- Thread starter Beanwood
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I wonder how the electronics compare to the single to three phase 240v boxes.
Indeed - but just to be absolutely certain I have tried the motor with my other inverter - and it still works smoothly.
Inverter was bought through Amazon back in January - so I'm following the steps they're advising - taking the top off, checking for damp or metal filings - it still has the error - so now it's back with them for next steps. It MAY come to nothing, but I need to ask...
Smouser
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I would like to learn a little bit more about the workings of VFDs too.
The cheap Chinese VFDs are always much lighter than branded VFDs. They must be cutting some 'non essential' components out.
I wonder if the 0.75kW VFD I bought have got the wrong sticker on it and that it might be a larger capacity drive. The components seem too big to be only for a 0.75kW rating.
I powered the drive up and the motor power can be set up to 4kW in the parameters. It might just be a generic control board not 'limited' to the drive's actuall capabilities. Who knows?
Those capacitors are larger than my ABB 2.2kw drives.
Agroshield
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Have a sniff around their website and see if physical case dimensions (and maybe weight) for any particular power rating agree with your idea:
1670 up to 2.2kW is same form factor: http://www.sakohz.com/Products/ski6700752.html
larger than 2.2kW, bigger box: http://www.sakohz.com/Products/ski670475k.html
Pete.
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That might be how they get the 415v output. I read about it somewhere, re-configuring the capacitor connections and the rectifier I think it was. Can't remember where mind...
I confess I have little idea how VFD's work.
I can see they are now low-cost & very popular for running 3ph kit off 1ph supply (instead of much bigger, more expensive Phase Converters).... but I've never used or even seen a VFD (let alone wired one up).
I'm interested in them though!
(I'm thinking the frequency is ramped-up to soft-start and provide a variable speed by a VFD - but how they utilise 1ph to run 3ph motors
I can see they are now low-cost & very popular for running 3ph kit off 1ph supply (instead of much bigger, more expensive Phase Converters).... but I've never used or even seen a VFD (let alone wired one up).
I'm interested in them though!
(I'm thinking the frequency is ramped-up to soft-start and provide a variable speed by a VFD - but how they utilise 1ph to run 3ph motors
Agroshield
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This is how I understand how they work.
The incoming AC voltage is first rectified to DC. If the VFD is single phase input, it uses a single phase rectifier, if three phase input, a three phase rectifier. That will give a DC voltage of sqrt2 (1.414) times the input voltage. The capacitors are used to smooth that voltage. Everything so far is roughly the same as a plug in DC power supply, just a lot heavier duty.
If you want to go from 230v single phase input to 415v three phase output, the voltage must be increased. What I am not sure of is how this is done or whether this is done pre- or post-rectification. It was Hopeful Dave on this forum who explained it somewhere.
As a result of the rectification, you have high power, high voltage DC available to you. This is switched very precisely and very fast by things called integrated gate bipolar transistors (IGBT). The IGBTs are controlled by a microprocessor. Everything in this stage is very similar to an inverter that you would buy for a caravan to make your 12v car battery power a 230v TV, except the caravan inverter has a much smaller feature set (is a lot less clever).
The AC sine wave produced is not continuous, but is a series of steps. However, it is stepped so fast that the motor does not care. This is a bit like the difference between an analogue record and a digital CD. For the microprocessor, it is as easy to produce three sine waves 120 degrees out of phase (three phase) from the DC as it is for the caravan inverter to produce a single one. If you wanted six phase, you could have it*.
The reason VFDs have come down in price is that high power IGBT technology has improved and can now be manufactured cheaply and robustly. On early VFDs, the IGBTs were expensive and delicate components. This is how one of the (now somewhat outdated) rules of VFD was born - on an old device, if you switched the load off while the IGBTs were sending juice down the wires, they would blow up.
Similarly, on the control (microprocessor) side, things have come on in leaps and bounds. Way more processing power for a fraction of the cost, so all the bells and whistles - soft start, overload protection, rpm display, electronic braking are now standard. Compare what you would expect as standard in a Nokia 3310 to what you expect as standard in an iphone.
This site has a lot of interesting stuff - http://www.vfds.org/insulated-gate-bipolar-transistor-887962.html and look at some of the related links in the sidebar.
* OBGT reference:
Pete.
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I confess I have little idea how VFD's work.
I can see they are now low-cost & very popular for running 3ph kit off 1ph supply (instead of much bigger, more expensive Phase Converters).... but I've never used or even seen a VFD (let alone wired one up).
I'm interested in them though!
(I'm thinking the frequency is ramped-up to soft-start and provide a variable speed by a VFD - but how they utilise 1ph to run 3ph motors
Think of them as a battery-powered signal generator.
They don't actually convert single phase into 3 phase. What they do is take single phase AC and rectify it into DC just like a car battery charger except at higher voltage, and store it temporarily in those large capacitors. Then they electronically generate three AC waveform outputs from that stored energy (called the DC bus) which mimic 3 phase AC well enough so you can run a motor from them. The benefit being that because you're generating the waveform you get to choose the parameters like frequency and voltage per frequency etc which makes them very versatile. You also get braking options which is when the VFD puts a load across the motor to slow it down and the energy generated also gets pumped back into the DC bus capacitors so that it's ready to use for re-powering the motor.
A nice understandable explanation
Great stuff! So it seems a great deal like the gubbins in an inverter-welder, or indeed a lot of other inverter kit (rectifying & creating digital multi-step sinewaves, but with extra bells & whistles).
I've not yet ever bought any 3ph kit myself, but a mate has a few v old Wadkin cast-iron things like spindle moulder & planer/thicknesser (which he's run with varying degrees of success off a basic static Transwave PH Converter).
I'm wondering if a VFD might the way to go for him (& that a VFD might add some much-needed added safety features like braking).
(Apologies to OP for the small hijack there - I got carried away!)
Seadog
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@Hopefuldave mentions it often, Pete.
Smouser
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All very interesting stuff.
The questions I have are:
1 . Why do some VFDs only have 2 large capacitors and others 4?
The question applies to single-phase 220v to three-phase 220v VFDs and single-phase 220v to 380v three-phase VFDs.
I have a Nflixin (also a cheap Chinese drive) that converts single-phase 220v to three-phase 380v rated at 2.2kW and that has only got 2 capacitors.
I have not opened this one up this morning to take a photo but you can see through the casing that there are only 2 capacitors.
2. Is there a way of determining a VFD's power output?
Let's say you don't have the model number or data plate and need to figure out what size motor the VFD can run.
As I have alluded to in a previous post, the 0.75kW Sako drive's components seem way too large to only be a 0.75kW drive. Even compared to branded VFDs.
3. Can you run a larger kW-rated motor on a lower-rated VFD?
Example 3kW rated motor and a 2.2kW rated VFD.
I know it is not recommended and usually, the recommendation is to ever so slightly oversize the VFD for the motor.
The way I understand it, the VFD will produce 2.2kW worth of power to the motor so the motor will obviously not make full power. However, for one of my lathes, it will be really rare to turn a massive piece of steel where I require the full 3 kW of the motor.
The reason I ask is, I have got a nice branded 2.2kW ABB VFD with loads of features and protection compared to a Chinese drive. I would prefer to run the lathe from the ABB but due to cost if I need a 3kW drive it will have to be a Chinese drive.
In theory, it should not damage the VFD as the VFD will only produce what it can produce (the VFD will probably run at 100% capacity the whole time though).
The questions I have are:
1 . Why do some VFDs only have 2 large capacitors and others 4?
The question applies to single-phase 220v to three-phase 220v VFDs and single-phase 220v to 380v three-phase VFDs.
I have a Nflixin (also a cheap Chinese drive) that converts single-phase 220v to three-phase 380v rated at 2.2kW and that has only got 2 capacitors.
I have not opened this one up this morning to take a photo but you can see through the casing that there are only 2 capacitors.
2. Is there a way of determining a VFD's power output?
Let's say you don't have the model number or data plate and need to figure out what size motor the VFD can run.
As I have alluded to in a previous post, the 0.75kW Sako drive's components seem way too large to only be a 0.75kW drive. Even compared to branded VFDs.
3. Can you run a larger kW-rated motor on a lower-rated VFD?
Example 3kW rated motor and a 2.2kW rated VFD.
I know it is not recommended and usually, the recommendation is to ever so slightly oversize the VFD for the motor.
The way I understand it, the VFD will produce 2.2kW worth of power to the motor so the motor will obviously not make full power. However, for one of my lathes, it will be really rare to turn a massive piece of steel where I require the full 3 kW of the motor.
The reason I ask is, I have got a nice branded 2.2kW ABB VFD with loads of features and protection compared to a Chinese drive. I would prefer to run the lathe from the ABB but due to cost if I need a 3kW drive it will have to be a Chinese drive.
In theory, it should not damage the VFD as the VFD will only produce what it can produce (the VFD will probably run at 100% capacity the whole time though).
Pete.
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My Clarkson grinder has (I think) a 0.3kw motor driven by a 0.25kw Omron VFD. The only thing that bothers it is if I turn the speed knob up too quickly, it cuts out with a fault. For a lathe with it's greater turning mass you might have to keep the accel and decel times conservative.
If your VFD has a display for running amps you'll see that it rarely gets anywhere near motor rating.
If your VFD has a display for running amps you'll see that it rarely gets anywhere near motor rating.
Little update on teh original post.
After consulting with the manufacturer (Through the supplier) they think.....It's faulty.
Messaged Amazon, and within 2 minutes have a return label, and I will have a full refund on the way once they have received it. Now I just need to wait for the money to come back in, so I can find another one to buy....
After consulting with the manufacturer (Through the supplier) they think.....It's faulty.
Messaged Amazon, and within 2 minutes have a return label, and I will have a full refund on the way once they have received it. Now I just need to wait for the money to come back in, so I can find another one to buy....
Smouser
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I have tested the Sako VFD I bought this morning.
All seem to work perfectly fine.
The motor I used for testing has got an unknown power rating, the data plate has been lost.
However, I believe it is either a 0.55 kW motor or 0.75 kW motor judging by various images from the internet.
I have got some other SEW motors too rated at 1.5kW but they are larger so the motor I have used for testing has definitely got a lower rating than 1.5 kW.
Source 1
Source 2
Not very technical but it is the best I can do with an unknown motor rating.
The below video is the Sako VFD running the motor wired in STAR for 380V. I took the fan cowling off so that I can loosen the brake as it did not disengage when trying to run the motor from the VFD.
Ps. @Beanwood sorry I have completely derailed your thread
All seem to work perfectly fine.
The motor I used for testing has got an unknown power rating, the data plate has been lost.
However, I believe it is either a 0.55 kW motor or 0.75 kW motor judging by various images from the internet.
I have got some other SEW motors too rated at 1.5kW but they are larger so the motor I have used for testing has definitely got a lower rating than 1.5 kW.
Source 1
Source 2
Not very technical but it is the best I can do with an unknown motor rating.
The below video is the Sako VFD running the motor wired in STAR for 380V. I took the fan cowling off so that I can loosen the brake as it did not disengage when trying to run the motor from the VFD.
Ps. @Beanwood sorry I have completely derailed your thread
Ps. @Beanwood sorry I have completely derailed your thread
Absolutely shocking behaviour - who'd have thought a thread on here could EVER go off on a tangent!!
(You're more than welcome)
I'm actually interested in results with your VFD - as I need a replacement...
Agroshield
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1. Sometimes it is packaging space. Sometimes economics of supply. Sometimes as a result of reliability analysis. To make a comparison you would have to look at the values on the capacitors and how they are wired (capacitors in parallel add - the opposite relationship to resistors).
2. Same size components can have different ratings. Many MOSFETs are in the same circuit board package but switching ability varies considerably. On the output side, there might be more IGBTs on a higher rated device. The manufacturer will want to rationalise part count. It is easy to solder different strength or a different multiple of components to a universal circuit board, something not easily detected unless you are an electronics whizz.
3. As Pete. says, most VFDs will display running amps. Some have the ability to connect an external meter. The VFD has enough inbuilt smarts to protect itself and trip if you try to overload it. If you are using the machine for profit, you will lose money if you keep having to reset it after it trips. If you are using it in a hobby environment and once a year get too greedy with your cut, it is no big deal.
In some sense you can determine the answer to 2 by doing what you suggest in 3. Connect the VFD up to a bigger motor, load the motor up in a known and repeatable way (that's what a dynamometer does) and monitor the amp draw. If the VFD can do 4 amps at 100% duty cycle, but not 5A at 100% duty cycle, you have found its capacity. It might be prudent, for peace of mind, to point an IR thermometer at the VFD while doing these tests.
Hopefuldave
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I suspect they do it exactly the way I have:
400v class often have two banks of capacitors wired in series because...
Big electrolytics that can take 600v at high ripple currents are *a lot* more expensive than ones that can take 400v.
By putting them in series the capacitors (or banks of two+) are each only seeing 330v or so, the voltage is split across them (like wiring two 6v bulbs in series to run off 12v) - there are resistors to help keep the voltage equal as well as bleed off the capacitors at power-off.
with 3-phase 400v input, the capacitors (or banks) in series are charged up as one with rectified 400v (about 600v peak) to give 600v bus voltage across them - this then goes off to be chopped up to make a simulated sine wave. This is how they deliver the 600v peak to power a 400v RMS (Root Mean Square) motor - this is easiest to think of as the "area between zero and the sine wave", averaged out the 600v peaks would be a continuous 400v (hope that makes sense).
With 240v input (single or wimpy American 3-phase) the capacitors can only charge to around 330v, so the inverter can't deliver the 600v peaks - so motors have to be wired for 240v (delta) instead of 400v (Star)
For 240 input 400v output, instead of charging the two capacitors (or banks) as one block, each is charged by the rectified 240v separately, one for positive 240v (+330v peak) the other for the negative half of the AC current, also giving 330v peak, but -330v.
The inverter's tapped across the positive and negative ends, giving more than the 600v or so required to drive a 400v motor.
To do this it's quite easy: take the incoming neutral to the point between the capacitors (or banks), the rectifier's + to +v, - to -v, giving 330v on each.
BUT: the capacitors are only being charged half the time on + or - AC waveform, so they have higher charging and discharge currents - either needing to be higher capacitance or reducing their lifespan, and in both cases the current through the rectifier increases. This is why they need to be de-rated, to allow the components leading up to the actuall chopping circuits to survive
My VFD was modded by finding the common point between capacitor banks and fitting a terminal to take mains neutral, feeding live to the rectifier - this is a "voltage doubler", it gives a less well-regulated "raw DC" on the capacitors, so I run a 5HP VFD into a 3HP motor - and it works.
I'm pretty sure (99.9% certain) this is how the commercial 240-415 VFDs do it - if they're thorough they'll fit larger capacitors and a higher-current rectifier, but not so much better that they can justify some of the prices, I doubt the Chinese makers of the reasonably-priced ones milk it as much as some in the US and UK...
Dave H. (the other one)
