"Digital Phase Converters" - Inverters for Multi-Motor Applications

I'm using the motor windings as a 1-1 autotransformer (the motor is connected to the Input L1-L & Star-N) to generate two additional phases of 240V on L2 & L3. (which would create a phase-phase of 415V, but not a phase-neutral of the same).

I think the mechanism that's now creating the 430V line-neutral voltages, is that the L1 winding has had insulation damage causing a partial short, effectively reducing the number of turns and creating a situation where windings L2 and L3 are acting as a 1.8x step up transformer; which also introduces a lot of instability into the system (of mutually stabilising magnetic-electrical fields) as a whole resulting in it immediately disintegrating under load.

Every "VFD" I've ever used has had an integrated logic controller, with the motor drive electronics not directly controllable without interfacing through that logic controller (nor would you want it to be, given the complexity of what's going on under there).

Indeed increasingly manufacturers are offering VFD's which also provide access to the unused capacity of an internal logic controller to provide general purpose I/O's to give additional control functionality akin to a PLC.

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This is in contrast to a soft-start drive or "choc-block" style frequency convertor (a la the aforementioned Eaton DB1) which is still a complex drive under the skin, but doesn't require an interface at all beyond momentary on and off buttons, or even a Delta-Star which is simple relay logic.

Having used VFD's extensively with rotating equipment (pumps and compressors), I really don't think that manual control by a pot (remote or otherwise) could be described as the design intent, even though it's entirely possible.

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I suppose this whole line of discussion is entirely semantic really, but I often find the semantics of how particular phenomena/components are described shape how I think about the overall system.

Edited By Jelly on 10/10/2022 11:28:39

Any expert opinions on the pros and cons of the different ways of creating 3-phase from single phase?

The ideal output would be three pure sine-waves of the same voltage and current rating timed 120° apart. Only one method I know achieves that, all the others are flawed, but I might be wrong!

• A Motor-generator set produces true 3-phase but is noisy and inefficient. They don't seem to be popular.
• A Static Converter uses capacitors to produces something close enough to 3-phase to start a motor, but once spinning the motor is mostly powered by 1 good phase with help from 2 phases not 120° apart or of equal voltage. Quiet, works reasonably well but not for everything. It's much more clever compromise than perfect!
• A Rotary Converter adds an idler motor to a static convert to improve the quality of the two iffy phases by using the idler as a kind of generator. Considerable improvement over a capacitor only converter, but 2 of the phases are are on the weak side, the idler is noisy, the motor costly, and spinning it wastes electricity. Still a clever compromise rather than perfect, but most 3-phase devices work with rotary converters
• Inverters that rectify AC in to DC and store considerably energy in capacitors from which 3 electronically switched outputs are pulsed 120° out of phase with each other with a form of pulse width modulation that creates what looks like a sine-waves to a motor, but are actually spiky. Many advantages including high efficiency, variable frequency (speed control), soft-start, torque control and other smart goodies. Disadvantages, affordable models only support one motor, they produce a lot of interference, and breaking connections to a running motor are liable to generate high-voltages capable of puncturing the motor's windings or the inverter's output stage. A variant uses the single-phase input to provide one of the 3-phases directly, and electronics to generate the other two. Not entirely clear myself how they differ from a VFD, but no speed control and the electronics seem more complicated due to the need to tie the two generated phases accurately to the real one. The Inverters aren't simple electronics; rather the switching is programmed to respond in fractions of a millisecond to the load as necessary to maintain phase and average volt/amps. They also detect overloads and other fault conditions, provide reverse, and other features. The size of the manual can be a major disadvantage!

My feeling is:

• Static Converters have a reasonable chance of working with one or more motors but there are no guarantees.
• Rotary Converters - more money - are likely to work with almost everything, but big and noisy.
• Inverters are efficient and provide high added-value. In our workshops speed-control is wonderful. Unfortunately but they're mostly one VFD per motor devices, which gets expensive when one owns several 3-phase machines, and Model Engineers are famously unwilling to spend money! Industry, who work motors more than we do and look at the bottom line over a few years have shifted to VFDs on a large scale. Most new 3-phase motor installations are powered by a VFD converting 3-phase to 3-phase because considerable savings are had compared with electro-mechanical motor control.

Grateful for corrections or additions to my understanding.

Dave

Two things:-

1. Inverters are generally more than happy driving multiple and variable loads with downstream switching. The output stages of a computer UPS, a solar/battery inverter or a modern welding set are almost identical in design to those in a motor drive VFD.
2. Having started my career at a manufacturer of large rotating electrical machines, I've often wondered if a rotary converter with a wound rotor (synchronous machine) wouldn't do a far better job than the common 'induction motor with capacitors' arrangement. It'd need an exciter and governor setup, but that's not difficult and it would have far better voltage and phase angle regulation.

Edited By Mark Rand on 10/10/2022 17:29:27

Many VFD and especially the cheap ones have a simple bridge rectifier and reservoir capacitor front end to derive the DC link voltage of approx 330V which is used by switched transistors to produce the 3 phase output. On a single phase supply current is drawn from the supply over a fairly short period at the peaks of the sine wave. When loaded the link voltage droops and there is high ripple current in the reservoir capacitors which can limit the maximum power available. This can be acceptable for a VFD starting a motor softly but the high very high load presented by a DOL motor start droops the DC link to the point where the VFD detects an under voltage condition. Typically a DOL motor start draws between 5 and 10 time the max load current but only for a short time as the motor accelerates from being stationary. If for any reason such as additional machine inertia, viscous drag from cold lubricant or mechanical load then the high load can last for several seconds. So for DOL use an VFD would need to be rated at 10x the motor max power rating. Even if the VFD has an active front end to reduce the amount of DC link droop the need to rate it for the DOL surge load would make it prohibitively expensive.

So the bottom line is that DOL starting is a challenging load for any supply and especially an active device such as a VFD or inverter

A passive static converter using and auto transformer and capacitors has better overload resilience and one with an idler motor even better at managing the DOL surge load but are less popular because of their size and noise.

There's no easy answer but the simplicity of DOL starting on a single phase supply leads to complexity elsewhere.

CS

^^^^^

That isn't the case from my experience.

Mark

Can you tell us what your experience has been?

CS

A "proper" inverter will run a motor just fine but may need to be oversized a bit to cope with starting loads. Such an inverter should have some start up surge capacity. They also have (or should have) output filtering. A VFD has little or no surge capacity becuse:
A. The load is not switched.
and
b. They have built in soft-start (speed / voltage ramp).

Some of the converters / inverters being sold are actually VFDs which have been de-rated and have the parmeters set to allow load switching. I'm guessing this is includes setting the overloads higher than normal.
They work but are not ideal and most of the ones I've seen advertised would require a lot of work to make a legal installation and a fair bit just to make them safe.

The other issue with using a VFD type "converter" is that the output is the electronic equivalent of a Delta transformer or generator. This means there is no neutral connection and in almost every case the load must be balanced across the phases. Adding a Delta-Star transformer will provide a neutral but does not solve the load balance issue.

There are lots of things that can be made to work but this does not mean that they are legal, safe or good design.

Robert G8RPI.

There is always the option of generating your own 3Ph, petrol or diesel powered, at least then it will be REAL 3Ph, maybe not an option for many, just a thought ! Noel

Running Hardinge lathe, Beaver milling machine, J&S 1400 surface grinder and Wolf pedestal grinder over the last 12 years or so, all fed by a single VFD/inverter. Initially Danfoss, later Teco. with DOL starting, plug reversing, dual speed motor and multiple machines operating at the same times.

Oh, and there isn't an issue with unbalanced loads. Inverters are constant voltage supplies, not constant current. I do have a neutral generating transformer, but in fact none of my machines and virtually no industrial machines need a 3ph 5 wire supply.

Edited By Mark Rand on 11/10/2022 18:11:11

No-one is suggesting that one would control the 3-phase bridge directly. On VFDs I have used the basic functions such as on/off, forward/reverse and frequency can be controlled from the front panel, via external components such as switches and potentiometers or from an external controller, usually via a comms link.

Conceptually the 3-phase control signals are simple. How the values are arrived at can be simple or complicated depending upon the control algorithm. Simple V/f control only needs a microcontroller but vector control ideally needs a DSP, preferably floating point to simplify the software.

Andrew

Mark,

I think that you have been lucky. The fact that your VFD seems to be rated to drive your full set of machines helps. If switching a single load the VFD must have enough overhead to start it. When you have other machines running their motors act as an energy reserve stabilising the VFD output. I guess your imbalance is less than 10% whidh is probably to small to notice.
How do you manage earthing and other safety requirements like RCDs?

Robert G8RPI.

Mark

I'd be interested to know what model of Danfoss and Tech VFD's you used so I can get some idea of the capability they have.

CS

I did seriously consider that, there was a 7.5kVA unit that just went for £400 on eBay, but it had clearly had a very hard life... The only other one which is suitable is a 10kVA unit with low hours going for about a grand, so less economic.

Currently, a diesel genny would be *very* competitive with grid prices too.

Unfortunately I live in close proximity to my neighbours, and with the best will in the world, "silenced" diesel generators are still obnoxiously loud in close proximity.

My take away from this thread is that it's technically an option, but at my scale it would be difficult to find a suitable inverter commercially, and I should be very cautious of being sold a large VFD which has been reworked to fill that gap, but producing EM interference and not meeting appropriate safety standards.

In light of that, I will probably persue a second hand rotary phase converter on the basis of it previously working just fine.

Out of curiosity what are the factors that you believe render these systems:

1. Unsafe
2. Non-compliant

I'm always just as interested to learn more about how NOT to do things for future reference, as to learn how to do something.

Jelly,

The unsafe bit comes in two areas,

1/ The units themselves are components and need to be put in a suitable enclosure with over-current protection etc. Without an enclosure many have exposed terminals (or covers that don't need a tool to remove), inadequate or no cable strain reliefs and they are generally not protected against dust etc.

2/ The equipment they are connected to wil have it's electrical safety based on it being connected to a conventional mains supply that is referenced to earth (even if neutral is connected). The output of a VFD is not normally referenced to ground. A VFD output typically has high frequency components that can affect filter components and cause leakage currents. They often have high voltage spikes that cn breakdown insulation.

For legality the big issue is interference (EMC /EMI). All VFDs need filtering. Some have filters on their mains inputs but it not common and may not be enough. They require careful grounding, often shielded cables and approriate connectors with 360 degree backshells. Even when the makes says they will meet emissions levels they will almost certainly mean industrial levels, if used in a residential area (technically just one home on the same substation or transformer feed) you have to meet the lower consumer equipment levels. From experience this can require double metallic cases, extra filters and special cables.
The filters cause another issue. Buy design they draw a large leakage current, well in excess of that allowed for consumer equipment. Often enough to trip a 30mA RCD. This can require a large and expensive transformer on the input. If you loose an earth connection the filters acn pas enough current to kill you even without a fault.
You can't wire a VFD output up to a wall standard wall socket because it does not have the protection required of a mains installation.

Just because you can hear your radio does not mean it is OK you relly could be affecting somone else. I once delt with a case where a piece of equipment with a 50Hz sinewave output generated by PWM like a VFD caused real interference to a ground approach radar on a RAF airfield. The radar operated on 10,000,000,000Hz and was completly wiped out by the interference. how could a 30kHz PWM making 50Hz cause that? It seemed unlikely even to me. It turned out to be harmonics. Mostly caused by the installer not using the prescribed cable layout but even with that corrected I had to design a special filter to get it down to an acceptable level.

Oh and another issue with VFD's generally even when properly installed is tht they can cause DC leakage currents in the mains supply. This current can saturate the current transformer in a RCD aor RCBO "earth leakage trip" on the household supply. Comminlly called "blinding" an RCD. There is no sign of this but it stops the RCD from working so a fault somewhere else in the houst that could cause a fire or electric shock that would normally cause the RCD to trip won't trip oit and this can have fatal results.

The RCDs most affected are "AC" types and are the most common. If you use a VFD connected to the domestic supply you should ensure that RCD(s) in the house are changed to ones not blinded by DC leakage (typically a type A but F or B may lso be suitable).

Robert G8RPI.

The spacing and number of harmonics are dependent upon pulse repetition rate and pulse width. How high the harmonics go, at a detectable level, is mainly controlled by the speed of the pulse edges. The faster the edge the higher the frequency of detectable harmonic.

Andrew

Hi Andrew,

It was a rhetorical question.... I can't give details but the interfence wasn't getting in the front end at 10GHz.

I have a comb generator that uses a fast diode to generate harmonics to at least 18GHz.

Robert G8RPI.

Thanks Robert,

Very glad I asked, the electrical safety bits were mostly what I expected, although I hadn't realised that the voltage spikes were so severe as to exceed the insulation breakdown voltage in some cases.

Nor do I think I'd ever had an explanation of why type AC RCB/RCBO's are unsuitable for circuits which are liable to DC injection either... even though I am aware that good practice is to use Type B when you have PWM equipment in the circuit.

The EMI aspect however I was entirely blind to, and the issue of leakage current from filters too (although it explains why 100mA RCD's are often recommended by sellers, even though they're only suitable for wiring protection, and not personal protection) so that's very interesting.