Cheap VFD

[Sonic EscapeParticipant @sonicescape38234]

I’m looking for a VFD for a 370W power feed motor from my milling machine. I’m thinking to buy this Delta VFD. I know that Delta PSU are fine so maybe their VFDs are also ok.

But how about the cheap VFDs like this one:

https://eur.vevor.com/variable-frequency-drives-c_10745/vevor-vfd-2-2kw-10a-3hp-variable-frequency-drive-for-3-phase-motor-speed-control-p_010905384151

They are available from many vendors and cost half the price of Delta. Anybody has any idea how good they are?

[Sonic Escape]

On 26 June 2024 at 08:59 Sonic Escape Said:…

They are available from many vendors and cost half the price of Delta. Anybody has any idea how good they are?

What do you mean by ‘good’ Sonic?

Electronics generally follow “Moore’s Law”, resulting in progressively more oomph at lower cost over time.  Thus an VFD made in 1970 cost as much as an expensive new car, came with a monstrously complicated manual, and required installers to do a few weeks training.  They were only found in industrial settings.   Half a century later the hardware and software are out of copyright and much improved, plus there is a huge market for VFDs.  It’s become relatively unusual to install a 3-phase motor without one, even when a 3-phase supply is available, because a VFD confers much better start/stop and running efficiencies compared with direct connection.

So prices have dropped, and particularly at the inexpensive end, many of the set-up complications have been hidden – they come pre-configured with sensible defaults.  BUT, VFDs aren’t plug and play consumer items.   The customer is expected to understand that they are components, needing to be wired appropriately, probably protected by an enclosure, and maybe filtered.    We’re at the tricky end of home-electrics, particularly if the machine has more than one motor, or any complications like star/delta, or complex control circuitry.

All that means there are a number of risks buying cheap:

• The item might be too cheap, or not meet local safety requirements.  (Not all imported electronics are well-made!!!)
• Faulty items sourced from abroad may be impossible to replace or get a refund
• No technical support.   (Apart from this forum!!!)
• Often sold by box-shifters, (like Vevor) who know nothing about the products they sell.

Many prefer to de-risk purchasing VFDs by buying them locally from a specialist.  UK consumer protection is strong, making it much less likely that dissatisfied customers are left out of pocket, whilst a local specialist is much more likely to provide support.   Some provide a kit with instructions, which is very helpful compared with receiving a bare box and Chinese manual.

There isn’t a simple answer to the question.  Anyone hoping to be told that there is no risk whatever in buying the cheapest possible alternative is in for a disappointment.   Truth is buying cheap is always a gamble,  In the case of a VFD, the outcome in part depends on the skills of the purchaser:  someone with a background in electronics and 3-phase power is well placed to sort out minor issues that might completely baffle a newcomer.     Of course newcomers can learn and might sort themselves out eventually, but if the goal is to fit a VFD that just works, probably quicker to flash the cash and get one that comes with support from a reliable source.   Alternatively, this being a hobby, many of us enjoy a challenge, in which case spending a week getting a poorly supported VFD to work will be enormous fun.

Dave

 

 

[KenLParticipant @kenl]

They work absolutely fine, I have four VFDs, three are recognised brands and one is a generic Chinese item identical to the one being sold in your link. The only difference I’ve noticed is the physical size of the generic one, they’re much larger than most of the recognised brands. The included instructions tend to be basic but good enough for most people and lots of info on the internet so not a problem.

[Robert Atkinson 2Participant @robertatkinson2]

I agree 100% with the comments made by Dave (SOD).
Of note Vevor no longer list ANY mains powered items on their UK website. Apparently this is because Customs and Trading Standards were seizing them all for being unsafe. Draw your own conclusions about the quality of Vevor products.

B.T.W you linked to much bigger VFDs than needed for a 370W motor.
Are you fitting a VFD for speed control or because of no 3 phase mains supply?

Robert.

[Robert Atkinson 2]

On Robert Atkinson 2 Said:

I agree 100% with the comments made by Dave (SOD).
Of note Vevor no longer list ANY mains powered items on their UK website. Apparently this is because Customs and Trading Standards were seizing them all for being unsafe. Draw your own conclusions about the quality of Vevor products.

B.T.W you linked to much bigger VFDs than needed for a 370W motor.
Are you fitting a VFD for speed control or because of no 3 phase mains supply?

Robert.

I need a VFD because I don’t have a 3 phase mains supply. And because I want to bypass the existing wiring for reversing directions and other stuff. I picked a bigger VFD because who knows in the future what I could use it for.

I’m not concerned to much about electrical safety. I don’t plan to use any I/O, just the existing buttons. I’m more worried about the quality of protections. For example in the Vevor VFD seems to be just one varistor comparing to 5-6 in more expensive models. But on the other hand, probably the VFD will run just 1-2 hours a month on average.

 

[Harry WilkesParticipant @harrywilkes58467]

Sonic you wrote an I quote ‘I’m not concerned to much about electrical safety’ is saving a few £’s worth you life !

H

[Harry Wilkes]

On Harry Wilkes Said:

Sonic you wrote an I quote ‘I’m not concerned to much about electrical safety’ is saving a few £’s worth you life !

H

Of course not 🙂 What I wanted to say is that I don’t see how that Vevor VFD could be an electrical hazard if I use only the existing keyboard and knob.

[Harry WilkesParticipant @harrywilkes58467]

Well Customs and Trading Standards were seizing them all for being unsafe then doesn’t this raise alarm bells maybe other brands for the Far East are better who knows until you get it and it lets out the blue smoke 🙂

H

[Dave HalfordParticipant @davehalford22513]

It may be the unit dumps excessive earth leakage, we don’t know, except we can’t buy them from a UK based supplier. I don’t understand why for lightly loaded units you have to over-rate the VFR. Maybe there’s a use for all the components they don’t fit.

[duncan webster 1Participant @duncanwebster1]

I’ve bought Chinese mains powered stuff twice, once direct from China, once from a UK supplier. Both were very suspect, the first I reckon was dangerous, the second had a moulded on plug which didn’t have a fuse incorporated as required by the BS. I reported the second to trading standards, but they never got back to me as to whether they had taken any action. I cut off the plug and fitted a proper one, but the whole thing expired shortly afterwards.

I know that a lot more of the stuff I buy probably comes from China, but if it’s imported by a reputable UK company who know what they are doing it’s more likely to be safe.

Mains electricity kills, you need to be concerned about it.

[Robert Atkinson 2Participant @robertatkinson2]

Something else to note is that the controls on the front of the unit are not intended for normal operational use. They are only intended for commisioning and testing. Additionally you can’t access them when the VFD is in the required enclosure.
It’s not just basic electrical safety. Interference is also an issue both emitted and received. Emissions can affect other people. Reception (susceptability) can cause un-commanded movement sudden stopping or failure to stop. This can cause injury.

Robert.

[RobinParticipant @robin]

I fitted 2 to my milling machine. The advert skipped the small print disclaimer, “Kindly NOTE: it doesn’t support external braking resistors“, but even so, excellent value for money and that new white plastic case looks great.

Robin 🙂

[David JuppParticipant @davidjupp51506]

Always seems to be the same here with VFD queries, people ask for advice but take absolutely no notice of those with technical expertise in the field.

[Sonic EscapeParticipant @sonicescape38234]

You made me curious about the other VFD I already have. It came with the lathe. It is a 4kW model, and one of the cheapest you can find:

I decided to have a look inside. Unlike the Vevor 1.5kW model, this one has 6 discrete IGBTs 25A/1200V (NGTG25N120) bolted on the heat sink together with a large square rectifying bridge.

I have no experience with VFDs but I wonder how this thing can boost the voltage to 380V without any coil? The 4 electrolytic capacitors are rated to 400V so there is no voltage doubler there. Anyway …

This is the main board. You can open in a new tab to see it in higher resolution. The active component are painted with a transparent lacquer. But not the PCB or resistors. This is strange. The large transistor on the top left must be the breaking switch, 3N150  2.5A/1500V N-MOSFET.

All small electrolytic capacitors are Rubycon. Not bad. That black relay is shunting the two 5W green resistors that limit the inrush current during capacitor charging. This control is not perfect. I know from experience that I can blow the fuse by turning off and the on the VFD supply.

The top 3 red film capacitors are connected between ground and the middle of each bridge leg. The bottom one is across the DC bridge input. IGBT drivers are TLP5701 photocouplers.

There are also two AMC1200 isolation amplifiers. This are expensive components. These are close to two shunts. I wonder why there are two. There is also a temperature sensor bolted on the heath sink.

All control and analog measurement part is on the small display board:

The CPU is an Sinowealth SH32F205, Cortex-M3 at 120MHz. A bunch of optocouplers, a relay output and many LF353 op amp.

The VFD has no filters on input or output. I don’t know how common is this. The Delta model has filters at both input and output. The guy who sold me the lathe told me that some capacitive touch lights in his home would sometimes turn on depending on the depth of cut 🙂 What I noticed is that DRO is sometimes incrementing without reason due to interferences. So I don’t trust it anymore.

That is all about this quick afternoon VFD dissection. I should have a comparable VFD from a more respectable OEM to draw a conclusion. Or maybe somebody can spot something interesting here.

[Robert Atkinson 2]

On Robert Atkinson 2 Said:

Something else to note is that the controls on the front of the unit are not intended for normal operational use. They are only intended for commisioning and testing. Additionally you can’t access them when the VFD is in the required enclosure.
It’s not just basic electrical safety. Interference is also an issue both emitted and received. Emissions can affect other people. Reception (susceptability) can cause un-commanded movement sudden stopping or failure to stop. This can cause injury.

Robert.

This is a good remark because I have a very noisy VFD that will work in the same time with the one I want to buy. They might start a fight 🙂 I need to add some filters anyway for the one I already have.

[Andrew JohnstonParticipant @andrewjohnston13878]

A few comments:

I doubt the VFD is putting out 380VAC phase to phase. A voltage of 220VAC times the square root of 3 gives ~380VAC, but you cannot get 380VAC phase to phase from rectifed 220VAC. I suspect this is specmanship.

You don’t need a transformer to generate the ~700VDC bus needed to generate a true 380VAC phase to phase supply. Either a seperate boost converter, or a modification of the input power factor corrector will do the job. But neither of these are likely to be found in a budget VFD.

In theory the bottom side IGBTs don’t need opto isolation. The top side IGBTs do need an isolated drive and an isolated supply on the gate side. The isolated drive needs a high dv/dt rejection ratio.

The two AMC1200 devices are measuring two phase currents. In theory, in a balanced 3-phase system, the third phase current can be calculated from the vector sum of the other two phases, the sum of all three being zero. The devices don’t seem that expensive, $1.45 in low volume.

I don’t know what a breaking switch is? The transistor/heatsink seems too small to be acting as a braking resistor.

It is common for VFDs not to have input or output filters. For a start it saves cost. Output filters are only needed in special cases. Mains input filters are normally provided seperately as part of the wider system design.

Andrew

[Robert Atkinson 2Participant @robertatkinson2]

Sonic said “The 4 electrolytic capacitors are rated to 400V so there is no voltage doubler there.”
Not necessarily. A standard rectifier  voltage doubler such as used on a universal 120/240 volt input power supply uses two capacitors. They are connected in series when doubling so each capacitor can be rated at less than the output voltagecapacitor can be rated at less than the output voltage.

The only problem with this is that it has a poor power factor and generates lots of harmonic currents. 🙁
An active circuit with an inductor and solidstate switch can boost th voltage, improve the power factor and reduce harmonics. This is oftem called a PFC (power factor correction) circuit. To meet regulatory standards any power supply of VFD over 500W needs some kind of power factor correction.

Robert.
Edit crossed with Andrew….

 

[Sonic EscapeParticipant @sonicescape38234]

I think if you rectify with 4 diodes AC you get a DC voltage multiplied with square root of 2. Anyway there is no coil inside. Only that small transformer for auxiliary DC voltages. This is a big specmanship I would say 🙂

Breaking switch I mean the transistor that connects the external breaking resistor. Or this is what I suppose.

Indeed you don’t need isolation for the bottom transistors. But maybe the cost saving was insignificant. I had a look on the external filters. If you consider adding two for both input and output, like the Delta model has, the VFD is no longer that cheap. At least the models I found are quite expensive.

[Robert Atkinson 2]

On Robert Atkinson 2 Said:

Sonic said “The 4 electrolytic capacitors are rated to 400V so there is no voltage doubler there.”
Not necessarily. A standard rectifier  voltage doubler such as used on a universal 120/240 volt input power supply uses two capacitors. They are connected in series when doubling so each capacitor can be rated at less than the output voltagecapacitor can be rated at less than the output voltage.

The only problem with this is that it has a poor power factor and generates lots of harmonic currents. 🙁
An active circuit with an inductor and solidstate switch can boost th voltage, improve the power factor and reduce harmonics. This is oftem called a PFC (power factor correction) circuit. To meet regulatory standards any power supply of VFD over 500W needs some kind of power factor correction.

Robert.
Edit crossed with Andrew….

 

I had a better look and actually they used a voltage doubler exactly like in your picture! That square 4 pin component is not a rectifying bridge but two high current Schottky diodes (0.4V drop each). Anode and cathode are connected together, see the A mark. And this point is connected to an AC input. Then on B mark, 3 thick wires go to the capacitors PCB. Two wires start from the other anode and cathode of the two diodes. And the third wire is the mid point between the two pair of series capacitors connected to the other AC input. This VFD is poison for the grid 🙂

[Sonic Escape]

On Sonic Escape Said:

On Harry Wilkes Said:

Sonic you wrote an I quote ‘I’m not concerned to much about electrical safety’ is saving a few £’s worth you life !

H

Of course not 🙂 What I wanted to say is that I don’t see how that Vevor VFD could be an electrical hazard if I use only the existing keyboard and knob.

Always good to ask the question ‘what could possibly go wrong?’  But before doing so, it’s important to assess the skills and talent of the questioner.   Here we are discussing Variable Frequency Drives, a family of electronic products that by fair means or foul, produce a simulation of the 3-phase power needed by many machine tools.   Advanced stuff, taking us into grown-up Electrical Engineering, and grown-up Electronic Engineering.

Forum members are a mixed bag, varying from those who know nothing about about electrics, up to those with advanced experience with avionic electronics.  Many are practical men, with a history of successful small-scale bodging, who tend to be dismissive of H&S or anything else that ‘gets in the way’.  Others have professional backgrounds, implementing large scale technical solutions required to be reliable and safe,  where folk are held responsible for their actions, and the penalties are high.

So if, in his innocence, Joe Public risks installing a substandard VFD in his shed, and burns it down, electrocutes himself, wipes out his neighbours telly, or simply finds the thing fails after a year or two, then so what?   This guy can carry on regardless, probably considering himself smart because his gamble came off.  Not so the professional: a hospital probably contains several hundred VFDs running 24×7, some of them driving important machines like MRI scanners, where downtime costs millions and risks lives.  Replacing all the VFDs with el-cheapo would initially be a cost-saving, but because several hundred are in play, reliability would soon be an issue.    Not a good idea to let man-in-shed buy VFDs for hospitals, because man-in-shed’s experience is grossly irrelevant.

Professionals don’t gamble, they’re trained to take calculated risks, though it has to be admitted they make mistakes too.   But done properly, there is an enormous difference between a gamble and a calculated risk.  The second takes a much broader view, including a historic analysis of of failure modes.   Worrying about the max speed he can run a plain bearing,  man-in-shed spins it up, and is happy if it doesn’t obviously overheat.  Unfortunately man-in-shed’s method is rotten to the core.    In comparison, the professional has access to a couple of centuries worth of bearing research, where millions of failures were reported and investigated.    He knows that over-speeding a plain bearing takes years off it’s life, and has graphs showing how many hours damage is done.   Man-in-shed assumes all is well because nothing obvious happened, but if he keeps doing it there will be trouble.

Bearings and electronics follow similar rules.   There are many ways an electronic engineer can arrange components on a board to convert single-phase into 3-phase.   As usual, the best way is usually the most expensive, so the circuit is likely to be full of cost compromises.   Keep cost down is generally a good thing, but it’s skilled work.   Unskilful or excessive cost-cutting is bad, and this characteristic of ‘too cheap’ products.   Hard to know if something is OK or not without having an expert do a careful analysis of the build, which is why electronic products are plastered with CE and other Certification marks, another system ‘Joe Public’ doesn’t understand.

Another problem is understanding who one is buying from, and whether or not a brand-name is worth anything!   Vevor are a ‘box shifter’, that is a company who source and sell ‘stuff’ to customers, possibly putting their brand on it.   What they have on offer covers the full range of commercial products, good, bad and ugly.  They’re not a manufacturer and don’t have any particular knowledge of what they’re selling.   Don’t expect any support from a box shifter:  if a product is unsatisfactory, the business model is to replace it or refund the cost.  The seller can’t explain the manual, or walk the customer through an installation.    This is a particular problem with a VFD because these are components, not consumer products, meaning the customer may require skills.

What could go wrong?   The item is a fire and shock hazard and both are greatly exacerbated if the unit is wired incorrectly.   The latter is a distinct possibility if the installer is unskilled:

• VFD outputs must be connected directly to the motor terminals, not switched in any way, such as by an existing contactor or safety interlock.   If a phase allowed to float, due to mis-connection, or switching,  the motor windings are liable to act like a spark coil, generating many thousands of volts easily capable of zapping the motor, electronics, and human!    Only safe if connected correctly, so the installer had better be careful.
• Machines with more than one motor require special consideration.   Most VFDs are only designed to run one dedicated motor, not a mixture of single-phase coolant pumps, machine lamps, and 3-phase motors.  Nor should one VFD be expected to drive two or more machines at different times.   Exactly how a given VFD will behave given a complicated mixed load is in the lap of the gods.
• Need to understand filtering, including knowing that the installer not having the equipment needed to detect EMC does not mean the installation is clean.  Much depends on what is nearby.  And filtering can have ‘interesting’ side-effects on earth trips, causing problems that might baffle a domestic sparky!
• Need to understand Star and Delta, changing motor connections as necessary
• And potentially more…

Plenty of forum members I’m sure capable of installing VFDs successfully, but please don’t tell everyone it’s easy after doing a few straightforward installations.   Think of the guy who struggles to wire a 3-pin plug, who has innocently bought one of those complicated mills with a multi-speed pancake motor and a weird mix of single-phase accessories and smaller 3-phase motors all hung off a proprietary controller, with no circuit diagram…

Dave

 

[Sonic Escape]

On Sonic Escape Said:

On Robert Atkinson 2 Said:

Sonic said “The 4 electrolytic capacitors are rated to 400V so there is no voltage doubler there.”
Not necessarily. A standard rectifier  voltage doubler such as used on a universal 120/240 volt input power supply uses two capacitors.

I had a better look and actually they used a voltage doubler exactly like in your picture!

Looks like I was wrong, the unit does output a 380VAC phase to phase 3-phase system. I hope there are suitable bleed resistors in parallel across the electrolytic capacitors.

I understand what the breaking switch is now, makes sense that it only needs a small heatsink.

Andrew

[John MCParticipant @johnmc39344]

I agree with Dave (SoD), we need to be careful talking about VFD installations, that is to say, not giving the impression that its easy.  The instructions for VFD’s originating from the far east leave a lot to be desired.  My last VFD installation, far eastern device, was complicated by mistakes in the instructions.  Luckily I spotted them, this forum was tremendously useful for sorting this out.

For someone not able to to wire a VFD, what do they do?   Easy enough to find a sparky to do work on domestic wiring but who do you go to for VFD’s?

I have an ulterior motive for asking that question.  A friend has just bought a Colchester lathe with a 3 phase motor.  He’s well aware of the benefit a VFD would be but will admit to struggling to fit a 3 pin plug.   He has asked around but cannot find anyone who will do the installation. Any ideas?

 

[Robert Atkinson 2Participant @robertatkinson2]

Andrew,

I woud not say you were wrong, just that you would never consider using such a circuit on a 4kW drive….

Robert.

[Robert Atkinson 2Participant @robertatkinson2]

John MC, where is your friend located?

Robert.

[Sonic EscapeParticipant @sonicescape38234]

I put the VFD up for sale. I don’t like it anymore after seeing what’s inside.

I’m going to buy two Delta electronics VFDs. A 2.2kW model and a smaller one of 0.75kW. They have EMI filters. I’m going to share them between a lathe, a mill and a vertical band saw. I’ll add a 3-phase socket to each VFD and connect what I want to use in that moment. The nice thing about this models is this remote control. I can easily move it to each machine when needed.

 

[Author]

Posts