Variable Frequency Drive (VFD) and Motor Operation

When did the mafunction start? Once this question is answered you can start tracking down causes. Was the PLC logic modified about the time the malfunction started, was a new employee hired that works with the equipment,...ect.

With so little information about the controls of the table just about anything is possible. What is the control signal to start the table turning? What signal tells it to stop? Are you using limit switches?

The problem can be with APFC . Improve your observation with the time period correlating your problem with Main voltage , current , frequency and power factor ( Incoming and outgoing at substation ) and percentage of transformer loading.

In designing APFC some manufacturers provide a portion of capacitors fixed and APFC connected for switching capacitors. This is done to ensure that during no load condition of the transformer the system will be highly inductive and the CT cannot sense as there is no load current . The fixed portion is decided depending on trial at site on different load conditions. So please make a through check of the power system and give your observation. You may be able to solve your problem by doing the corrections in respect of all possible variation during the time period your machine misbehaves .

Dear sir,

This problem is started when we started m/c after one months near about 15 days before. We have not changed any logic or timer of PLC prog.There are 8 stations which operates on this table with there many sensors. After completion of cycle of every station , Table rotates for next operation. For STOP command there is one sensor on gear box.

Voltage, frequency is OK when problem occurs. Our load is fluctuating so there is frequent switching of cap. banks.

Strange thing is only that; why this problem occurs in night only? Is there possibilities of spikes,impurities in supply?

VFD are notorious for inducing harmonics (1st, 5th, 7th,11th) throughout power systems they are connected to. These harmonics can be reflected/injected into the PLC I/O and seen as valid signals that can "trigger" TTL gates and cause equipment to operate erratically. Also, bad shielding on signal cables, poor/inadequate equipment grounds, and improper placing of signal cables adjacent to high current/high voltage conductors will also cause noise induction. Attach a recording oscilliscope to your I/O conductors and monitor the signals for noise. Any noise spikes with a magnitude of 75% to 85% of your cpu and/or I/O voltage rating will take your system to "no-man's-land" (threshold voltage) and cause uncontrollable reactions.

Friend,

Don´t limit your trouble-shooting to the APFC and plant-wide systems only. You can have a "bug" in the PLC program that didn´t surface until recently. This could be because of some change in number of operators or other signals, dates, or other factors. I have occasionally found bugs in programs I have written. (One was found when I was doing some changes for a system installed 12-months earlier, it had not caused any mal-functions because the particular combination of inputs and data conditions had not occurred yet. I fixed it, of course.)

It may be that workers on that shift are doing something slightly differently than workers on other shifts. It may be that the program is doing some time-of-day functions. It may be that any connections to a host computer system are acting differently because of other host computerr activities during the stated hours.

Other possibilities can be in the physical wiring of the system, with some damaged wire´s insulation touching another terminal or ground due to a different arrangement of equipment or personnel during the hours you mention.

Troubleshooting can be very difficult, but it can also be quite satisfying when you isolate and fix the cause. Don´t limit yourself to the obvious possibilities.

--JMM

i think it is due to harmonics generated by the APFC contractors ,if possible you can select to manual mode and see the VFD operation.

Put a UPS in line with your ac supply to the PLC. Then you will know which way to look to solve your problem.

If Rpm of motor is controlled by analog voltage given to VFD then check the analog Voltage during the specific period. This analog voltage is controlled usually by PLC.

Also note down the basic parameters of the VFD like motor frequency, voltage, acc time and dec time etc and then autotune the VFD once and enter the parameters which you have noted down.

During switch off of VFD, The drive would attain the maximum frequency so high speed is attend.

Vijay G.R

(Have you though someone on that shift might be winding you up, covering sensors up etc.)

Sounds like an I/O problem eg. sensor, encoder etc.

What enviroment are you in? is it wet or dusty?

Can you monitor your logic and enable commands?