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current surge stalling engine

davit

Aug 16, 2017
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hi carrying on from my previous post below,
i made a 24v generator using a 4 stroke petrol engine to charge my bank of batterys,
my problem is even though its 7.5 hp engine the 24v alternator pulls so much power it stalls the engine, to get it to work i cannot energize ( not good i know ) the alternator untill the engine is hot and reving hard ,
so my queston is is there a type of rheostat switch i can use to hold back / vary the charge until some power has gone into the batterys, so i could then turn up the switch ,
the alternator can push out 55amps -- 24v
many thanks
davit

https://www.electronicspoint.com/forums/threads/hi-need-help-with-a-dc-charging-issue.295075/
 

crutschow

May 7, 2021
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to get it to work i cannot energize ( not good i know ) the alternator untill the engine is hot and reving hard
What do you mean by "energize"?

Does the alternator have a separate voltage regulator or is it built in?
If you can access the field wire, you can add a resistor in series to reduce the field current which reduces the alternator output voltage/current.
 
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davit

Aug 16, 2017
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hi thanks for the reply
the is the modern type all built in with reulator, i have a switch and warning light just like in a car, but also the alternator will self exicite with high rev,s, and charge even with the ign / alt warning light turned off , just need to hold back the the current for a few minutes to a lower reading then turn it up full gradualy, ( to help the engine ) this is what i am hoping for just not sure if this is fesable
 

Bluejets

Oct 5, 2014
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Output would be limited by whatever load you have connected.
I'd say you have something connected up a-about or the alternator is cactus.
 

davit

Aug 16, 2017
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thank you for the reply
the batteries would be about 50% discharged when i switch on the alternator causing the engine to stall from a moderate to high rev setting
thanks
 

Bluejets

Oct 5, 2014
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just need to hold back the the current for a few minutes to a lower reading then turn it up full gradualy, ( to help the engine ) this is what i am hoping for just not sure if this is fesable

Can be done but to start with it would be much easier with and external regulator.
You would need an understanding of the operation of the alternator and be able to sort out the regulator wiring.
Then make up a controller to manually control the rotor input.
 

crutschow

May 7, 2021
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Have you tried connecting the load with the engine at idle, and then increasing it's speed?
 

davit

Aug 16, 2017
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Can be done but to start with it would be much easier with and external regulator.
You would need an understanding of the operation of the alternator and be able to sort out the regulator wiring.
Then make up a controller to manually control the rotor input.
its a new alternator costing £150 so i will have to stick with it, with there being just the one output + lead i was kinda hoping that if i cut that line and connected wirewound rheostat switch in series that would hold back the charge to a trickle till some juice went into the battery, then slowly turn up the switch allowing more through and hopefully by then engine could take the load,
thanks for the interesting video link, way beyond my knowledge though LOL
THANKS
 

Alec_t

Jul 7, 2015
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55A @ 24V is less than 2hp. I'm surprised a 7.5hp engine stalls.
Could you put a bit of reduction gearing (belt and pulleys) between the engine and alternator?
 

Harald Kapp

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The 7.5 hp from the engine may be available in the optimum operating point only. Much less will be available during idling or at very low or very high speeds. Do you have the hp vs. rpm characteristic of the motor at hand?
Here's an example from another site:
Y7lkM.gif

As you can see max power in this example is available at 5500 rpm whereas during idling (1000 rpm) only a fraction of ~ 20% is available. For your motor 20 % translates to 1.5 hp only which is less than the power requirementof the alternator:
Output power Pout = 55 V × 24 V = 1.32 kW equiv. 1.77 hp.
Taking into account losses within the alternator (efficiency around 60 %) the input power to the alternator would have to be at least:
Pin = 1.77 hp / 60 % = 2.95 hp.
If your engine delivers only 1.5 hp during idling, it is no wonder it stalls.
 

davit

Aug 16, 2017
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55A @ 24V is less than 2hp. I'm surprised a 7.5hp engine stalls.
Could you put a bit of reduction gearing (belt and pulleys) between the engine and alternator?
YES i believe that would work , but i have a lack of space to do that, and weight is getting an issue, thanks
 

davit

Aug 16, 2017
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The 7.5 hp from the engine may be available in the optimum operating point only. Much less will be available during idling or at very low or very high speeds. Do you have the hp vs. rpm characteristic of the motor at hand?
Here's an example from another site:
Y7lkM.gif

As you can see max power in this example is available at 5500 rpm whereas during idling (1000 rpm) only a fraction of ~ 20% is available. For your motor 20 % translates to 1.5 hp only which is less than the power requirementof the alternator:
Output power Pout = 55 V × 24 V = 1.32 kW equiv. 1.77 hp.
Taking into account losses within the alternator (efficiency around 60 %) the input power to the alternator would have to be at least:
Pin = 1.77 hp / 60 % = 2.95 hp.
If your engine delivers only 1.5 hp during idling, it is no wonder it stalls.

hi harald
its a chinese built engine ( honda ripp off ) no spec details available except the 7.5 hp sticker , as i have said before if i rev the machine to almost max (guesing 3-4 rpm) with the alternator freewheeling then switch battery feed on the engine cuts down but accepts the load and keeps running, i know this is really bad for the alternator but its the only way i can get charging,
do you think my rheostat idea would work ?? allways charging with an intiall trickle going through ,,thanks
 

Harald Kapp

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if i rev the machine to almost max (guesing 3-4 rpm) with the alternator freewheeling then switch battery feed on the engine cuts down but accepts the load and keeps running,
Sounds exactly like the problem i described in my post.
allways charging with an intiall trickle going through ,,thanks
Why not simply put a switch between alternator and battery? Turn off the battery when starting the motor. Once the motor runs at speed, turn on the battery circuit to charge. Why should this be bad for the alternator?
A rheostat suitable for 55 A will be a messy big thing.
 

davit

Aug 16, 2017
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Sounds exactly like the problem i described in my post.

Why not simply put a switch between alternator and battery? Turn off the battery when starting the motor. Once the motor runs at speed, turn on the battery circuit to charge. Why should this be bad for the alternator?
A rheostat suitable for 55 A will be a messy big thing.

yes this is what i do at the moment , i use an isolater switch, i was hoping to stop the big jolt when i switch on which i have to get right ie engine revs and temperture, i have this coming from ebay
20k Wirewound Rheostat Variable Resistor
https://www.ebay.co.uk/itm/20k-Wirewound-Rheostat-Variable-Resistor-/363419254523?nma=true&si=iOGlcuKFP1Iyc7BpihIGMxzKpkM%3D&orig_cvip=true&nordt=true&rt=nc&_trksid=p2047675.l2557
 
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Bluejets

Oct 5, 2014
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Why won't you just give everyone a break and show what you have there and a diagram of what is connected where.
While you are at it a link to the specs of the alternator.
Thanks.
 
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