Danfoss VLT Variable Frequency Drive Exposed



A dysfunctional tour of an dysfunctional Danfoss variable frequency drive!

Feel free to post constructive comments on what parts are what or how it works, none of this “you don’t know what your talking about” crap pls 😉

One of many I’ve had as scrap, this is a mid-size one. some are so big and heavy they come on their own separate pallet!.




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  1. James Jones

    @elect0033,
    Send a list of questions you have, I access to many experts (myself included) in the field of LV and MV VFDs as well as DC Drives and switchgears. The big tech stuff now is harmonics control with filters (active and passive) or phase shift transformers (6 pulse and 18 pulse). The variable and constant torque as well as vector drive is big in today's industry. Let me know what info you need.

  2. Jorge Barrera

    I work at Danfoss, technical support, the "hot line". I think thats a VLT 6000, a HVAC, these family is obsolete. But I can tell you, Danfoss is the only one that adds an extra filter in charge of regulating the harmonics in the electrical system. Thats why it can get so freaking expensive, cause they worth it. And of course if you buy one fo these (ofc not anyone can buy one of these) you have the safety that youre buying quality and service satisfaction.

  3. Aussie50

    @MaNiAcBALL G'day!, yes these are a great drive!, it is a 6000 series.
    I've seen external power filtering options with these too but never picked one up.

  4. Aussie50

    @Nspect I'm not really sure, just mixed terminology I think. I used to call them that too but as you say, a VSD is like what I use on the Milling machines at work, 2 cone shaped pulleys that open and close to vary the drive ratio.

  5. COMPUTER 1

    THANKS ! I learned more about the internals of a VFD than my entire 3 1/2 year career as an industrial processing equipment mechanic taught me – even going to classes at company expense. I would love to have those diodes and hook them up to a 555 circuit with a 0-5 ohm pot to use as a reverse style DC motor controller to run a DC motor. Would that work ?

  6. Aussie50

    @mrnewagemotor I'd never dealt with a VFD before I did this vid so you might want to see some of my more recent ones for more info.

    the 'diodes' are actually IGBT bricks, but in theory you could control the output pulse width to drive the motor.
    I'm not sure about building a DC drive from scratch tho.

  7. bakupcpu

    Those 3 big thing are probably dual mosfet or transistor the 3 phase rectifier should be around thought. And yes they send a semi sine wave to the motor those 3 big round thing are RF choke to prevent high frequency to make false reading on the amp mesuring device. Hope i brought some help.

  8. Aussie50

    @bakupcpu Thanks for the tips, IO have learned a lot from making this vid.
    the 3 bricks are Insaulated Gate Bi-Polar Transistors and the white thing I held up towards the end is the rectifier that feeds them

  9. olestetter1

    @bakupcpu The 3 big round things are coils for protection for the IGBT output stage. They limit a sudden currentflow. The RFI coils and capacitor are situated on the dc bus side( the board with the big white capacitors).

  10. kevvywevvywoo

    The chokes are there for radio emissions compliance, they arent necessary for the actual function of the drive. The little white critter is the 3-phase input rectifier. There is a sticker on the front that says 'decanter feed pump'. They use those in Sewage Treatment Plants but also Fruit Juice factories, it's part of a centrifugal seperator. Either way i hope u washed your hands before eating your sandwiches!

  11. Aussie50

    @kevvywevvywoo yeha I've learned a lot about these drives over the last 12 months!

    lol yeh some are from treatment plants, I always wash up after handling old junk regardless.

  12. tyreza79

    wowow; ok thats a master piece: you could covert it in to a very strong induction heater: which it very useful
    wow variable frequency ?
    how much is the frequency variabilty ?

  13. Aussie50

    From what I understand the load sensing is done by the current transformers that the output cables run through (blue or black rectangular devices with a hole in the center for the cable to run through) one of those or its associated components could be bad and telling the drive that the motor is pulling lots of amps when it isn't (I'm guessing your motor is fine?)

    what you unplugged is probably the board with the CT's on it. trace the wires back and see if any of the CT's are short

  14. William Squires

    The power blocks act as a 3-position switch; it can connect the output to gnd, +V, or Hi-Z (High impedance – essentially an open circuit). By controlling the input signal to the switch, it creates a "rotating" current between any two of the three phase outputs. So, if you pull "U" to ground, "V" to the positive supply, and "W" to Hi-Z, then switch "V" to ground, "U" to Hi-Z, and "W" to +V, and finally "W" to ground, "V" to Hi-Z, and "U" to +v, you turn on the windings in order; "U->V", then "V->W", then "W->U", and back to "U->V", you've created a "rotating" magnetic field in the motor.


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