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Old 22-06-2020, 08:08   #1
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Wanting advice with undersized generator

We have a Kohler 13 KW single phase 220 volt generator onboard. We also have a Bauer Mariner 320 compressor with a 7 KW single phase 220 volt motor.

All the experts tell me that the generator is too small for the startup power required for the compressor motor. I have been trying to find a solution for this without incurring the cost of buying either a new generator or compressor. However, the many different technologies confuse the hell out of a non technical person like me;

  • Variable Frequency Drives,
  • Part-winding starters,
  • Wye-delta starters,
  • Solid-state starters with automatic bypass contactors,
  • Autotransformer reduced-voltage starters.

Can somebody please help?
What can be done to have the above machines work together?
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Old 22-06-2020, 08:11   #2
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Re: Wanting advice with undersized generator

If the compressor has an unloader so the motor isn't starting with the compressor output pressurized, there's a decent chance it'll work. Personally, I'd try it and see how it does before adding stuff to the compressor to reduce the startup load.
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Old 22-06-2020, 08:19   #3
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Re: Wanting advice with undersized generator

What’s the duty cycle on the compressor?
Are you wanting to run other loads at the same time as the compressor?
What experts? Marine electricians?
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Old 22-06-2020, 08:34   #4
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Re: Wanting advice with undersized generator

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Originally Posted by rslifkin View Post
If the compressor has an unloader so the motor isn't starting with the compressor output pressurized, there's a decent chance it'll work. Personally, I'd try it and see how it does before adding stuff to the compressor to reduce the startup load.

Thank you - I will find out if there is an unloader.
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Old 22-06-2020, 08:37   #5
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Re: Wanting advice with undersized generator

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What’s the duty cycle on the compressor?
Are you wanting to run other loads at the same time as the compressor?
What experts? Marine electricians?

I don't even know what a duty cycle is!?
No, we do not have to run other equipment whist the compressor runs.
Experts = a whole bunch of people, electricians, suppliers, etc.
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Old 22-06-2020, 08:38   #6
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Re: Wanting advice with undersized generator

Did you actually try it? Worse case is a temporary stall, just be ready to flip it off. The initial surge is big, but it may come right out of it.
Also, I wonder what the startup draw would be with all the pressure out of the compressor tank? With no "push back" pressure, wouldn't it be very low? Or an option of a valve on the compressor line to open until the motor runs is an option.
I have an old shop compressor that has an undersized motor, maybe 1 time out of 10 it wont start up, (Tank empty) I have a permanent wrench on the air fitting so I can crack it open and it starts right up when the motor was stalled....
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Old 22-06-2020, 08:41   #7
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Re: Wanting advice with undersized generator

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Originally Posted by Adelie View Post
What’s the duty cycle on the compressor?
Are you wanting to run other loads at the same time as the compressor?
What experts? Marine electricians?

I searched the manual - not a word on "duty cycle".
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Old 22-06-2020, 08:45   #8
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Re: Wanting advice with undersized generator

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Did you actually try it? Worse case is a temporary stall, just be ready to flip it off. The initial surge is big, but it may come right out of it.
Also, I wonder what the startup draw would be with all the pressure out of the compressor tank? With no "push back" pressure, wouldn't it be very low? Or an option of a valve on the compressor line to open until the motor runs is an option.
I have an old shop compressor that has an undersized motor, maybe 1 time out of 10 it wont start up, (Tank empty) I have a permanent wrench on the air fitting so I can crack it open and it starts right up when the motor was stalled....

No, we have not tried it. The generator is onboard, the compressor is in my workshop. Will be several months before we will be able to install the compressor.
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Old 22-06-2020, 08:48   #9
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Re: Wanting advice with undersized generator

Modern compressors, even cheap ones, empty the cylinder and hoses leading to the tank when the motor shuts off. The first compression then is not against tank pressure. If you don't have that, Cheechako certainly has a practical solution. Get that sucker spinning and then close the valve.

The usual easy cheap way to get the genset over the hump would be a $10 starter capacitor on the hot line leading to the compressor.
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Old 22-06-2020, 08:58   #10
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Re: Wanting advice with undersized generator

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.......... The usual easy cheap way to get the genset over the hump would be a $10 starter capacitor on the hot line leading to the compressor.

Can you provide more details on this solution please. But make it simple so that me stupid will be able to follow.
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Old 22-06-2020, 09:04   #11
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Re: Wanting advice with undersized generator

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Can you provide more details on this solution please. But make it simple so that me stupid will be able to follow.
Lots of info and videos on this subject in RV sites. I'm not an electrician, nor do I play one on TV, but a capacitor is (in concept) an extra little battery in the line that "dumps" all its energy into the line at once when it's energized. It boosts the juice to get the motor over its start-up load.
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Old 22-06-2020, 09:12   #12
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Re: Wanting advice with undersized generator

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Originally Posted by tkeithlu View Post
Modern compressors, even cheap ones, empty the cylinder and hoses leading to the tank when the motor shuts off. The first compression then is not against tank pressure. If you don't have that, Cheechako certainly has a practical solution. Get that sucker spinning and then close the valve.

The usual easy cheap way to get the genset over the hump would be a $10 starter capacitor on the hot line leading to the compressor.
Yeah, or... can you get a capacitor start motor to replace the one on the Compressor I wonder?
Or brainstorming again... How about a solenoid valve and time delay relay that opens the air line for a few seconds until the motor is running?

OP: do you have a Fluke (or similar) current draw tester? One of those ones that you just measure outside the electrical wire? You could test the compressor and see what it actually draws at startup maybe?
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Old 22-06-2020, 09:14   #13
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Re: Wanting advice with undersized generator

It may be worth your while to consult an electrician and ask if a soft start capacitor will get you over the starting surge. They are inexpensive and easily installed.
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Old 22-06-2020, 09:24   #14
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Re: Wanting advice with undersized generator

I had that same compressor running off a little Panda 5KW gen set (actually 4.7 on a good day)

It would load it on start up but quickly recovered. You can make it easier on the gen set by going to any electrical supply store and telling them you need a start cap (capacitor), tell them the voltage and current and they will recommend one, it is easy to install (just ask them how to hook it up) just make sure it is covered so the terminals are not exposed as it will shock you even if no power is applied.

Also bleed off the air before starting, I just opened the water drains and the fill valve.

You should have no problems.

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Old 22-06-2020, 09:30   #15
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Re: Wanting advice with undersized generator

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Originally Posted by Cheechako View Post
OP: do you have a Fluke (or similar) current draw tester? One of those ones that you just measure outside the electrical wire? You could test the compressor and see what it actually draws at startup maybe?
The Fluke will not work. it will not be able to measure the startup current. This peak current last for too short.
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