Author Topic: Variable Frequency Drive?  (Read 1691 times)

Offline nabzim

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Variable Frequency Drive?
« on: July 15, 2017, 09:11:09 PM »
Hi all,
After building a half-bridge sstc (and after reading a little bit about VFDs), I feel like I could potentially build the same circuit (half bridge) with a few modifications, to use as a VFD to power AC induction-motors with full speed control.

Any feedback/suggestions of what modifications the circuit would need? ( I'm fairly certain that I don't need to run off of a feedback-loop like the sstc)

Offline Hydron

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Re: Variable Frequency Drive?
« Reply #1 on: July 16, 2017, 12:36:42 AM »
Gate drive would not be possible via transformers without more complex circuitry - VFD will not be 50% duty cycle like a SSTC.

Other than that the concept is reasonably similar - both are hard switching half bridges (3 of them in the case of a VFD).

Offline Mads Barnkob

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Re: Variable Frequency Drive?
« Reply #2 on: July 16, 2017, 09:32:26 AM »
You are spot on in your discovery that power electronics / inverters / power conversion bridges are often identical in many applications as Tesla coil, induction stove, audio amplifier, variable frequency drives, power supplies, etc etc.

As Hydron says the biggest difference is how you drive the bridge, which controller, timing, phasing of outputs etc that you have. For a 3 phase induction motor you would need to time the phases 120 degrees from each other and have a variable frequency and PWM control table to stay within a certain V/Hz ratio.

As illustrated here and with their description for a single phase output:


source: http://www.vfds.com/blog/what-is-a-vfd

Quote
The blue sine-wave is shown for comparison purposes only. The drive does not generate this sine wave.

Notice that the output from the VFD is a “rectangular” wave form. VFD’s do not produce a sinusoidal output. This rectangular waveform would not be a good choice for a general purpose distribution system, but is perfectly adequate for a motor.

If we want to reduce the motor frequency to 30 Hz, then we simply switch the inverter output transistors more slowly. But, if we reduce the frequency to 30Hz, then we must also reduce the voltage to 240V in order to maintain the V/Hz ratio (see the VFD Motor Theory presentation for more on this). How are we going to reduce the voltage if the only voltage we have is 650VDC?

This is called Pulse Width Modulation or PWM. Imagine that we could control the pressure in a water line by turning the valve on and off at a high rate of speed. While this would not be practical for plumbing systems, it works very well for VFD’s. Notice that during the first half cycle, the voltage is ON half the time and OFF half the time. Thus, the average voltage is half of 480V or 240V. By pulsing the output, we can achieve any average voltage on the output of the VFD.
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Offline nabzim

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Re: Variable Frequency Drive?
« Reply #3 on: July 19, 2017, 07:39:35 PM »
Gate drive would not be possible via transformers without more complex circuitry - VFD will not be 50% duty cycle like a SSTC.

Other than that the concept is reasonably similar - both are hard switching half bridges (3 of them in the case of a VFD).
Any idea what kind of more-complex circuitry? I would have guessed maybe just a different ic chip for an interrupter, but in this case it would be attached to the enable-pin of UCCs, so technically I guess you could call it an "enabler" instead, haha.
Also I'm only really interested in driving a single phase motor.

As for Mads' comment, the thing that I don't know how to do is create a waveform like those second two, where there's multiple positive pulses then multiple negative pulses. Is there a special IC chip that performs this function? That is where I am lost
« Last Edit: July 19, 2017, 07:42:37 PM by nabzim »

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Re: Variable Frequency Drive?
« Reply #3 on: July 19, 2017, 07:39:35 PM »

 


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