[WTA] vVI, VFD? CSI? all can use for PLC?

[crying_baby]

hi sry if im asking a dumbo question but i wanna know if whats the difference in the three drives above i mean practically, is tehre a preference over to use VFD for ac motor control over VVI , CSI etc?


ya i know, the detail of how it operates but is there such preferences over such issue

thank u in advance if som1 could enlighten me on this. thanks!!

 

[Tom Jenkins]

VVI is a Variable Voltage Inverter

CSI is Current Source Inverter

I'm not familiar with VVI, but CSI is an older type of unit and isn't often used in new applications. Most new inverters are PWM - Pulse Width Modulated.

All three are just types of VFDs - Variable Frequency Drives. Unless you have a very special application you will find that a PWM drive from any major manufacturer will be adequate and most cost effective for your needs. Since the PLC is simply providing start/stop and speed commands the type of VFD won't affect the PLC programming or control.

 

[Leadfoot]

The primary difference is how the output is switched to create the variable voltage and frequency.

VVI, variable voltage inverter, and CSI, current source inverter, are older methods. CSI must have a motor connected for it to operate. VVIs can run with out a motor connected. These drives basically have a DC drive front end to convert the AC to a variable voltage DC. This is part of how the output voltage is controlled. PWM drives have DIODE RECTIFIER front ends and have a FIXED DC bus voltage. The output voltage is controlled by the width of the pulses, AKA PULSE WIDTH MODULATION.

PWM is todays prefered method. You can do SSSSOOOO much more with PWM.

VVI is also refered to as SIX STEP. Of course all VFDs us a SIX STEP method of commutation. Presently I do not know of anybody using VVI on new equipment. Many VVI machines are still out there and working just fine. Most are repairable but the cost to repair one typically costs more than a new PWM drive.

 

[DickDV]

In practical terms, VVI and CSI are old technology using slow output switching devices to convert DC back to AC.

Because of their slow switching, the best these drives could do is simulate a sine wave by producing a large step positive, then another large step positive on top of the first step, then stepping down to the first step and then going to zero. The negative part of the sine wave was simulated the same way only with the polarity reversed.

This crude sine wave caused the motor to be very noisy at low speeds and also limited the motor's ability to rotate smoothly and develop its proper torque. In fact, at very low speeds you could actually see the shaft cog from one position to the next in time with the six voltage steps.

When these drives were state-of-the-art they were used on fans and pumps quite successfully because there was no need for low speed performance but they were not usually suitable for industrial machine drives.

PWM has changed all that with drive/motor systems now able to produce smoooth rotation and full rated torque at speeds at or near zero. Using anything else today would be a mistake.

Looking back, there are only two things that come to mind that were better in the VVI/CSI days. You rarely had EMI/RFI problems with the motor leads and motor insulation was not stressed as much on inverter power.

 

[Leadfoot]

This crude sine wave.......

This is definately an UNDERSTATEMENT.


I like showing customers the true waveform on the motor leads and watching them step back in dis-belief.

 

[crying_baby]

thanks thom, thanks leadfoot and dickdv, i reli appreciate it. but i haf came across that, VVI is used to control Multiple motors applications and VFD are used in open loop motor control aint this apply too.

 

[DickDV]

PWM drives are suitable for multiple motor applications and general drive applications open or closed loop depending how your system is arranged.

 

[crying_baby]

aaa i get it dickdv, thanks for all teh advice !! i'll google for more information! haf a nice day!!