Wanting advice with undersized generator

[Jack C]

I use a microprocessor bases soft start unit to run a 16,000 BTU AC unit from a Honda 2000i. You run the AC on shore power about 11 times and the unit learns the compressor start characteristics. Then it works when powered from the generator. Much better than capacitor start. It really limits the inrush. Cost between $250-$300 a few years ago. Think it was from micro-air. You will probably need a bigger unit

[Scubaseas]

There's no unloader on it but if you remove the power (unplug) to the ACD (automatic Compressor Drain) it will open all stages to air. I doubt the compressor would start with a load on it (like pressure in the lines) but maybe if you start from Zero PSI it would work. I'd try it before going to soft starters and the like.

[Bill Seal]

Put a check valve at the tank inlet and a manual bleed valve between it and the compressor. This way you won't have to drain the air out of the tank.
(This is what the automatic unloader does)

[HopCar]

Many years ago I wanted to run my 5000 btu air conditioner with a Honda 2000 generator. No problem I thought, it only takes 600 watts to run it. Would not start it! I learned it takes a lot of power for just a fraction of a second to get it moving.

Look on the compressor label for LRA or Locked Rotor Amps. That’s about how much power you need to start it. In my case it worked out to about 3000 watts. The LRA was 28 if I recall correctly.

I solved the problem very easily by adding a Supco Hard Start Capacitor. Less than twenty bucks. After adding the Supco I could run it on a 1000 watt generator. https://www.amazon.com/Supco-SPP6-Ha...2849164&sr=8-5

[tkeithlu]

Revelations, did you get your question answered such that you can move forward? I saw your request for clarification after my post, but then other people had answered for me. To summarize, lots of AC motors come with starting capacitors, but if your compressor does not, go to an electrical supply house and ask for a 7KW 220v motor starting capacitor. Alternatively, ask an electrician to get you one. It goes in the hot wire to the motor, in series with the motor. All it does is store up some juice and dump it when you start the motor.

If boat wiring is leaving you trampled, let me recommend this book. It helped me get over some rough spots; I really did not understand AC circuits despite Electrical Engineering 101 55 years ago.

https://www.amazon.com/s?k=Charles+W...ref=nb_sb_noss

[motretu]

+1 on the fluke meter not being able to record the compressor start current.

my guess is that if the compressor usually started on a 5kw genset (as posted above), then it should easily start on your 13kw unit, especially if it is a non-inverter type.

the numbers you need ("locked rotor" or "start current" for compressor & max momentary (measured in milliseconds) current draw for genset) should be available from the manufacturers.

at the end of the day, i'd do a temp install of the compressor and see what happens.

[Bill Seal]

http://www.capacitorguide.com/motor-starting-capacitor/
Read this and you'll find out why you can't just add a "starting capacitor" to a motor that wasn't designed for it.

[BruceS]

I can't see it working as most big workshop compressors start up every so often just to keep the pressure up. If you only have a job (spraypainting, buffing, grinding etc) that will last an hour you could start it with no pressure (or tap turned off) and wind down the 'blowoff' valve so air just escapes instead of shutting off the motor. Then the motor would just keep going until YOU stop it when the job is finished.

[Reefmagnet]

Compressors are the worst for starting from generators. Rule of thumb is it takes 3x their rated load to start. Some gensets are designed to provide up to 3x their rated load for short durations whereas others can't get past the rating on the plate at all. The best thing to do is plug it in and see what happens and then go from there.


If it doesn't work then I'd consider installing a clutch or some other system that will allow the motor to start and reach operational speed before the load is applied.

[Westcliffe01]

What is the RPM of the single phase motor ? www.Surpluscenter.com has lots of cheap 3 phase motors. If you replace the single phase with a 3 phase motor like this https://www.surpluscenter.com/Electr...or-10-3173.axd then add the single phase to 3 phase VFD you can then set the start ramp speed so that the startup load is spread over a longer period, gradually loading up the generator and you will have a nice system. The example motor is 7.5hp 208V 2870 rpm motor for $189.


A 3 phase motor starts beautifully and usually runs a lot quieter than a single phase motor and with the VFD you can choose to speed up or slow down the running speed. Single phase stuff is as crude as hell and sort of "lowest common denominator" stuff built for use at home where there is no 3 phase where the manufacturer was too cheap to include the VFD as an option or a lot of informed people would take it.

[Dsanduril]

Originally Posted by Revelations

Thank you - I will find out if there is an unloader.

The Mariner 320 manual says that the condensate drain is used as the unloader. In units < 4kW it is manual - you have to open the drain. In units > 4kW it should be automatic - the drain is open when the motor is off, it gets closed electrically once the motor has started. It does depend on option levels, but overall the manual says to use the condensate drain as the unloader.

[Djarraluda]

In simple terms, a “standard” starter is called Direct On Line and it is exactly that, flick the switch and power is connected, it will typically take 7 times running current for a few seconds as it overcomes the inertia to get things spinning.
You don’t say which country you are in, but approx 230 volt single phase is Europe where approx 220 volt 3 phase is US.
If you are on single phase, then Star-Delta or Auto Transformer are not options. I am not aware of soft starters or variable frequency drives for single phase but that does not mean they are not available.
Generator (actually Alternator) set designs will often mount a bigger alternator on smaller engine to give protection for starting loads. Frame size of alternator will take the heat from those starts better than one that is otherwise matched to the engine.
So, first query is Are you talking Single or Three phase?
Roger

[Dsanduril]

Originally Posted by Revelations

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

[Djarraluda]

OK, so I did not read the original post adequately.
7kW single phase is a massive motor, quite unusual in many places as we tend to go 3 phase at a lot smaller size, generally because of that starting current.
So, Star (or Wye)-Delta and Auto Transformer are simply not options.
Look for a soft starter single phase but I would follow the other advice first, see if the load will be OK as the compressor starts unloaded. Big motor but low power needed to get out spinning still needs high torque. You may be lucky but if not, then source your soft starter accordingly.

[svrevelations]

Thank you for all the advice and options! I guess the next step would be to connect everything first and take it from there. Much appreciated!