Soft starter/vfd help

burcusa

Hi,
I am new to this board as I just moved into an off grid house about 3 months ago. I have a 24 V system with generator back up. My generator is a Perkins 10.5kw diesel generator (40A/leg). I just bought a new table saw. When I bought the saw I asked the salesperson about energy requirements and they said it need a 230V/20A circuit (but only used 13.8A at full load). So I wired that circuit in directly off the generator. Tried to start the saw and it stalls out the generator.

So I call the manufacture of the saw and they tell me that the electrical requirements that they advertise only apply to on grid (no mention of this anywhere in the owners manual). They tell me that they think there is a work around, but would not tell me what it is. They also let me know that I can't return the saw. Next I call the store I purchased it from and they to tell me I can't return the saw and they give me no help in how to set it up so that it will work.

So I am on my own. After many hours talking to folks and internet research, I find that maybe a soft starter or a VFD will be the solution. I see that Schneider electric sells many options. So I call Schneider electric and the square D tech line and am told that they think this product would work but were not able to help me choose which one to order (they did offer to help me if I have installation questions though.. ). Anyways, does anyone here have expertise with these products? If so, am I on the right path? If so, which of these products soft starter or VFD will work best with my application, and how do I choose which size of the product to purchase?

Thanks in advance,
Mike Burcusa

mcgivor

Welcome to the forum 

The motor inrush can be as high as 10× the run current, this inrush will cause the generator to try match the demand but the diesel engine cannot react fast enough to support the inrush and start up current causing it to stall. So you are correct a soft start will reduce these currents, ramping up which in turn allows the generator to provide the demand graduaiiy. The generator should be able to support the load under normal run conditions once over the "hump", the soft start should be sized appropriately to the horse power of the motor, being rated above the motor name plate rated HP to lessen the demands on the electronic components, for example a 3hp motor should use a soft start with a 5hp rating for example, or with a current rating higher then the motor full load amp rating. Any offerings from reputable manufacturers, Schneider/Square D, Eaton, Siemens, Danfoss, Hitachi, ABB etcetera are good choices,  VFD's tend to be more expensive and generally used on 3 phase equipment with more complex control options such as reverse and input capabilities, none of which are needed on a saw.

Estragon

On mobile site, so can't see if you have system details in a sig line, but any chance you could run the saw off battery/inverter and use the genny to recharge?

Battery/inverter can generally handle surges better than generators, and some inverters have a generator support feature in which they can augment generator with battery rather than simply passing generator current through.

bill von novak

burcusa said:

Anyways, does anyone here have expertise with these products? If so, am I on the right path?

Let me suggest a much simpler solution - a starting resistor.  Get a small value, high wattage resistor (around 1 ohm) and wire it in series with the saw.  Wire a high power normally-closed pushbutton to short out the resistor.  To start, press the button, turn the saw on, and then when the blade starts to spin release the button.

If the saw does not spin use a smaller value resistor.
If the saw still trips the generator use a larger value resistor.

peakbagger

Hard to find a VF drive for single phase motors. Far better off with 3 phase motor but expect its a custom motor for the table saw.  The odd part is you can feed a 3 phase VF drive with single phase (if the correct option is ordered.).

That starting resistor sounds intriguing and a lot cheaper.  

jonr

I agree with trying a resistor.   Maybe one like this.  The load is much greater than 300W, but it's brief.

https://www.ebay.com/itm/1-ohm-300-Watts-High-Power-Ceramic-Tube-Resistor-300W

burcusa

Thanks everyone for the helpful comments. I am really interested in the resistor option. Will this work with the magnetic starter on the saw? Or will I need remove the starter and put a much simpler switch in?

Then just to be clear, do I write the resistor and closed push button in before the switch?

Thanks again!
Mike

bill von novak

burcusa said:

Thanks everyone for the helpful comments. I am really interested in the resistor option. Will this work with the magnetic starter on the saw? Or will I need remove the starter and put a much simpler switch in?

Then just to be clear, do I write the resistor and closed push button in before the switch?

Thanks again!
Mike

Magnetic starters are usually used to allow simpler on-off switches to be used; that way the switches don't have to carry full motor current.  You would want to put the resistor between the magnetic starter (relay) and the motor, and then bypass that resistor with the NC button.