One Lucky Sparky

Hi Guys,
For the last week at work I've been setting up an increaser gearbox for testing on the test bench I made for the purpose. We had a 7.5Kw 3 phase motor (10hp) driven by a 7.5Kw VFD and the motor couldn't run the gearbox to full spped before the VFD cutout. Anyway we upgraded to a 15Kw motor and 15Kw VFD today, the sparkie was told many times not to hook the 3 phase input to the output terminals on the VFD as it would blowup. When finally everything was connected and the VFD programmed it was time to hit the start button where the LCD screen just said' OC'- Over current-. Well I was on the phone to the guru in no time flash and I told the sparkie to take out the motor connections and reboot the VFD. Well the VFD didnt power backup again and while the sparkie was checking voltages I told him to check the open motor wires... YEP they were the 3 phase input wire and how the thing never blewup has both me and the guru who sold us the unit in amazment. The only reason we can put down is the supply line is only 4mm wire with a 75 metre cable run and with the reverse circuit that wire couldnt pass the short circuit current needed to make a 5K unit let the magic smoke out in glorious fashion with the stae manager looking on. After a heap of tests the VFD was proven to be OK so I put the Vee belts on the motor and drive and slowly turned the frequency pot to 50hz. Where the input speed was 1705 rpm, tomorrow I need to tweak the max frequency to 60 hz so I can get upto 1800 rpm on the drive spindle.

The spec's of the gearbox on test, the unit is bolted to a V12 Cat diesel with an input speed of 1750 rpm, the output speed is 4,500rpm and the motor develops around 750hp.

Well tomorrow is the big test infront of the clients and all vibrations on the gearbox will be tested.

Cheers Bryan

Sorry Bryan, you lost me. Why didn't you just hook up the 15kW vfd to the 7.5kW motor?

And, why wouldn't you connect the 3 phases from the motor to the vfd? How else would you connect it?

And, why are you using a 15kW motor to test a gearbox that is supposed to handle 750hp?

Maybe I've misread what you posted but I'm sure I'm not the only one.

Just looking for clarification.

Mike.

So why are you spinning up the gearbox any way?

To date the worst overall electricians I have worked with are typically the licensed ones. Apparently that license means they no longer have to read or understand electrical codes or schematics and are free to half ass their way through jobs while getting premium pay rates.

The unlicensed home owner knows he doesn't know everything and therefor gets the code books and necessary information before he starts doing anything. That concern for doing it right often times gives their work a far better quality when its done.

Ok the reason the gearbox was setup for testing the client stated it had major vibrations and in the desert it could be heard over 1K away under load. Surprisingly I have overhauled 11 of those box's now and everyone is working great, the one thats back was the secondlast one I did. The one that replaced it was the last one I did and is running perfect, we had a condition monitoring mob around today and we did the test at 50hz where the output was doing 4,457 rpm. They did say off their tests the primary gearset has big vibrations so I took the top cover off the gearbox today and we didnt find any damage. Tomorrow the investigations will start and with all the other work I have to spare time will be gold'n.

Pommie,
We did try the installed 7.5Kw motor to run the setup but drawing over 30 amps on 415 volts was too much for the 13 amp rated motor. When the sparkie opened the cover on the wiring box burnt enamel was smelt by me 10 metres away. So a near new 7.5kw motor might be my next wind genny project. Anyway with the 15Kw motor the current draw going through the frequency range to get to 50 hz was near 40 amps at times, the full load current dropped to 21 amps at 50hz so the 15kw motor and VFD was justified.

On reading the VFD manual today where it can be controlled via RS485 these VFD's might be a good way to learn RS485 and I can use one on my cnc project for variable speed etc.

Cheers Bryan