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fuel pump inductive spike?

rv8ch

rv8ch

Well Known Member
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The other recent thread on fuel pumps triggered a question - I know we put flyback diodes on the starter and master relays to handle the "inductive spike" when turning them off, but why not the fuel pumps?

Here's a couple of videos talking about the phenomena:

https://www.youtube.com/watch?v=LXGtE3X2k7Y

https://www.youtube.com/watch?v=OE9TvfyC5EY

I have the Andair PX375-TC and it's working great, and perhaps it has a snubber diode built in since the suggested wiring diagram does not include a diode, but wanted to check with the VAF brain trust to see it's something I should consider. I guess I could break out the handy-dandy USB oscilloscope to see if it shows a spike...
 
rv8ch said:
The other recent thread on fuel pumps triggered a question - I know we put flyback diodes on the starter and master relays to handle the "inductive spike" when turning them off, but why not the fuel pumps?

Here's a couple of videos talking about the phenomena:

https://www.youtube.com/watch?v=LXGtE3X2k7Y

https://www.youtube.com/watch?v=OE9TvfyC5EY

I have the Andair PX375-TC and it's working great, and perhaps it has a snubber diode built in since the suggested wiring diagram does not include a diode, but wanted to check with the VAF brain trust to see it's something I should consider. I guess I could break out the handy-dandy USB oscilloscope to see if it shows a spike...
Click to expand...

Closing a contact does not create arcing in the contact, but opening it can. The diode is not there to deal with inductive load. It is there to shunt the release of energy from the collapsing magnetic field (which is synonymous with release of stored elec energy) so that the stored energy doesn't create an arc across the switch contacts when you open them. Motors use energy to create motion, so no stored energy. All the magnetic energy is consumed by the spinning object. Relays store energy in a magnetic field to hold an object in place. It is not really consumed and therefore that magnetic field tries to go somewhere when the source disappears.
 
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I dont have a diode on my master or starter relays they are 200 amp continuous duty single pole single throw relays, is everything on my bus at risk of a spike without a diode on that master, wait, only things that would be powered on when the master switch comes on so things with a switch should be safe, so now need to find out what comes on with master. What about the starter, how does a spike affect it?
 
Ruready said:
I dont have a diode on my master or starter relays they are 200 amp continuous duty single pole single throw relays, is everything on my bus at risk of a spike without a diode on that master, wait, only things that would be powered on when the master switch comes on so things with a switch should be safe, so now need to find out what comes on with master. What about the starter, how does a spike affect it?
Click to expand...

Here is an article that explains why a diode is needed on the start and master contactors.

https://www.kitplanes.com/aircraft-wiring-4/
 
Ruready said:
I dont have a diode on my master or starter relays they are 200 amp continuous duty single pole single throw relays, is everything on my bus at risk of a spike without a diode on that master, wait, only things that would be powered on when the master switch comes on so things with a switch should be safe, so now need to find out what comes on with master. What about the starter, how does a spike affect it?
Click to expand...

The flyback spike is generated when the contactor coil is de-energized. Suppressing the spike with a flyback diode will increase the life of the switch that activates the coil.

Also ref posts #8 and #10 where the flyback spike from a start contactor or Facet boost pump can trip a B&C regulator's OV crowbar.

Off topic but some contactor info from the Cole Hersee data sheet:
  • The common 24021 starter contactor is intermittent duty, 750A make, 100A break, 10 s on, 20 min off, 3.8 ohm coil.
  • The common 24115 master contactor is continuous duty, 85A, 17-1/2 ohm coil.
 
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Switch contacts bounce while closing, so there is some sparking, but nothing compared to when opening an inductive load.
Most of the inductive energy is dissipated across the opening contacts.
The voltage spike seen by avionics is of no concern to modern equipment designed to DO-160 standards.
Read Bob Nuckols' article: http://www.aeroelectric.com/articles/avmaster.pdf
The biggest danger from inductive current is to the controlling switch. To minimize sparking,
connect a diode across the coil with the banded end connected to positive.
 
Simple DC motors use a ‘commutator’ (basically a switch) to switch the polarity of the spinning electromagnets every time it goes past the face of the fixed magnets. Since large inductances don’t like to switch currents rapidly, the coils are designed for relatively low inductance. But low inductance also means less stored energy, so less sparking when you open the switch.
 
The V spike from a start contactor is enough to trip the OV circuit of the B&C regulator.
 
Inductive spikes

lr172 said:
Closing a contact does not create arcing in the contact, but opening it can. The diode is not there to deal with inductive load. It is there to shunt the release of energy from the collapsing magnetic field (which is synonymous with release of stored elec energy) so that the stored energy doesn't create an arc across the switch contacts when you open them. Motors use energy to create motion, so no stored energy. All the magnetic energy is consumed by the spinning object. Relays store energy in a magnetic field to hold an object in place. It is not really consumed and therefore that magnetic field tries to go somewhere when the source disappears.
Click to expand...

True about motors, but diodes still have a role here. Namely stopping over run motion. On a fuel pump it is no issue, but on the flap or trim motors, diodes stop the motion when power is removed, rather than skating on a bit further. In those cases, the diodes must be installed not directly at the motor, as they get driven in both directions, but connect the diode ahead of the reversing switch.
 
My alternator field CB was popping fairly often (overvoltage trip) years ago and I finally realized it was happening when I cycled the Facet boost pump switch. I put a diode across the pump, problem solved. If you can duplicate the problem, install the diode. Don't overthink it...
 
N49ex said:
... on the flap or trim motors, diodes stop the motion when power is removed, rather than skating on a bit further. In those cases, the diodes must be installed not directly at the motor, as they get driven in both directions, but connect the diode ahead of the reversing switch.
Click to expand...

  • I'd like to see a reference that explains this. I would have thought the motor would become a generator whose voltage polarity would not conduct through the diode but rather want to charge the battery.
  • Can you provide a schematic showing how a diode is wired ahead of the reversing switch and still sees the motor?
  • I know a flyback diode on a solenoid like a relay or contactor or fuel injector slows it reverse motion (which is bad in all three cases) upon power off but solenoid motion reverses whereas motor motion coasts in the same direction.
 
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