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alec(at)alecmyers.com
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 Posted: Fri Jan 19, 2018 12:05 pm Post subject: suppression diodes |
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on a similar subject to recent discussion, why don’t we use a suppression device across the load of the starter contactor (i.e. the motor) to prevent arcing at the contactor contacts?
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nuckolls.bob(at)aeroelect
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| Posted: Sat Jan 20, 2018 7:25 am Post subject: suppression diodes |
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At 02:04 PM 1/19/2018, you wrote:
| Quote: | --> AeroElectric-List message posted by: Alec Myers <alec(at)alecmyers.com>
on a similar subject to recent discussion, why don’t we use a suppression device across the load of the starter contactor (i.e. the motor) to prevent arcing at the contactor contacts? |
Excellent question!
Shucks, starters draw a lot of current. If one
just 'tapped' the starter button and hit the
contactor with the starter's 500-1000A inrush, doesn't
that 'charge' the starter's inductance to astronomical
heights?
You betcha . . .
Let's consider a turbine starter motor with a dc
resistance of say 0.015 ohms. Let us presume
further that the loop resistance for wiring,
battery contactor, starter contactor and battery
is also on the order 0.010 ohms.
When the contactor closes, we have 25v/0.025-ohms
or 1000 amps of inrush current. Terminal voltage
on starter drops to 15v.
Energy stored on an inductor is Joules(Watt-Seconds)=Inductance
(Henries) x I(Amps)squared/2
Needless to say, 1000 squared is a pretty big
number. If the starter contactor bounces during the
first few milliseconds of the spin-up, the potential
for arcing is considerable, but the air gap during
a bounce is tiny; the discontinuity interval is short.
Heat energy developed is relatively low and within the
operating limits for dime-sized contacts. Any transient
excursions for field collapse are impressed across the
contact gap and do not propagate out onto the bus.
Once the bouncing has subsided and the motor spins
up, current draw falls dramatically. Here's a plot
I found in some document out at Beech about a bazillion
years ago: https://goo.gl/4cL4ff
It's the voltage-current vs. time plot of a cranking
event on a turbine engine . . . more specifically,
a B400 Beechjet. Turbine cranking curves are interesting
because the engine takes so long to spin up . . . it
doesn't get to the 'light off' rpm for 15-20 second!
We can readily see that starter current at light-off
has fallen to under 300 amps, about 30% of inrush.
At 300 amps across the motor's 0.015 ohms, we can
see that the motor is actually RUNNING on Volts=
300A x 0.015Ohms or about 4.5 volts. At this time,
voltage applied to the motor is 18-19 volts.
This means that the motor's COUNTER EMF or CEMF
is 18-4.5 or about 13.5 volts.
If the battery contactor opens at this point, the
initial arc striking voltage is 4.5 volts and the
current is 300A. A much lower value than the numbers
during inrush. At 300A, stored energy on the starter's
inductance is about 10% of that experienced during
the inrush event.
As it turns out, it's quite easy to build contactors
that are tailored to withstand the inrush event.
See https://goo.gl/RGGhgG
The flimsy looking flat moving contact has
very low area, high force interface with the
stationary contacts. i.e. HIGH PRESSURE
Further, its mass is low and spring rates are
low to minimize bounce. The intermittent duty,
high force solenoid offers strong reduction
of contact erosion during the starter inrush
interval . . . similarly, the light mass of the
moving contact assembly offers rapid acceleration
and good contact spreading velocities at de-energization.
The design has TWO contact in series which means that
during the opening sequence, contact spreading velocity
is essentially doubled.
This design minimizes potential arc damage to the
contacts as the magnetic field in the motor
collapses. Again, the energy is expended in the contact
gaps and does not propagate out onto the bus.
This last fact was not well understood by many in
the vehicular DC power system world . . . it seemed
only natural that this energy intensive event at
the starter motor would produce massive 'spikes'
potentially deleterious to vulnerable electronics
in the vehicle . . . it has been proven not to
be so. It turns out that ALL known and anticipated
perturbations on the ship's bus for ALL operating
conditions are well inside the legacy testing
limits for aircraft hardware as described in DO-160.
See https://goo.gl/6gSsSL
Hence, the starter motor is not the big bear in the
woods that legacy hangar lore would have us believe.
It IS a high energy even easily managed by the starter
contactor design and not a threat to the rest of the system.
So if an avionics tech says you've got a $1000 repair
bill on an in-warranty radio because a 'spike got it',
you need to talk to his/her supervisor.
Bob . . .
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