Generator Auto Start - When to Stop Cranking - Page 1
Forum section: electronics / projects

author: frostheave
author_rank: Contributor
timestamp: February 06, 2019, 11:21:40 pm
content: Hi All,I would like to build an auto start circuit for my home backup generator.My question is, what is the best output to sense, to decide when to stop cranking the starter?Should I monitor the 120 Vac output? Should I use some type of tachometer? Something else?Is there an industry standard that should be followed?Thanks for your help.

author: Psi
author_rank: Super Contributor
timestamp: February 06, 2019, 11:32:30 pm
content: Maybe crank current would be much lower once the engine is turning itself over?

author: Bratster
author_rank: Frequent Contributor
timestamp: February 06, 2019, 11:38:51 pm
content: Tachometer /RPM would be the best measure in my opinion.Sent from my Fi Moto x4 using Tapatalk

author: tkamiya
author_rank: Super Contributor
timestamp: February 07, 2019, 03:19:11 am
content: I would check oil pressure. If it's not building up, you better shutdown the engine. (assuming this is 4 stroke) Otherwise, I'd use hole-sensor or something and measure the crankshaft movement.Also, if it doesn't start after X number of attempt, I'd stop.

author: Psi
author_rank: Super Contributor
timestamp: February 07, 2019, 06:12:48 am
content: I wonder if you could pickup something from the knock/detonation sensor. Assuming this engine has one.

author: johnwa
author_rank: Frequent Contributor
timestamp: February 07, 2019, 06:30:12 am
content: I faced this problem when fitting out my fire pump for automatic start. It is a diesel, so I couldn't tap off the ignition circuit, but I found I could get a fairly good tacho signal from the charging coil for the starting battery.The starter solenoid is controlled by a microcontroller, but I also took the precaution of AC-coupling the drive signal, so it will still shut off eventually, even if the program crashes.

author: JVR
author_rank: Regular Contributor
timestamp: February 07, 2019, 12:36:48 pm
content: I've done this for a 10kVA diesel genset. Just use the frequency output of the alternator. Once the alternator is producing ~35+Hz, stop cranking.

author: JVR
author_rank: Regular Contributor
timestamp: February 07, 2019, 12:39:19 pm
content: To add, you also want to monitor the voltage / frequency of the output during runtime, as you will want to disconnect the load if RPM starts to fluctuate (due to low fuel) or if the system is overloaded.

author: Andreas
author_rank: Super Contributor
timestamp: February 07, 2019, 08:58:13 pm
content: Quote from: frostheave on February 06, 2019, 11:21:40 pmFor cars it is usually when the low idle RPM are reached or when a timeout (10 seconds) has passed.(besides the enable conditions like neutral gear / clutch is open ...)

author: tatersalad
author_rank: Contributor
timestamp: February 07, 2019, 09:27:29 pm
content: what kind of generator are you working on? I converted a military diesel over to be automatic and to my surprise it will disengage the starter for you. I do have a 120v relay to sense line voltage so if it doesn't come up it will stop cranking heat the glow plugs for 30 seconds and crank again.

author: frostheave
author_rank: Contributor
timestamp: February 08, 2019, 05:03:27 pm
content: 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

author: schmitt trigger
author_rank: Super Contributor
timestamp: February 08, 2019, 05:30:40 pm
content: 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.

author: fourtytwo42
author_rank: Super Contributor
timestamp: February 08, 2019, 05:40:03 pm
content: 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

author: NorthGuy
author_rank: Super Contributor
timestamp: February 08, 2019, 06:46:25 pm
content: Quote from: frostheave on February 08, 2019, 05:03:27 pmUnless 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.

author: ejeffrey
author_rank: Super Contributor
timestamp: February 08, 2019, 07:39:05 pm
content: Quote from: frostheave on February 08, 2019, 05:03:27 pmEasy 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.

author: frostheave
author_rank: Contributor
timestamp: February 08, 2019, 09:42:42 pm
content: Quote from: schmitt trigger on February 08, 2019, 05:30:40 pmYes, that would be a possible alternative if the relay method does not work.Thanks.

author: frostheave
author_rank: Contributor
timestamp: February 08, 2019, 09:49:57 pm
content: Quote from: fourtytwo42 on February 08, 2019, 05:40:03 pmYeah, those glow plugs are important to use when it's cold.Your point is well taken. I want to make the system as idiot proof as possible!

author: frostheave
author_rank: Contributor
timestamp: February 08, 2019, 09:59:11 pm
content: Quote from: NorthGuy on February 08, 2019, 06:46:25 pmThanks NorthGuy.Nothing fancy about the gen. I will be using an Arduino for the controls. Is there some advantage to monitoring frequency vs. voltage?

author: frostheave
author_rank: Contributor
timestamp: February 08, 2019, 10:09:58 pm
content: Quote from: ejeffrey on February 08, 2019, 07:39:05 pmThanks a lot ejeffrey.I have a separate transfer switch that will be controlled by the Arduino in order to transfer between commercial and generator power. Yes, the programming will include a generator warmup and cool down period as well as only transferring loads when the generator is running and warmed up.I was wondering how I could sense the frequency and your transformer solution is so simple! If my voltage/relay method does not pan out, I'll definitely give this a try.Bob

author: Bratster
author_rank: Frequent Contributor
timestamp: February 08, 2019, 10:24:32 pm
content: Quote from: frostheave on February 08, 2019, 09:59:11 pmIf this is a basic generator, frequency will be very strictly tied in with RPM.I'm guessing the generator running RPM will probably be 3600 for 60 Hertz.That's what the ones that I have installed at various sites are.Sent from my Fi Moto x4 using Tapatalk

author: tautech
author_rank: Super Contributor
timestamp: February 08, 2019, 10:52:29 pm
content: Quote from: frostheave on February 08, 2019, 09:49:57 pmIMO a microswitch on the governor throttle linkage somewhere could be the way to go if you're prepared to do a little experimentation and metalwork for a bracket on which to mount it.The governor won't move at cranking speeds but it will immediately on engine firing as the RPM's lift.Take a moment to observe its behavior then decide if it will suit your needs.I'd probably cascade the microswitch into a small 12V relay which then breaks the starter solenoid winding.Old school I know but you won't need to worry about needing any snubbers controlling the back EMF from the solenoid winding that might fry semiconductor switching of the solenoid.Still you might need a reverse bias diode on the small 12V relay winding as this is where you'd likely switch in for when mains power fails.

author: frostheave
author_rank: Contributor
timestamp: February 08, 2019, 11:40:23 pm
content: Quote from: Bratster on February 08, 2019, 10:24:32 pmAH HA, I see.I wonder if it is linear, i.e. the frequency would increase 1Hz for every 60 rpm increase.Thanks for the insight.

author: frostheave
author_rank: Contributor
timestamp: February 08, 2019, 11:47:18 pm
content: Quote from: tautech on February 08, 2019, 10:52:29 pmNow that is a very interesting idea. I like it.Unfortunately, I am working away from home for the next 2 weeks so I won't be able to try these great options until I get back.Thanks for the KISS idea.

author: tautech
author_rank: Super Contributor
timestamp: February 09, 2019, 12:01:48 am
content: Quote from: frostheave on February 08, 2019, 11:47:18 pmSometimes we just overlook the piss simple remedies.As to just how to use a microswitch, well you could just let the governor arm press against it or use a pull type and connect it to the arm with a linkage or light spring.In either case you need enough travelandforce to activate the switch without influencing the governor arm unduly so that it effects how it controls engine RPM, that BTW as mentioned is normally 3600 or just a shade more to manage loads and maintain correct mains frequency.

author: den
author_rank: Contributor
timestamp: February 09, 2019, 09:56:58 am
content: Current sensing coil on the high-voltage spark plug wire would do a good job making an RPM signal. One could use a Schmitt trigger to convert the signal to a digital form. Small bonus: if ignition fails - you do not fill you crankcase with fuel, trying to start the engine.

author: NorthGuy
author_rank: Super Contributor
timestamp: February 09, 2019, 11:31:03 pm
content: Quote from: frostheave on February 08, 2019, 11:40:23 pmOf course. One revolution is one cycle in AC. Some big generators run at 1800 rpm - they produce twoAC cycles per revolution.After it starts, It may slow down or speed up a bit when the load changes. May even go below 50 Hz when it tries to start a motor, may even die of overload.If your goal is to protect the loads, then you probably need to monitor both frequency and voltage, which all can be done through a single ADC.Assuming your generator has grounded neutral and your electronics is grounded too, you don't need a transformer to monitor voltage - a voltage divider will do.