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Current transformer help please

I have a CNC lathe, a Miyano, that I just bought. The machine was made in the early 90s. I ran it in a different shop for the last year and it worked fine. The shop had only single phase power available, like my shop. But in the old location the machine ran on solid state phase converter and in my shop it runs on a rotary converter. The line voltage at the old location was 235 volts. In my shop it is 245 volts. The machine is made to run on 230 volts. My phase converter is well balanced and the manufactured leg measures 245 volts. The problem is one three phase fractional hp motor that powers a chip conveyor. The power supply wires for this motor are wound through some sort of current sensing transformer. Thes are regular 14 gauge stranded insulated wires that appear to be hand wound through the current sensing transformer. So I'm thinking that with the higher voltage running through the wires the voltage output from the transformer is a little high and it causes the machine to alarm out on the chip conveyor. I disconnected the output from the current xmfr and the machine now does not throw an alarm, but this also removes the protection. I'm thinking that if I take a couple turns out of the xmfr then the voltage output would be lower and I can then still use the xmfr to protect the motor. Will that work? Thanks, Eric

Reply to
etpm
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I'm over my head, it seems your saying all three phases run through the current sensing transformer. Didn't know that worked. I think we need to know how many turns of the 14 Gauge stranded wire are wound through the current sensing transformer? Per phase? From that someone can decide how many turns to remove. Mikek

Reply to
amdx

I was able to push the wires aside enough and using a mirror and flashlight I was able to read on the device that it is an Omron SET-3A current converter. Looking at the data sheet at omron.com I see that the SET-3A device connects to another device and the two together operate as a unit. The sheet also says that the setup won't work with waveform distortion. I'm thinking now that maybe my rotary phase converter must have too much waveform distortion and that's the real reason for the problem. Anybody have any thoughts about a device that I can use to replace the current converter? Maybe just a circuit breaker? Afterall, the motor just runs the chip conveyor, which just pulls all the metal shavings (chips, AKA swarf) out of the machine. And the motor is exposed so I can feel it to see if it's too hot. I can see if the magic smoke is coming out too, but I don't want to wait that long. Thanks, Eric.

Reply to
etpm

The alarm is going off because the motor is drawing more current than the system designer thought reasonable??? You should be able to measure that and compare to the motor specs???

Changing the measurement system won't fix that, it'll just make it easier for you to ignore the fault condition.

Who knows how the system works...you haven't disclosed enough info to tell...but...if it's a current transformer, the trip point is related to the number of turns. If there are three turns on the primary, and you remove one, you move the trip point up 50%...maybe... depending on how they configured the secondary and sense circuit.

Have you considered the possibility that there really is a fault? Maybe something got bent in transport and is causing extra load on the motor?

You don't say how you're measuring the voltage and "balance". If the harmonic content differs between phases, the current sense mechanism may be affected by that?

Random modification of alarm trip points based on recommendations you got from an anonymous "expert" here on the interweb is risky.

If you have employees, you might want to consult with your attorney about the benefits of defeating safety alarms.

Reply to
mike

So, what you're saying is, it has a current sensing system to detect jam ups ? In which case, there should be some point of calibration you can set.

Are you sure you don't have a Max Current pot some where on the drive board? If it even has a real drive! Maybe It don't ?

Also, I guess this could of been some make shift sensing system, too.

I guessing that maybe there is a 3 phase rectifier in there somewhere converting this to DC for a reference. In that area, there should be a way to calibrate this. Maybe adding a R across this rectified circuit to load it down.

Jamie

Reply to
Jamie

Get a 1:1 3 phase transformer for it. pass the power through that and then supply the electronics with that. The transformer will correct that issue, if that is really the problem.

Jamie

Reply to
Jamie

Greetings Jamie, The current converter does indeed connect to a device that controls the contactors for the motor and I can adjust it. I identified the device last night. I'm hoping that adjusting the thing for a higher current will work. I did put am amp clamp on the leads to the motor and it is drawing less than the nameplate current. If adjusting the device doesn't work then I'm thinking that putting a circuit breaker with heaters in it in line with the motor to protect it while bypassing the existing wiring. This should protect the motor but will not turn on the alarm lamp. Eric

Reply to
etpm

It might be good idea to get the supply voltage down to the rated nameplat= e voltage for the machine. That in turn should reduce the current into the = chip conveyor motor and possibly clear up your problem. 3 simple 15V 'bucki= ng' transformers on the input to the machine should do nicely to reduce the= supply voltage to the rated value. Neil S.

Reply to
nesesu

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