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mike mike is offline
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Posts: 634
Default Current transformer help please

On 4/3/2012 9:43 AM, wrote:
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?

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 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.