Triac Tripping Breaker at high RPM

[jamesgates1000]

Just replaced the spindle bearings on my Denfored Triac, was running nice, until...

I made a mistake the other night and picked up the wrong tool offset, rapid 15mm into ali with a 6mm spot drill at 4000rpm. Got a 1008 spindle drive fault, the 10a timed fuse had blown so I replaced it. I then turned the machine back on and once I hit the green on button it tripped the breaker in the garage workshop. Reset breaker and replaced the fuse again and the machine turned on fine. Ran program fine until it picked a tool and tried to run at 4000rpm, this caused the main breaker for the whole building to go. Turns out that if I try to run the spindle above 3300rpm it will trip this main breaker.

I can still run the machine at lower revs but would like to find out why I can't run the spindle faster. Spindle sounds okay, maybe a bit crunchy occasionally.

Has anyone had a similar problem and could give me a hint, is this most probably a motor(SEM) or spindle drive(Sprint 1600i) issue?

Also does anyone know why the machine fuse and the garage breaker fail to trip before tripping the main breaker.

[moray]

In my experience of an old connect cnc lathe with DC spindle, those are the symptoms of a failing drive. It had the same problem, and a new drive cured it. The SCRs start to fail short circuit, so the mains essentially starts seeing a fast switching dead short.

The main breaker being tripped is likely to be down to either it's seeing more current and reaching it's trip limit before the local breaker, or it's a faster acting breaker.

The same thing was wrong with my Triac, but as you'll of seen from my thread over on mycnc, I used it as an excuse to retrofit an AC servo. Somebody did link to a new sprint drive in one of the Triac threads over on mycnc, and they're aren't that expensive new, but I can't remember if it was my thread or Chaz's.

[jamesgates1000]

Hi Moray,

Thanks for your help again. It did report as spindle drive fault when I first changed the spindle bearings, as the top parallel bearing doesn't actually touch the spindle shaft so I pushed it against the spindle slightly so the bearing would spin. Think this pressure put extra load on the spindle. Do you think that it was the crash that damaged the drive, or just getting to the end of its life?

Found a Sprint drive for £220 so not too bad, need to replace it as I have quite a bit of work coming up.

Is it a big job to set up or can I just measure the resistance on the pots on the old one and use that to set the new one?

May be worth getting a faster acting local breaker too for the future just in case.

[Martin]

It sounds more like a fault on the spindle motor to me. I presume you have a SEM motor?

It may be worth checking the actual speed of the spindle using a tachometer if you can get hold of one. if it's over 4000 rpm you can turn it down on the max speed pot.

It's is possible for the motor to flash over & deposit carbon inside the motor which may need to be cleaned out by a motor rewind company.

[jamesgates1000]

Took the motor to servo repair place in Bristol, apparently there was a washer loose rattling round inside, which was causing the motor to short. Luckily it didnt damage it enough to need a rewind. Having it re furbished as it has been taken apart. Hopefully this will solve the issue then I can return the spindle drive card if it is not needed.

[jamesgates1000]

Got the motor back fully refurbished and still getting the power tripping problem. Will try the replacement sprint drive board, is it fine to just set the pots by eye against the old board?

[Martin]

The top speed is quite critical to not go beyond.

Also make sure you set the 4 switches the same.

[jamesgates1000]

Just to wrap this up....

Power was still tripping at high speed and if I took a medium size cut, got a £7 tachometer from ebay to check the rpm and it was running fast. (3600rpm when set to 3000rpm)

Dialled it back a bit but it was still tripping power taking cuts that it did before.

So I finally got round to changing the sprint 1600i board, used thje tachometer to set the max speed just under 4000rpm, and now no problems at all.

Thanks for all your help with this.

[Variable777]

Just wanted to add my experiences here.

My Triac was blowing fuses at around 2500rpm. It was flashing over inside the spindle motor and then blowing the 6.3A fuse (top left group). It also took out my house breaker as well. The problem was probably historic as I found a packet of fuses in the cabinet from the previous user.

I am indebted to Steve Oddy at Denford and Jim Lock at Sprint Electronics for their advice during the resolution of this issue.

As the control board was doing a good job up to 2500rpm I was advised that the Sprint 1200 board was probably ok. This indeed turned out to be the case. I spent some time seeking a company who could deal with the motor (SEM MT 30U4-36) and after a number of false starts sent the Motor to B&T Rewinds (Dave Phelps, 0121 359 8398, dave@bandtrewinds.co.uk) in Birmingham. They diagnosed a faulty Tacho winding and low permanent magnet field. They overhauled the motor with new bearings and a commutator skim, rewound the tacho and had the magnets sorted. Total cost £624 including VAT. B&T Rewinds cannot deal with motors with encoders but are ok with wound tacho's. They were very helpful throughout the process and have given a 12 month warranty on the motor. Be aware that if the armature winding had been faulty also then it would have been another £300.

The fuse for the Sprint 1200 is a ABC-15-R and is available from RS part number 848-2687.

Good news is that I have been spinning the motor up to 4000rpm with no issues and the problem appears to be sorted. One small alignment issue I had was not realising that the speed override pot controls the rpm between 50 -120% - I had set it to max and could not get the rpm range to set correctly.