VFD and cable 3

[buzzp]

We have a long run of cable (2000') from the vfd to the motor that will be directly buried. I realize there can be large 'transients' at the motor end due to the distance traveled. I know some motors are classified as VFD duty, does the same apply to the cable feeding the motor which the vfd is running? Are there any special requirements for this cable as far as a UL category or others? I suppose if the amplitude of the transient is known then the cable could be selected that way. These are 50HP 460VAC pump motors. Any help would be appreciated.

 

[Skogsgurra]

buzzp,

I think that motor reactors are necessary. Chose reactors with 2 or 3 percent voltage drop and make sure that the cable capacitance does not resonate with the reactors (it will make the cores quite hot). One way of avoiding ringing is to connect low ohm resistors parallel to the reactors. try 22 ohms 200 W first and see if that is enough. The criterion is that the output voltage from the reactors is free from ringing.

The reactors serve three purposes: 1 they reduce the du/dt of the PWM signal so that the travelling wave is suppressed. 2 they reduce the risk of inverter tripping due to overcurrent when charging the cable. 3 they reduce the risk of ozone production in the cable itself - ozone will reduce cable life significantly.

Have a look at the ABB tehchnical guides, and Baldor. They say a lot about these things.

One thought: 2000 feet is equal to some 600 metres in my world. That is an awfully long cable. Make sure that you do not lose too much voltage due to RxI. May need special consideration.

 

[buzzp]

Thanks for the info. I had not considered reactors yet. I will definately have to look at installing some since they would definately help with the motor life. Do you know of any requirements (as far as standards like UL, IEC) that address VFD cable?

I used to install oil field equipment with drives. Some of these wells were 10000' deep. The drives had reactors built into the bottom of the them. I do remember the cable was a flat cable. However, I believe it was flat to reduce the chance of smashed cable (between casing and motor) more than anything else. I can not think of any other advantages to this flat cable.

I will also check the vendors sites you suggested. Thanks again. Have to give you a star for reminding me about reactors.

 

[JatTiw]

Hi Buzzp,
I am quite sure with the cable length for your application you will have the problem of cable charging current due to harmonices (result of PWM) in the system. A suggestion , Think about installing some active filter system

 

[buzzp]

A reactor would definately be in order. Keep in mind this not a project I have been involved with but I should have thought about reactors from my rookie days in the oil field.
The added benefit is there should not be any special cable requirements outside of the norm. Thanks for the replies.

 

[Allphase]

http://bwcecom.belden.com/college/PDF/VFD_Flyer.pdf

http://www.nema.org/index_nema.cfm/1427/0DDF742A-8790-4A21-88D139CFC069648B/

Application Guide for AC Adjustable Speed Drive Systems

 

[dadfap]

buzzp

the reactors in the bottom of the drive may well have been part of the DC link filter circuit not anything to do with the application mentioned above.

Your comment on the flat cable is correct, it is electrically a pain in the *, but does allow additional clearance in the restrictions of a well bore.

All the best

dadfap

 

[Skogsgurra]

buzzp,

You may experience high losses in the resistors I mentioned in my first answer. With these cable lengths, you should use the lowest possible switching frequency you can get from the inverter.

Most inverters can be set to run with 1,0 or 1,5 kHz, but are often used with 2,5 or 4 kHz. Going from 4,0 kHz to 1 kHz will reduce the heat a lot. A 50 HP motor will not have any problem running with 1 kHz switching and if this is a "down the hole" application, the whining sound will certainly not be heard by human ears.

 

[dpc]

I seem to remember a previous thread discussing the merits of a special cable with equally spaced ground conductors for this application. Maybe someone remembers this or a search would turn something up.

Any chance you could get the AFDs closer to the motors?

dpc

 

[buzzp]

I am not sure of the range of carrier frequencies of this particular drive. I did not know they were up to 4kHz. Last time I knew (about 4 years ago)they were up to about 2Khz. Guess things change fast. I have passed along the information to the responsible party. They were already ahead of me and ordered a reactor with a filter.

Dadfap, I believe you are correct that the inductors in the bottom of the drive are for the DC link. I will have to dig out my old spec sheets on these drives and see exactly what they were for out of curiosity. Like I said, I worked their fresh out of college so some stuff did not stick. The drives I installed were not the PWM type but PAM. Still, I think the same issues arise but not with the frequency of the PWM type. I installed Centrilift drives. However, this application is for a PWM drive. Thanks for the help.

 

[Skogsgurra]

dpc,

Belden has some special cables for VFDs, but they are more for reducing EMI and - to some extent - the residual voltage on the motor frame. These cables have a reduced PE conductor (earth or shield, if you like) impedance. Belden also claims that their VFD cables do not get destroyed by ozone that is created by partial discharge inside the cable (due to capacitive HF current in the voids between the cores).

I do not think that they will help much in this application, though.

Sorry folks. Have to rush now. There are some bottles to be uncorked. I wish you all a Happy New Year!

 

[jbartos]

Suggestion: Visit

http://www.transcoil.com/#klc

or equivalent,
for:
As the technology of Pulse Width Modulated (PWM) Drives are incorporated into various applications and processes, the increased energy savings and decreased maintenance on Drives can be offset by increases in Motor failures. The dV/dTGuard product family has been designed as an engineered solution for motor failures due to the reflected wave phenomenon. Designed to be installed at the output terminals of the Drive, dV/dTGuard Output Filters provide effortless installation, convenient accessibility and enhanced motor performance and durability. Available in a UL/CUL version or in our original version....

 

[dccap]

Can Some one tell how to size the output choke for reducing the dV/dT and reflected wave in VFD operated motor?

 

[jbartos]

Suggestion: Visit

http://www.transcoil.com/#klc

(click on KLC Brochure)
for:
Considerations when Specifying and Selecting KLC/KLCUL Filters
KLC and KLCUL filters may be used in single Drive and Motor applications as well as a single Drive controlling
multiple motors. In each case, there are some application guidelines that need to be followed in order to insure
the optimum performance of your KLC or KLCUL Filter. Filters are sized based on the full load amps of the
motor or motors.