>operate in current limit as the (average) voltage increases from some low
>value. If the voltage reaches the set voltage limit, then the controller
>becomes voltage-limited and holds the motor voltage from exceeding the
>setpoint while the current drops off.
>
>For instance, the 'Zilla offers the ability to set a maximum (average)
>motor voltage limit and also has an adjustable current limit.
>
> > I guess if DC series-wound regen were possible,
> > the Zilla would already have it.
>
>Regen is possible with series-wound motors. I believe a reason why the
>'Zilla doesn't offer this feature is because Otmar's experiments with
>series motor regen suggest that it is hard on brushes/comms, especially at
>the sorts of voltages the 'Zilla users are likely to run. Curtis
>offers/offered a regen controller (1221R) for series motors, and Otmar once
>published instructions on how to modify a standard Curtis for regen
>operation (though he advised against doing so based on his experiences with
>the setup). Not sure if these instructions are available on the Café
>Electric site; I think they may originally have appeared as an article in
>Home Power?
>
>A "simple" regen scheme I saw described in a paper some time back involved
>assumed a pack composed of 6V floodies and used a couple of contactors, a
>beefy diode, and a power resistor (a length of allthread with washer
>"fins") to connect one 6V battery to the motor field for excitation and to
>parallel the two halves of the pack during regen to allow regen to lower
>speeds/at lower motor RPM.
>
>Cheers,
>
>Roger.
>
>_______________________________________________
>For subscription options, see
>http://lists.sjsu.edu/mailman/listinfo/ev

_________________________________________________________________
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Discussion Starter #2
Re: [EVDL] EV controllers- controlling both voltage and current

Phil Marino wrote:

> I agree with Morgan that you can't control current and
> voltage independently in a series motor. What I mean
> is that you can't arbitrarily apply a desired voltage
> and a desired current at the same time.
>
> But, you can certainly limit both, as you suggest. I
> don't think this is what he meant, though.

Well, I'm not sure what he meant, but I'd hope that if he is halfway
through an EE program he has encountered and understands Ohm's law!
Since it isn't possible to force an arbitrary current to flow through
*any* given impedance (motor, resistor, etc.) while forcing some
unrelated arbitrary voltage across it I wouldn't expect that to be what
he was asking. ;^>

The *only* thing he could do that resembles what he asked about is to
have independent control over the controller output voltage and current.
As he increases the voltage (limit), the current will increase (in a way
defined by the impedance of the load) until it reaches the present
current limit. If he continues to increase the voltage command/limit
without adjusting the current limit, the current remains fixed and the
voltage increases as the motor speed increases (since the motor is a
load whose impedance varies with RPM). If he turns up the current
limit, then the current increases until it reaches the new limit, or the
until the voltage bumps up against the present voltage limit, and so on.

Cheers,

Roger.

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