Portable Generator Internal Circuit Breaker vs Inrush Current

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kwired said:
One thing I will point out you mentioned inrush (or whatever you want to call the peak current at starting) may be 54 amps assuming voltage input remains stable. Reduced voltage soft starting methods intentionally reduce that current surge. If the source has high enough impedance, there is undersized conductors, etc. you also get some that effect that will reduce the surge current. Sounds like the starting ability of your generator may be high enough it isn't going to be too significant, but that is something to consider. a generator often has more severe voltage drop than utility power when you suddenly put a larger load on it, particularly if large enough load that prime mover temporarily slows down during that surge.
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the motors have 54 starting amps which means if both turn on at the same time the starting amps is 108. Do I put the soft start at the load size of the generator? I cannot modify manufacturer's equipment because the motors are in the unit.
 
Tainted said:
How do I find the voltage dip based on my situation?
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Please find these data:
  1. %Xd of your genset
  2. Maximum SkVA of your load
  3. Rated kVA of your genset
The formula to use is:
Vdip =(%Xd of gen)/(%Xd of gen+(Rated kVA of gen/Max SkVA of load))
Hope that helps.
 
Tainted said:
the motors have 54 starting amps which means if both turn on at the same time the starting amps is 108. Do I put the soft start at the load size of the generator? I cannot modify manufacturer's equipment because the motors are in the unit.
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What I was trying to say is if generator impedance is high, the voltage drop you will experience makes sort of a soft starter in a way on it's own, if so they won't draw that 54 amps when starting on generator if output voltage drops significantly.
 
kwired said:
What I was trying to say is if generator impedance is high, the voltage drop you will experience makes sort of a soft starter in a way on it's own, if so they won't draw that 54 amps when starting on generator if output voltage drops significantly.
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Oh I see. It's hard to get information I need from this portable generator... I'm debating whether or not I should roll the dice on this one....
 
Tainted said:
I'm designing a backup system for (2) 2HP motors.
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Do you have the motors? Could you perhaps do 2 things to help your situation? If you use 2 VFDs with long acceleration (effectively a soft start, but you seem to be single phase making those less common) AND start them sequentially rather than simultaneously, you'd have a good chance.
 
GeorgeB said:
Do you have the motors? Could you perhaps do 2 things to help your situation? If you use 2 VFDs with long acceleration (effectively a soft start, but you seem to be single phase making those less common) AND start them sequentially rather than simultaneously, you'd have a good chance.
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the motors are internal to the unit by manufacturer
 
What kind of loads are they driving? That possibly makes a difference as well. Something that takes longer to accelerate may give you more trouble than something that accelerates easily.
 
Tainted said:
It’s a sewage grinder
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Probably easy acceleration unless of course it is jammed.

When you say sewage grinder I am thinking maybe is also a dual pump lift station where only one pump normally runs at a time, they alternate running, and only if the first pump can't keep up with demand or if first one failed for some reason would the second one kick on?

If so and this is only for emergency standby you possibly can turn one pump off so that you won't have the risk of tripping the 50 amp breaker on generator. I really don't think you will have trouble starting just one of those motors it is when starting the second one that you might risk tripping that supply breaker.
 
kwired said:
Probably easy acceleration unless of course it is jammed.

When you say sewage grinder I am thinking maybe is also a dual pump lift station where only one pump normally runs at a time, they alternate running, and only if the first pump can't keep up with demand or if first one failed for some reason would the second one kick on?

If so and this is only for emergency standby you possibly can turn one pump off so that you won't have the risk of tripping the 50 amp breaker on generator. I really don't think you will have trouble starting just one of those motors it is when starting the second one that you might risk tripping that supply breaker.
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If it is something that you start and let run - then you obviously are going to be there to start it, could have some trouble starting the second motor, but once you get it started is probably going to be fine. If it automatically starts while unattended is where there could be troubles
 
kwired said:
If it is something that you start and let run - then you obviously are going to be there to start it, could have some trouble starting the second motor, but once you get it started is probably going to be fine. If it automatically starts while unattended is where there could be troubles
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There are 2 floats in the system, if a float is high enough it activates 1 of the motors, and then if the second float is high enough it activates the second motor while the 1st motor is on. If both floats are high it means both start together
 
Tainted said:
There are 2 floats in the system, if a float is high enough it activates 1 of the motors, and then if the second float is high enough it activates the second motor while the 1st motor is on. If both floats are high it means both start together
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Exactly what I was figuring. Most those types of systems there is alternating control that starts the "other motor" every cycle. Should the first motor fail to start the second starts when second float level is reached. They usually never both run other than if there is an abnormal condition, so this likely means you really only need to worry about starting just one motor from the generator for the most part. They would only both start at same time if level is high when you first start up the system, but you could disable one or the other if it can't handle both starting at same time.
 
kwired said:
Exactly what I was figuring. Most those types of systems there is alternating control that starts the "other motor" every cycle. Should the first motor fail to start the second starts when second float level is reached. They usually never both run other than if there is an abnormal condition, so this likely means you really only need to worry about starting just one motor from the generator for the most part. They would only both start at same time if level is high when you first start up the system, but you could disable one or the other if it can't handle both starting at same time.
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What are the chances that both will start at the same time? The possibility is still there
 
Tainted said:
What are the chances that both will start at the same time? The possibility is still there
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Not likely, unless there is an extended power outage, since it’s a portable, and someone will be there to start it and transfer, it would be easy to lockout the second pump until the tank is pumped down below the second stage start level. After that, it would be a heck of a lot of sewage being dumped in at one time to start both pumps at the same time.
 
Tainted said:
What are the chances that both will start at the same time? The possibility is still there
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They should only start at same time if level is above second float when you initially start it up after power had been lost. In that case someone is likely there to supervise startup to make sure things go ok. Once up and going they are not that likely to start at same time without some malfunction being the cause.
 
kwired said:
They should only start at same time if level is above second float when you initially start it up after power had been lost. In that case someone is likely there to supervise startup to make sure things go ok. Once up and going they are not that likely to start at same time without some malfunction being the cause.
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See post #35 LOL!
 
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