Buck Boost transformers on VFDs
- Thread starter LMAO
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- Placerville, CA, USA
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- Retired PV System Designer
Yes, V line is greater than V bus only near peak. At issue was whether it is always correct to say that is the only time the diode will conduct.
V diode is not always V line minus V bus when there are other elements in the circuit.
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And that what seems to be getting ignored or maybe just not understood.
or irrelevant to the point
statement: conducts at V line peak > V bus
response: "incorrect"
in fact, correct
disagree
the statement was conduction occurs at peak, V line > V bus
this is true, not "incorrect"
this accurate for the vast majority of commercial vfd's
My posts seem to be "going away", being disappeared. I wonder.......
Anyway, we were making commercial variable speed drives from the early seventies, usually for industrial applications. Had they not been commercially viable we wouldn't have been commercially viable. Three phase converters had a 120 degree conduction angle. That was the basis for the semiconductor thermal calcs were done to size them, the heat sinks, thermal switches, enclosure ventilation, and cooling medium* flow requirements.
*Mostly it was forced air but on some of the bigger units we had de-ionised water and on the higher voltage units (up to 11kV) we used silicone transformer fluid.
Horses for courses.............
If you can manage with only inductors and no capacitors in DC link, on output filter etc in your converter, then 120 degree conduction angle is possible imo.My posts seem to be "going away", being disappeared. I wonder.......
Anyway, we were making commercial variable speed drives from the early seventies, usually for industrial applications. Had they not been commercially viable we wouldn't have been commercially viable. Three phase converters had a 120 degree conduction angle. That was the basis for the semiconductor thermal calcs were done to size them, the heat sinks, thermal switches, enclosure ventilation, and cooling medium* flow requirements.
*Mostly it was forced air but on some of the bigger units we had de-ionised water and on the higher voltage units (up to 11kV) we used silicone transformer fluid.
Horses for courses.............
It isn't just possible. It works. With both.
Any diagram?
winnie
Senior Member
- Location
- Springfield, MA, USA
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- Electric motor research
Given a conventional diode rectifier (no gates, no control circuits...) and a capacitor filter, the diodes only conduct when forward biased, and thus current only flows when the supply AC exceeds the capacitor voltage. In small systems with big filter capacitors the diodes very clearly only conduct at the 'peak' of the AC cycle.
Assuming a fixed supply, rectifier, and load, the conduction angle of the rectifier will change with the size of the capacitor; as the capacitor value increases the conduction angle decreases.
This also goes to the point that Jraef made about impedance in the supply to the rectifier. Impedance reduces the current flow once the supply voltage forward biases the diodes, thus increasing conduction angle.
I have no doubt that the drives Besoeker describes were designed to have 120 degree conduction angles, through proper selection of supply impedance, capacitor size, and second order characteristics (such as capacitor esr).
Besoeker: in the drives you describe, with input rectifier conduction angle of 120 degrees, I presume this is at full load. I would expect the conduction angle to decrease at partial load, as the DC capacitor retains voltage exceeding portions of the AC supply
-Jon
Assuming a fixed supply, rectifier, and load, the conduction angle of the rectifier will change with the size of the capacitor; as the capacitor value increases the conduction angle decreases.
This also goes to the point that Jraef made about impedance in the supply to the rectifier. Impedance reduces the current flow once the supply voltage forward biases the diodes, thus increasing conduction angle.
I have no doubt that the drives Besoeker describes were designed to have 120 degree conduction angles, through proper selection of supply impedance, capacitor size, and second order characteristics (such as capacitor esr).
Besoeker: in the drives you describe, with input rectifier conduction angle of 120 degrees, I presume this is at full load. I would expect the conduction angle to decrease at partial load, as the DC capacitor retains voltage exceeding portions of the AC supply
-Jon
Unfortunately I had a computer failure and lost a lot of drawings. But it is fairly simple. The inductor is in series with the DC and the capacitor is across it.
Never mind. Make a free hand sketch to explain the working of your converter if you are pleased.
Thank you for your polite and considered response.
Mods, bear with me for wandering a bit off topic.
The chokes depended on the application but usually down to about 30% load. Harmonics were a concern even in the early days. This is something you know when you are designing power electronic circuits. Discontinuous current is not your friend and we had to provide no negative tolerance on harmonics (and efficiencies). Getting the arithmetic wrong was a big no no. Financially.
A somewhat related tale from a few decades back. We built the VSDs for a pumping station in St Albans*. Seven in total and the biggest two were just 226kW. For harmonic mitigation, we made them 12-pulse. For contract purposes, we had to do a before and after harmonic survey. Much to my surprise, there was no measurable difference and I did the tests over a 24-hour period. What slowly dawned on me was the location. The station was in a residential area but fed by its own 11kV/400V transformer. Thus, the point of common coupling was at the 11kV supply which is where I had to do the measurements. And they exceeded the G5/3 requirements that were the UK standard at that time.
Colour me stupid but what slowly dawned on me was the location. Residential. My background, as I have mentioned, is power electronics design, manufacture, install, and commission. Power electronics was the dirty beast when it came to harmonic pollution.
But, but.....
Even back then there was a lot of non-linear residential kit. And associated harmonics. Nothing big, just a lot of it.
*St Albans is a pretty town with a majestic abbey and dates from Roman times.
Is there any confusion between unfiltered three phase converter in which conduction angle 120 degree and filtered three phase converter in which conduction angle less?
I've had dealings with/designed different sorts of variable speed drive systems. Most, if not all, have some sort of filtering even if that is the motor itself.
Electric-Light
Senior Member
550v is +15% from 480v. It's not a huge stretch and possibly something drive manufacturer maybe able to approve with a possible warranty restrictions depending. Capacitor life is significantly reduced when you get close to the rated voltage so 550v occasionally is different from running close to it around the clock.
Small is relative to the capacity of the power source. The kind of load you describe is very much frowned upon on small systems when the Ipk is disproportionately high relative to transformer current capacity. Something as small as 3/4 hp can cause problems with home generators. Elevators are also known to case problems with emergency generators despite having great margin kW capacity wise. Older electronic ballasts and desktop PCs for commercial offices often used a large line choke that weighed as much as the rest of the power supply to smooth out the rising edge.
This is now done with a high frequency power converter and you will find it in electronic ballasts and power supplies that that need to maximize circuit capacity. This technology allows light weight welding inverter to tap close to 1.8kW from a standard outlet. without it, it would need a 240v circuit.
Small is relative to the capacity of the power source. The kind of load you describe is very much frowned upon on small systems when the Ipk is disproportionately high relative to transformer current capacity. Something as small as 3/4 hp can cause problems with home generators. Elevators are also known to case problems with emergency generators despite having great margin kW capacity wise. Older electronic ballasts and desktop PCs for commercial offices often used a large line choke that weighed as much as the rest of the power supply to smooth out the rising edge.
This is now done with a high frequency power converter and you will find it in electronic ballasts and power supplies that that need to maximize circuit capacity. This technology allows light weight welding inverter to tap close to 1.8kW from a standard outlet. without it, it would need a 240v circuit.
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