Electronics > Projects, Designs, and Technical Stuff
Generator Auto Start - When to Stop Cranking
frostheave:
Thanks to all for your replies!
A little more info. The generator is a small 4KW, 7HP, gasoline engine. It does not have a tach output. It does have a low oil sensor but not an oil pressure sensor.
Trying to keep this as simple as possible, here are my thoughts on your suggestions.
1) No tach output requires some type of sensor.
2) Crank current requires a sensor, or maybe read battery voltage?
3) No existing oil pressure sensor. Retrofit probably not an option?
4) As far as I can tell, there is no knock/detonation sensor.
5) Trying to measure a tacho signal from the charging coil for the starting battery may be possible with no special sensors?
6) Measure AC output frequency. Once the alternator is producing ~35+Hz, stop cranking. Requires measuring Hi Voltage frequency.
7) 120 Vac relay attached to generator output.
It seems the simplest first attempt would be to use a relay with a 120 Vac coil.
a) Begin cranking.
b) Monitor state of relay.
c) If relay closes, continue cranking for a short time then stop cranking.
d) If relay does not close and it has been cranking for 5 seconds, stop cranking, wait for 10 seconds, try again, goto a)
e) Check state of relay and make sure it remains closed, i.e. generator is running and 120 Vac is being output.
Of course this process will require some experimentation. The first thing being to attach the relay coil to the generator 120 Vac output and see when it closes. Does it close while cranking but the engine hasn't started? That would be a problem. I'll report back with my findings.
Thanks again for your suggestions.
Bob
schmitt trigger:
2) Crank current requires a sensor, or maybe read battery voltage?
There are many "clamp on" current sensors available on Amazon and Ebay, which should be minimally invasive if you decide to follow this route.
fourtytwo42:
I feel I have to share a funny story regarding this! I was once involved in a site that had a large (100Kw+) standby set outside in a courtyard but it's controls were in a basement with no windows. Somebody had the bright idea of rigging up a baby alarm so the operator in the basement could hear when the engine fired.
This worked fine till one day it transpired an operator forgot to switch on the heater plugs before cranking. Those of us upstairs became aware something odd was going on as clouds of white smoke continued far longer than the normal few seconds to the extent that outside the building became a fog! The operator continued for so long eventually the starter batteries could no longer turn it over.
I imagine the said operator was retrained (possibly by having to personally change the batteries) as it never happened again :)
NorthGuy:
--- Quote from: frostheave on February 08, 2019, 05:03:27 pm ---5) Trying to measure a tacho signal from the charging coil for the starting battery may be possible with no special sensors?
6) Measure AC output frequency. Once the alternator is producing ~35+Hz, stop cranking. Requires measuring Hi Voltage frequency.
--- End quote ---
Unless you have a fancy variable speed inverter generator, the AC frequency is the tachometer. As soon as a threshold frequency is achieved, you stop cranking. But also run a timer - don't crank for too long if there's no AC, and if this is the case, shut down the generator and disable automatic starts until a human inspects it.
After the start I wait some time to let the generator warm up without a load, then I turn on the load relay.
ejeffrey:
--- Quote from: frostheave on February 08, 2019, 05:03:27 pm ---6) Measure AC output frequency. Once the alternator is producing ~35+Hz, stop cranking. Requires measuring Hi Voltage frequency.
--- End quote ---
Easy enough to do with a small transformer. A standard 12V power transformer will allow you to get the voltage and frequency with an isolated measurement. This will also let you monitor while running.
Does your generator have built-in load switch that only connects the output once at the target RPM? That would make sense to avoid loading the generator at startup and to avoid producing too low voltage/frequency during startup. If so, then your simple relay solution is probably the best.
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