rogerrabbitt wrote:ok the biggest equipment i run is a low pressure vtwin compressor
it's 3kw but it has to start with an open valve until it kicks in properly.
the diving compressor turns over maybe five times then labours and blows a fuse.
and it's huge i'm no big feller but i have trouble lifting it.its about 25 percent bigger physically than the other 3kw motor i have
11 inch pully.
First thing is make sure that your compressor is turning easily and smoothly when rotated by hand and no belt on it. Also when you try running it, make sure no other electrical items are on at the same time, thus sapping off some of your available amps. Also check your incoming voltage with a meter! If it is much lower than the motor rating it will cause a higher amp draw and might stall out the motor under load. Also start the compressor running with the condensate drain valves open to let the rpm get up before "loading" the compressor by closing the drains. The 1st and second stages have larger pistons and cause more initial strain on the motor than the others.... so be sure they are draining and not compressing.
Important consideration: Your motor could have a bad "start capacitor" which gives a single phase motor a little extra voltage kick during the high power (amp) draw of starting the equipment turning. If you can have that checked first it might save you a lot of trouble. Also, I assume that the motor runs ok when not connected by belts to the compressor... if you haven't tried that do it now. If that's ok, proceed to check the start capacitors. I see two attached to the motor. If you have an older volt/ohm meter with a needle instead of digital, you can give it a basic test but it's not foolproof: disconnect all power to the motor. Access the capacitor terminals, but note that if you have tried running it recently, the cap (capacitor) may be "charged" and give you a nasty jolt, so short across the terminals with a screwdriver to discharge it while only touching the plastic handle. Then note what wires go where and write it down. Remove wires from cap and turn ohmmeter on. Touch red wire to one cap terminal and black wire to the other while carefully watching the needle. It may jump a little. This has given a tiny charge to the cap from the battery in the meter. Then reverse the red and black wires and watch closely as you touch the terminals with them again. You should see the needle jump slightly as it discharges its stored charge. This is a rudimentary check and does not tell the full story, but if no "jump" on that second step you may have a bad cap. If needle jumps a little proceed to next idea.
Don't worry about physical size as related to power of the motor, as different frame types and even motor age can vary widely in that respect.
Next you might try the other motor if it has a smaller drive pulley... or put a significantly smaller pulley on your existing motor and see if that works better. It will then require less power to turn the compressor, thus drawing less amps and maybe not overloading your circuits.
Try all that for now and give us a report.