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VFD problem with 485 Comm. Ports Filter Needed


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I am new to the Forum,


I am an Automation Tech, the works for an Oil Company in Texas.

We have ROD PUMP oil Wells with Baker 8800 PUMP Off Controllers on them.

They sense when their is no Fluid in the Well and Turn the Pumping Unit Off.

Most of the pumps run with 20-40 HP Electric Motors that are 3 phase.


Our Company Electrical Dept. started to put VFD on 480 vac single phase power locations converting

it to 3 phase output.

They were using static phase converters before.


One problem with the Electric Motors on an oil well rod pump well application is the load changes some.

The Pumping Units have to be balanced to try and balance the load.


The Problem we are starting to see is when the Pump off Control has the pumping unit running we

see trash on out 485 comm ports and the Pump off units reboots every few minutes.

( The POC are small RTU with CPU in them )

Everything is OK when the Unit is on but the Pumping Unit is not running.


The ? is what filter is needed to control the interference from the VFD?

Our Electrical Dept. is not using any special motor or wire to hookup the VFD to the Motor.

I don't know if the problem is harmonics or RF ?


If I need to find out more Info to answer the ? better let me know.




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Hello Leland


Welcome to the forum.

VFDs work by rapid switching of the output stages to create a PWM output voltage. This rapid switching generates a lot of noise that can cause interference problems if the installation is not undertaken in a manner to minimise the noise generated.


I have a paper that I can send you on installing VFDs to minimise interference. If you want a copy, send me a Private Message (click on the new messages link above right and create a new message addressed to marke) with your email address and I will forward you a pdf copy.


Basically, the noise is generated by the output stage and is radiated by the cables between the drive and the motor.


Connect the drive to the motor with a screened cable with the screen connected to the motor frame and the drive frame. Use either screened glands, or clamps to terminate the screen to the motor and drive.

Earth the motor to the supply via the drive rather than direct to the main switchboard. The drive is the supply for the motor, not the main switchboard. i.e. connect earth from motor to drive and then drive to switchboard. This will help reduce earthloops and radiation area.


Some drives are designed to minimise EMC (noise radiation) and others are not. The good ones have built in filtering and the bad opnes do not. If the drive does not include internal filtering, you will need to add an external filter on the input to the drive. - Follow the manufacturers recommendations for this.


Additional gain can be made by passing the output cable through ferrite rings. On small drives, you can use one large ring with multiple turns. Note, all three phases (but not earth) pass through the same ring.


Best regards,

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...and just in case you don't already know, as a fellow American:

Screened = shielded

Earthed = grounded



One of the most common problems I see here in the US is people using normal wire on the output of VFDs and putting it in conduit like the manual says, but they fail to notice that the last 3-6 feet of flex conduit going to the motor peckerhead is now completely non-metallic. Older sealtite used to have interlocked steel flex inside, the better stuff also had a copper layer running throughout that ensured a continuous ground. In the last 10 years or so however, Code began allowing the newer seal-tight flex that has NO metal in it and electricians don't stop to think of the ramifications for VFD circuits. You can still use it, but only if you use shielded motor cable inside. Belden and most of the other major wire mfrs now offer this cable here in the US.

"He's not dead, he's just pinin' for the fjords!"
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