The purpose of an undercurrent relay in an aircraft's starter/generator system is to:
A. Provide a backup for the starter relay.
B. Avoid excessive charge and resultant heat within the electrical system.
C. Disconnect power from the starter/generator and ignition when sufficient engine operating speed is achieved.
D. Keep current flow to the starter/generator under the circuit capacity maximum.
Starter-generator technical question
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Re: Starter-generator technical question
Answer C is correct
The high current required to start an aircraft engine turning is enough to hold the undercurrent relay closed and supply current to the coil of the starter relay and the ignition relay. When the engine starts, the current through the series starter winding decreases enough that the undercurrent relay will no longer remain closed. The starter and the ignition relay contacts will open, disconnecting current to the starter winding and de-energizing the ignition exciters.
The high current required to start an aircraft engine turning is enough to hold the undercurrent relay closed and supply current to the coil of the starter relay and the ignition relay. When the engine starts, the current through the series starter winding decreases enough that the undercurrent relay will no longer remain closed. The starter and the ignition relay contacts will open, disconnecting current to the starter winding and de-energizing the ignition exciters.
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co-joe
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Re: Starter-generator technical question
I believe, but as telex says, an undercurrent relay would be c).
Essentially the starter is an electrical load while it's driving the engine. As soon as the engine takes over driving the starter, the current drops to zero. Remember, current is determined by the load.
Essentially the starter is an electrical load while it's driving the engine. As soon as the engine takes over driving the starter, the current drops to zero. Remember, current is determined by the load.
Re: Starter-generator technical question
Liberalism itself as a religion where its tenets cannot be proven, but provides a sense of moral rectitude at no real cost.
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co-joe
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Re: Starter-generator technical question
The RTFQ part of the question to me is that an under Voltage relay would be to isolate a failed or malfunctioning generator from becoming a load. Not sure such a thing exists off the top of my head, but that would lean towards b).
Re: Starter-generator technical question
Thanks, I figured that it was C but was not sure. Been a long time but...does the King Air have this and is it in the electrical diagram somewhere?
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co-joe
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Re: Starter-generator technical question
Not that I can think of. The Starter Gen switch is manual. My understanding is that if you were to forget to turn off the Starter after the engine reaches 50% N1, it draws no load at all. Where you'll have a problem is when you try to bring the Gen on line it won't. I thought it would cause the started to burn out, however I've been told that no damage occurs.
Later models like the 350 have a HED (Hall Effect device) which can isolate the Starter Gen from the rest of the system under high load, or even momentarily during cross gen start. The 1900 calls it a "high current sensor". I believe it's the same thing under a different name. I'm no expert on either of those and really late 200's are foreign to me, my experience goes up into the 80's serial numbers and that's about it.
Later models like the 350 have a HED (Hall Effect device) which can isolate the Starter Gen from the rest of the system under high load, or even momentarily during cross gen start. The 1900 calls it a "high current sensor". I believe it's the same thing under a different name. I'm no expert on either of those and really late 200's are foreign to me, my experience goes up into the 80's serial numbers and that's about it.
Re: Starter-generator technical question
Thanks,
I think that there was a guy in Montreal a few years back that forgot to select both start switches off and ended up airborne and IFR with a loss of all electrics.
I think that there was a guy in Montreal a few years back that forgot to select both start switches off and ended up airborne and IFR with a loss of all electrics.
