[ARC5] BC-455 adventure

[J. Forster]

Hi Mike,

Another reason a dyno goes wonky with a series resistor is the source
impedance of the armature circuit goes up. This essentially causes
positive FB as follows:

Suppose a 24 V Dyno is running on a 24 VDC supply with a series resistor.
If the HV load goes up a bit, the armature draws more current. Because of
the high supply source impedance, the voltage across the motor, armatuse
and field decreases. The reduction in the field reduces the back EMF of
the armature causing more motor current to flow, which further reduces the
field, and so on. Positive FB.

I think that is the cause of the instability you observed. This is why
controllers for DC Shunt motors keep the field constant and use a stiff,
variable supply for the armature.

Best,

-John

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> On 6/16/2012 4:52 PM, J. Forster wrote:
>> I have a model of the dyno and all the equations. It'd be easy enough to
>> run the numbers in Excel. IMO, running the machine at reduced field is
>> not a good idea.
>
> Heh...I recall the grad student leading the power lab in 1961
> momentarily dropping the field in the ~50hp motor he was demonstrating -
> a much abused lab machine.  It took off like a scalded cat with a
> terrible scream.  He switched the field back on in time, though.  I
> don't think they run labs like that for every EE student like they did
> in my day. Just a dinosaur...sniff... :-)
[snip]