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learning about inrush at startup

LEOK

Active member
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Location
Gainesville/Fl
This is not about any particular generator, mostly about learning about how they work and loads. This is new 36kva Perkins turbo/meccalte single phase alternator and a Deep Sea 7320 line monitoring controller

learning curve about loads. The 36kva genset set shutdown, Deep sea controller watches everything. The logs showed over and under voltage, frequency, and current. About three hours in while still waiting for power Co to put the meter back in the over voltage crossed the limit setting. So that lead to investigation on the AC, Turns out the starting load is huge locked rotor is 150 amps. The settings on the controller were set for zero tolerance so when house load increased due cooking diner that threshold was crossed. So not have any real experience with the controller a deep read on the manual was needed. Turns out the limits have tolerance setting so I opened them from zero to five seconds. There was another setting called short circuit with Amp setting of 200 amps. with setting called Electrical trip IDMT or shutdown I change it to Shutdown from trip and raised it to 300 amps. Knowing that large motors look like short circuit at start up. Loaded the new config and as soon as the AC came on the gen shutdown, log showing short circuit. So more reading, turns out IDMT is power curve of time versus overload. The higher overload the less time it will allow a run. So tomorrow we will try the 5 second delay and Electrical Trip setting enabled. When we choose this particular Alternator I spoke with MeccAlte and the engineer explained the rotor construction. He recommend the model with iron core rather than the Aluminium explaining how the dedicated single phase iron core can handle loads better especially motor that start under a load like pumps. Who would have thought the surge from an AC that only draws 7k watts running would surge past 300A on startup.
 

Coug

Well-known member
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Olympia/WA
so a 30 amp aircon is passing 300 amps at startup? That seems a bit steep. Normally I would expect around 150-180 amps (5x-6x inrush), but it's not impossible. If it isn't a brand new unit I'd try replacing the start capacitors on it to see if that helps. If it IS a new aircon then I'd be calling the company and asking them where it should be.
And yeah, iron core for generators help reinforce the magnetic field as the iron becomes part of the field, where the aluminum won't. I didn't even know anyone did them with aluminum cores.
 

LEOK

Active member
125
26
28
Location
Gainesville/Fl
Its on 60 Amp breaker, and the label says MIN CKT AMPS 39.1, new Trane 4/17. It starts the unit with no trouble, it just the tuning of the line monitoring controller. We didn't put the current transformers on the utility side, should have then we have logs on power Co also.
 

Triple Jim

Well-known member
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83
Location
North Carolina
I'm not surprised at the starting current. I worked with some big blowers that had 3-phase Baldor motors. They took a long time to come up to speed so it was easy to see the starting current. Ranging from 5 to 15 hp, they all started out at 10 times the running current. In my case I ended up having to go to physically larger sand filled time delay fuses that tolerated longer times of overcurrent because the smaller ones I tried would blow before the current dropped.
 
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