Re: Mariss' power supply circuit

--- In CAD_CAM_EDM_DRO@y..., bjammin@i... wrote:

> At 03:14 AM 6/20/02 +0000, you wrote:
> >> Shorting the armature of a motor while it is spinning at high
> >> speed will also cause 50-100 times rated current to flow.
>
> I'd like to see the math on that.
>

OK...

25HP industrial DC motor with 500V armature, rated current
is 25HP * 746 watts/HP / 500Volts = 37.3amps
The nameplate values will be a little more, maybe 38-39A

The motor is 96% efficient, and about half the losses are
iron and field winding losses. That means armature copper
loss is about 2%, so the IR drop at rated amps is 2% of 500V.
So the armature resistance is 2% * 500V / 38A = 0.26 ohm.

Motor is spinning at rated speed (1750 RPM), so the armature
counter-EMF is 500V. Short the armature, and the current is
limited only by armature resistance.
Current = 500V / 0.26 ohm = 1923A

1900 amps through a 38 amp motor is gonna bust something!

> >> with a short, there is no current limit.
>
> Sure there is; the windings themselves have resistance.
>

Yes, but the resistance limited current is very high.
The current limit I was referring to was the current
limit function of the drive, which reduces it's output
voltage to prevent the motor current from getting too high.

> >> of one case where the semiconductors in a DC drive failed,
> >> shorting the armature of a 25 HP motor while it was spinning
> >> a roller on a paper machine (high inertia load). The fault
> >> current generated enough braking torque to snap the shaft of
> >> the motor (around 2" in diameter)!
>
> I hate to sound doubtful but I am.

It happened - I heard the story from the field service tech who
had to replace the drive and motor. I did the failure analysis
on the drive. (The snubber resistors were sized to handle the
line notches created by the drive, but not the notches from the
other 10 drives on the same power line. The excessive dissipation
in the snubber resistor caused it to unsolder itself from the PC
board. Then the unsnubbed voltage spikes caused the SCR's to
fail shorted.) This was about 12-14 years ago when I was at
another employer. The drive in question was a third party drive
that our systems division had installed at the paper mill. I
was working in our development group designing a drive to replace
the third party products. This incident caused a re-evaluation
of our own snubber designs <grin>.

> The impulse generated when shorted to a stop is no bigger than
> the impulse when shorted to start, IE, no bigger than the
> impulse generated the instant you put it under power from rest.

Do you start your servo motors by connecting them directly across
your DC power supply (without a servo amp), with a large inertia
load on them? Maybe you can, with small motors - this is where my
experience with large motors may lead me astray. Large efficient
motors have resistances that are very small (that's why they are
efficient). Maybe little motors have enough internal resistance
to protect them. However, I think that even small motors would
draw far more than rated current if suddenly connected directly
across the DC supply. Servo motors rely on the drive to limit
their current under stall and starting conditions.

>
> > Due to poor installation of bus bars in a large coal-fired
> > generating station near here (Labadie plant near at Labadie,
> > MO) two sets of bus bars carrying the output of two 840 MVA
> > alternator sets at 14 KV flew off their insulators and shorted
> > the two alternators out. The alternators were ripped partly
> > out of the floor
>
> This is a bit more believeable, as the tie-down bolts for these
> big alternators aren't really huge. But it didn't break the
> drive shafts, did it?
>
> >The rotors of these alternators are HUGE, of course, a solid billet
> >of steel about 18" diameter and 6 to 10 feet long!
>
> No lams? Darned odd that.

They are synchronous machines, so the rotor has a DC field current
only. No AC flux, no need for laminations. Which is a good thing,
given the enormous mechanical stresses on the rotors.

>
> Regards, Hoyt
>

John Kasunich