Variable Frequency Drive or VVVF

[adel_48]

modulation index vvvf

Hi,

Does any one has any resources for VVVF circuits that are used in elevaors

Thanks in advance

 

[IanP]

hitachi vvvf

You can get a lot of info on Variable Speed Drives from sites such as Hitachi, PDL Electronics, Siemens etc.
I don't think you would like to built one, do you?

Generally, these devices can operate in several modes: hoist (elevators), process control (pressure, speed...). This is selected by software means.
Depending on the load you select size of VSR and because they are quite complicated and relativel not expencive I would just buy the right one.... 8)

 

[adel_48]

vvvf control basics

Thank you very much, however I want to build one. That is why I really want the circuits of these drives.

Or at least does anybody know any circuit or even the methods to convert DC into sinewave with variable amplitude and variable frequency.

 

[IanP]

how to build a vvvf

What I can tell on this subject is this:
Type of modulation: PWM, usually between 5 and 10 kHz.
In the beginning of each cycle the pulses are narrow, the more you approch the peak of "sinewave" the pulses are wider and again their width starst to decrease and is 0. The more often you repeat this cycle the higher the output frequency, and most of the VSR will have the range from 0Hz to 150Hz.
Remember, this is not "sinewave", it series of pulses, but the avreage current and voltage (because of PWM) will have that shape.
Very difficult task is to "silence" these devices.
I always use additional input and output filters and always shielded cables.
The most expencive components are the power transistor blocks; in some instances when they were damaged replacement was almost identical with the cost of new device.
So I wish you good luck and if I come across a diagram I will let you know.

 

[usernam]

Well like IanP said you can use PWM. One method is sinusoid PWM(SPWM). I'm attaching a figure of it as applied to a 3-phase DC-AC inverter.
Basically in SPWM a sine wave of frequency f say, is compared with a sawtooth wave. When the magnitude of the sine is greater than that of the sawtooth the upper switch in the inverter is turned on and Vao is +ve.(Vao is basically the voltage of the bus corresponding to phase A with respect to a point O. The potential of O doesn't really matter since we are basically interested in Vab the line voltage and Van the phase voltage.) When the sine is less than the sawtooth, Vao is -ve. Now over one cycle of the sine wave Vao varies in a manner such that it's fundamental harmonic is a sine wave. This fundamental has a frequency equal to f. The magnitude of the fundamental is proportional to m the modulation index given by Vs/Vt. Where Vs is the sine wave peak amplitude and Vt is the sawtooth peak amplitude. So by varying these two parameters you get VVVF.

These are the basics of sinusoid PWM. SPWM is not very popular however since it is not too easy to implement. Usually something called space vector PWM is used. but thats another story.

Hope this helps. Ask if you need anymore help.