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dooola2020
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Join Date: Feb 2014
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PWM in VSD

02/18/2014 2:36 AM

hello , this forum is really very good i can't stop reading ......

we have received new Toshiba VSD after reading of the manual the point of PWM i couldn't understand

This VSD is 18 PWM with phase shifting transformer .. i couldn't get the point of using phase shift transformer except harmonic reduction ... Am I right ???

i do case study with bridge rectifier and notify result of disconnect one phase and two phases at

one phase dc voltage induced reduced by 1/3

and effect of two phase disconnection no induced voltage .... How???

i connected two o/ps of two rectifiers in series without phase shifting found that very high current passing through .... Why??

Please I need ur advise to understand the concept of it

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PWSlack
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#1

Re: pwm in vsd

02/18/2014 5:30 AM

Did the manual arrive with it? How did the phone call with Toshiba's Technical Helpline go? Why is there a need to mess about with it other than simply hooking it up in line with the manual and the advice received on the telephone bearing in mind that this may affect the manufacturer's warranty?

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Fredski
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#2
In reply to #1

Re: pwm in vsd

02/18/2014 8:17 AM

read the manual???? why would that step be taken?
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PWSlack
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#4
In reply to #2

Re: pwm in vsd

02/18/2014 9:07 AM

It's quite underwhelming when this teeny-weeny and so vital step gets ignored in the quest to tear open the packaging and play with the newly-arrived toy. So much stuff gets returned in the weeks following the holiday seasons that retailers really have to be careful about customers who try and return things that are dud through their own making.

Caveat vendor.

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lyn
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#3

Re: PWM in VSD

02/18/2014 9:03 AM

Hook it up, according to the manual, and see if you can make it work.
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JRaef
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#5

Re: PWM in VSD

02/18/2014 11:35 AM
  • This VSD is 18 PWM with phase shifting transformer .. i couldn't get the point of using phase shift transformer except harmonic reduction ... Am I right ???

Correct. Even order harmonics cancel each other out, and in 3 phase systems, odd order harmonics that are a multiple of 3 (triplen) cancel each other out. So by using 3 phase shifting transformers and 3 diode bridges (called an "18 Pulse" front end), more of the odd order harmonics that are non-triplen get combined into multiples of 2 or 3 and are significantly reduced.

  • i do case study with bridge rectifier and notify result of disconnect one phase and two phases at one phase dc voltage induced reduced by 1/3 and effect of two phase disconnection no induced voltage .... How???

No idea what you are talking about here or how it relates to an 18 pulse front end. If you feed a VFD with single phase power, you CANNOT use an 18 pulse front end, so what happens on the DC side if you do is a moot point. But in general when you remove one phase of a 3 phase H bridge rectifier, the peak voltage remains the same, but the ripple increases significantly, so you need almost twice as much bus capacitance to smooth it. Current however will increase through the bridge by the square root of 3, so if the diodes were sized for a full bridge operation, they will become over loaded. So for a single phase supply to a 3 phase drive, you must de-rate the output by 1/2 in order to handle the extra current and extra bus ripple.

If you disconnect 2 phases out of 3, how is current going to flow in the bridge? That's why there is no output.

  • i connected two o/ps of two rectifiers in series without phase shifting found that very high current passing through .... Why??

I have no idea how you would accomplish this in a VFD, you would have needed to disassemble it to do that. Why would you do such a thing? What were you trying to accomplish? But as to "why"? If you connected them in "series" to each other, what was on the combined output? Current is determined by load, not supply.

Step away from the drive if you need to use it, you are likely damaging it with your experimentation. If it's only purpose was this sort of experimentation, then OK, but understand that once the magic smoke comes out, it is only good for dissection.

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dooola2020
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#6
In reply to #5

Re: PWM in VSD

02/19/2014 7:53 AM

Thanx JRaef for ur concern

Only papers delivered was schematic diagram and controller programming

This is the link for schematic diagram

http://www.mediafire.com/download/s1fouo3awf7cewu/518+to+815+KVA+SWD+dwg.pdf

If you look at the drawing

Page7: there are two o/p one come from phase shifting trans and the other came from customer power supply (My 1st question about important of phase shift trans plus harmonic reduction)

This case study which i made was about page8 to see effect of phase removal and effect of phase shift trans

This is the experiment by measuring o/p by AVO meter to get RMS value

dc voltage o/p in case of 3ph = 540vdc

dc voltage o/p in case of 2ph = 360vdc

and i connect dc1- to dc2+ to get series connection and once i put power on it very high current pass this connection even there is no load applied even i reduce input voltage feeding bridge rectifier

The reason i made this to see what effect of connecting two o/ps in series and why he connect them in parallel here ((page 8))

*Step away from the drive if you need to use it, you are likely damaging it with your experimentation. If it's only purpose was this sort of experimentation, then OK, but understand that once the magic smoke comes out, it is only good for dissection.

Thx for ur advise but all experiments i have done is far from my drive I just trying to understand what i have to do in case of troubleshooting
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PWSlack
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#7
In reply to #6

Re: PWM in VSD

02/19/2014 8:41 AM

The answer to that one is to pick up the phone and give the supplier a bo!!ocking for not sending through the installation and commissioning manual with the equipment. Don't pay the invoice until it arrives.

Why is there a need to play around with this thing, BTW? Usually, they just get installed and just work. The sorts of voltages described look pretty painful/fatal in the wrong hands...

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