“Digital Phase Converters” – Inverters for Multi-Motor Applications

[JellyParticipant @jelly]

So my Rotary Phase Converter has begun to play up after being moved, and whilst working through trying to find the ultimate cause of the issues I'm debating other options.

I don't want to go down the VFD route as:

• I'd need at least 8 of them which would eclipse the cost of just getting a 3-Phase connection from my DNO (at £3.6K it's not actually unthinkable).
• Given the machines I run, and the torque limitations inherent in running motors outside their design frequency I wouldn't gain enough meaningful benefit from the speed control to justify the expense.

However, there are a number of 240V to 415V inverters with sufficient capacity which are cheaper than a replacement Rotary Phase Converter.

 

My question is whether I could use a single 5.5kW or 7.5kW inverter as a "digital phase converter" to power multiple 16A 3P+N+E sockets on a 3-phase circuit, with a maximum single motor load of about 3kW?

I'm unclear to what extent (if any) the inverters can cope with the temporary overload condition created by starting a motor DOL, which I would be doing.

Edited By Jelly on 08/10/2022 23:42:59

[JellyParticipant @jelly]

Are Inverters a genuine alternatives to Rotary Phase Converters

[SteviegtrParticipant @steviegtr]

Are you thinking of running all the machines at once. If not then you could just draw what is needed at one time. I would not think of overloading a Inverter. Underload no problem. Although when setting up a Inverter you input the Max current of the motor to use. Which would be difficult if you had multiple motors with different FLC.

Steve.

[JellyParticipant @jelly]

I'm unlikely to use more than one machine at once, but I'd like to be able to go from running my Lathe (3kw) to my Mill (2kw), and turn the coolant pumps (0.35kW) and feed motor (0.25kW) on and off freely.

When you say you wouldn't think of overloading an inverter, what counts as overload?

When I start a motor DOL, the inrush current is going to be 4× – 6× full load for a tenth of a second, dropping back to ~0.1× FLC over 2-3 seconds…

Do I need to size an inverter to deal with the power requirements at inrush (so 18kW for a 3kW motor), if the inverter isn't also acting as a soft-start mechanism.

[SteviegtrParticipant @steviegtr]

That is exactly what an inverter does.You set the up to full speed time at what you want. Example 5- 10 secs or whatever. seconds. So the inverter ramps up the power slowly which would annul the inrush if DOL. As long as the FLC is not more than the rating of the inverter. Hope this helps.

Steve. P.S You will probably now get lots of other advice. Contrary to mine.

From a retired industrial Electrician.

[JellyParticipant @jelly]

That sounds like it won't work for my use-case then, as sharing a single remote inverter would mean continuing to use the DOL starting circuits in the individual machines.

I would need to over-ride the DOL starter circuits of my machines and provide remote switching of the Inverter at each machine, which is almost certainly more costly than the money saved by using an inverter over a RPC.

[SillyOldDufferModerator @sillyoldduffer]

It depends on the specification of the inverter. Small inexpensive VFDs generally aren't up to being shared because their electronics assume one permanently connected load and are liable to pop if hot-switched between motors. They also expect to be parameterised to match the characteristics of an individual motor for best efficiency and torque and don't cope well with several different motors singly or in combination.

Bigger and more expensive inverters are more likely to support mixed loads and hot-switching but it can't be assumed. There's more than one way of making 3-phase from single-phase, and each method has advantages and limitations that might not suit a particular workshop. Check the inverter specification matches what you do in your workshop.

Paying £3.5k for real 3-phase may be the best option in a workshop with several 3-phase machines, especially if two or more are powered up at the same time. Real 3-phase doesn't care much about load variations and there are no electronics, capacitors or noisy rotary converters to go wrong.

Dave

[Clive SteerParticipant @clivesteer55943]

Jelly

It may be better to find the problem with your converter than change your system.

A static converter is essentially a passive device consisting of an auto transformer to boost 240V supply to 440V and a number of capacitors in parallel to "generate" voltage on the 3rd phase wire. A rotary converter is the same but with a permanently connected 3 phase idler motor. The idler motor improves the balance of the 3 phases and stores energy which can cover starting loads of other motors by acting as a generator. Each motor connected to the 3 phase supply will also act , to a certain extent, as an additional idler motor. The most common cause of a failure in a static/rotary converter are the capacitors as these degrade over time although a loose connection cannot be ruled out if failure is coincident with change of physical position. On some static converters, especially the higher rated ones, a phase voltage monitoring relay is used to switch in extra capacitors to maintain phase voltage balance when motor starting loads are applied but rotary converters don't need this feature as the extra starting energy is provided by the idler motor.

What is the nature of the issue with your rotary converter?

CS

[Clive FosterParticipant @clivefoster55965]

Jelly

I've had a 10 hp "plug & play" digital phase converter from Drives Direct running my machines for 15 years or so with no problems. It just does what it says on the tin. Slotted straight in to replace an RPC. Wiring arrangement probably isn't acceptable under current regulations but its safe with individual plugs and disconnectors for each machine. I have the (expensive) box of smoothing inductors normally advised for CNC use on the output side to kill any interference before it hits the power lines.

I have 9 machines ranging from 1/2 to 3 hp. Including a WW2 era 3 hp two speed motor on my P&W lathe with its splendidly steam punk oil immersed switchgear. I think I have had about 9 hp running at once but normally its one machine in use and maybe the Rapidor power saw (3/4 hp) chopping stock for the next job with the 3 hp motor on the hydrovane compressor kicking in occasionally. I'd be a bit dubious running a reciprocating compressor of the same nominal capacity of the hydrovane due to the heavy draw during run up.

The Drives Direct systems are expensive but I understand there are some significant internal modifications to the innards and programming to ensure the box doesn't fault out. Trying to start machines when connected to a VFD running at 50 Hz will cause a modern inverter to fault out and stop.

Drives Direct say their boxes can safely start a single machine up to half the nominal power rating. I reckon 1/3 (ish) is a more sensible de-rate.

I got my box second hand as a temporary measure whilst I found an electrician to hook up the three phase incomer I'd had installed. Nowt so permanent as a temporary job. Gave up on electricians after about 5 no shows!

I've done the replace with individual inverter maths a time or two and came out with rather better figures than you last time. If starting in 2022 individuals would be the way to go for me. Reckon I'd actually do better than break even selling the Drives Direct box and installing individual units and changing the two speed motor.

Clive

[Anonymous]

If by an inverter the OP means a VFD as a replacement for a converter there are a number of issues.

One, the output of a VFD is PWM where the pulse width changes every pulse so that the fundamental is a sine wave. Running PWM waveforms around the workshop without proper shielding could cause all sorts of interence problems. Filters are available to filter out the higher frequencies. They are conceptually simple, but are expensive as they are carrying full load currents.

Two, VFDs have a number of internal parameters that are tied to a motor specification. The parameters provide soft start and overload protection among other features. Not tying these parameters to individual motors may cause problems.

Three, VFD outputs do not like being switched when running, it's an easy way to let out the magic smoke.

I'd either try and fix the existing converter or swallow hard and install proper 3-phase. I did the latter and am glad I did so. I connect up my machines and they just run, as does all the control gear. So I can get on and make parts not faff around with electrics.

Andrew

[Mark RandParticipant @markrand96270]

I've been running my workshop with a single inverter for the last 12 years. Pedestal grinder, drill sharpener, Hardinge lathe, Beaver milling machine and J&S surface grinder. The original was an old Danfoss 23kVA monster that I got for £10, the current one is a Teco Speecon modified by Drives Direct to have a voltage doubler inside and sold as a '7.5hp digital converter'. It's quiet, efficient and completely reliable with DOL starting, plug reversing and multiple motors on multiple machines.

I have two additions to it to improve things.
1) A sinewave filter from REO to eliminate motor noise and any possibility of electrical noise.
2) A star to star transformer acting as a neutral generating transformer so I can run any 240V devices built into machines (5 wire three phase instead of 4 wire three phase). This isn't actually neccessary for any of the machines, but I already had it.

Inverter output stages and protection have improved unrecognisably over the last 40 years, when most of the horror stories and urban myths date from. The worries about inverter outputs and protection not being capable of handling multiple loads and switching are just not relevant any more.

 

Edited By Mark Rand on 09/10/2022 10:30:44

[Mike PooleParticipant @mikepoole82104]

3 phase 110V 200Hz power was generated on site with a motor generator set to supply portable tools that were safer and faster for drilling grinding and discing. A static inverter was investigated as the motor generator was costly to run and air and battery tools were increasingly popular. After 60years of use the system was abandoned and the inverter system was never installed. The supplier of the static inverter must have been confident that it could have done the job of supplying multiple tools with variable loads. In another situation we installed a 10KW UPS to supply a monorail which again had multiple motors running with variable loading. There is not a problem creating a 3 phase supply but a VFD is a device that is usually more sophisticated than just supplying 3 phases and will not be happy with multiple devices and load switching. One situation where a VFD was used with two motor was a special case when two identical motors were mechanically coupled to work in place of a large motor. As the unit to be powered was a scissor lift and the lowered position needed to be as low profile as possible the two slim motors were used. The manufacturer specified that the motors used must be factory built units as a rewound motor could be 10% different from a OEM unit. Supplying 415V 3 phase from a single phase supply is not really practical. VFD units are available that output 415V but a complex input stage is required to charge the DC link of the VFD to a voltage high enough for 415V to be output. I suppose it would be possible to use a transformer to step up a single phase 240V supply to a high votage and rectify it and smooth it to provide a DC link equivalent voltage but that would be a very expensive transformer rectifier supply. What is theoretically possible and what is cost effective are often poles apart. The very reasonable price of VFD units will often make them a winning solution even if the machine controls will require modifying. At some point the expense of installing a 3 phase supply may be economical but a machine equipped with a VFD can be better than having a fixed speed motor.
Mike

[Clive SteerParticipant @clivesteer55943]

Although I've not done this myself I see no reason for an idler motor to be connected to an inverter to suppress switching and load transients that my "upset" an inverter.

CS

[noel shelleyParticipant @noelshelley55608]

Since the fault occured after the device was moved I would look for loose or faulty connections first ! A rotary converter is switch gear, a pilot motor and capacitors. Next I would check the capacitor values AFTER they have been discharged, one or more may have gone out of spec. Finally the motor, connections or insulation damage. It is not hard to build your own rotary converter. Good Luck. Noel.

[John HaineParticipant @johnhaine32865]

**LINK**

[Robert Atkinson 2Participant @robertatkinson2]

I have serious reservations about the Drives Direct units. They say a lot in their ebay listings and on their website but what they don't say is more interesting. They clam their units a CE marked, EMC and LVD compliant. However this is misleading at best.

1/ These units are components and should not be CE (or UKCA) marked.
2/ A number of units on ebay at the moment are clearly not LVD compliant due to exposed mains terminals
3/ The manual for the DSA series of phase converters indicates that AC reactors, input filters and ferrite cores to meet EMC and harmonic emissions limits. DD do not list these items in their ebay store or supply them with the unit.
4/ The DSA unit manual also says a 100mA RCD should be used. This is illegal for any circut connected to a outlet of 32A or less in the UK.

While these units can be made to be compliant it is not a case of just putting a mans lead with plug on it and connecting to a motor. DD don't care about any of this because they know it is the users responsibility to comply with the regulations. As a COMPONENT supplier they have no responsibility.

Robert G8RPI.

[JellyParticipant @jelly]

Posted by Clive Steer on 09/10/2022 10:00:50:

Jelly

It may be better to find the problem with your converter than change your system.

…

What is the nature of the issue with your rotary converter?

CS

Initially it was that the output voltage on L2 and L3 was low, at 182V & 184V, I traced that issue back to the connections with the balance capacitors which I used to tune the voltage between phases.

Since I did that, the issue has switched and is now overvoltage on L2 and L3 (430V and 418V) at idle, dropping to extremely low if a load greater than about .35kW is started.

The motor is now also extremely noisy when running as a idler, (it's quiet when being spun up by the pony motor, which gets it up to full speed before the system is excited, so it's not the bearings).

The resistance over winding W1-W2 is now 1.5 ohms whilst the resistance over U1-U2 & V1-V2 are both 5.2 ohms (which is what W1-W2 was previously).

​

Based on all of the above, my judgement is that I've disturbed something which has let the magic smoke out of the motor (which I got second hand, after 12 years in continuous use).

Given that:

• a direct replacement motor would be both hard to find (11kW motors are not normally manufactured in 240/415) and exorbitantly expensive, and
• that even a somewhat smaller motor for use as an idler would cost more than the price of commercially available phase converters (especially on the second hand market),

It seems sensible to weigh in the motor and part out the control cabinet in order to subsidise the cost of a replacement phase converter…

​

I was quite proud of the phase convertor I'd built, but it's simply not economic for me to repair at this point, unless I just wait for a suitable idler motor to appear on the used market… Which is not really desirable.

[JellyParticipant @jelly]

Posted by Andrew Johnston on 09/10/2022 10:14:35:

If by an inverter the OP means a VFD as a replacement for a converter there are a number of issues.

…

I'd either try and fix the existing converter or swallow hard and install proper 3-phase. I did the latter and am glad I did so. I connect up my machines and they just run, as does all the control gear. So I can get on and make parts not faff around with electrics.

Andrew

I don't mean a "VFD" as I've always taken those to be a specific type of motor drive which is intended for PLC control to adjust motor speed within acceptable ranges.

I do mean a converter using the same kind of PWM to generate AC from a DC input, which in turn comes from rectifying a different AC input; my understanding was that some units which were capable of and designed for use as a convertor, rather than as a motor drive, were now available.

From what i've read thus far, there are such units out there and some forum users seem happy with them, but they're not actually designed for that use, it's just that if they're sufficiently overspecified as to not be too stressed, they cope with the use case.

​

I'm reticent to install 3-Phase as I know I'm likely to move house in the next 2-3 years, and would need to shell out hundreds if not thousands on top of the cost of the DNO install, because it would also see the supply point moved to a completely different (and really awkward) location on my property.

If the economic climate was a little sunnier right now, I would probably be a bit less of a miser about it and just get it installed, but whilst it's (just about) the kind of spending involved is not nothing, it's more than I've ever paid for a car, or indeed any personal purchase other than the house itself!

[Anonymous]

Posted by Jelly on 09/10/2022 23:03:26:

I don't mean a "VFD" as I've always taken those to be a specific type of motor drive which is intended for PLC control to adjust motor speed within acceptable ranges.

That is incorrect, a VFD is just a variable frequency drive. The output frequency can be varied by several means. Could be a PLC, but I expect most people on here just twiddle a knob on the unit or use a remote potentiometer to provide an analogue voltage input. Alternatively a VFD can just be used to provide a 50Hz 3-phase output.

If the OP is planning to move then I would agree that installing 3-phase isn’t sensible.

Andrew

[Clive SteerParticipant @clivesteer55943]

Jelly

As you say a replacement 11kw motor will be difficult to come by. However I'm puzzled as to how you achieved the boost from 240V to 440V. Did you use a transformer or by using the motor windings?

Although this may not be a conventional configuration you could use a single VFD to provide a 3 phase supply bus to all the machines and convert each machine so that the DOL contactor and motor protection circuit is separately powered as the contactor circuit is usually powered at 240/110V from a transformer connect between two phases. Then each machine can be "engaged" before the VFD is run up on a soft start. You may need a small idler motor on the bus to absorb transients should you operate a machines EMO or inadvertently disengage its contactor. Usually a VFD needs to be mounted fairly close to the motor to reduce radiated interference but the idler motor would clean up the "supply". If the idler motor is a 6 or 8 pole machine then the noise it generates will much less than if you use a 4 or 2 pole motor. Although this is starting get more complicated an auxiliary dry loop contact on the machines motor contact could be used to "start" the VFD.

CS

[Clive FosterParticipant @clivefoster55965]

Jelly

Pure, fixed frequency, electronic single phase to three phase converters basically don't, and never will, exist as a commodity item for motor drive duties.

Due to the need to supply high short term currents during start up the innards of a pure, fixed frequency converter needs to be rated to handle at least double the motor power. If you want a universal "plug and play when installed by an electrician one" three times is better. Someone is bound to shove it onto a reciprocating compressor without a leak down manifold so it has to restart under serious loads!

The inherent soft start of a VFD box running up to frequency from zero allows it to be matched to motor power with a relatively limited short term high current capacity. So all the expensive components that handle the high power end can be smaller and cheaper. These days the control gubbins, display et al are well sorted and cheap so a fixed frequency converter would, allowing for the smaller market, be at least 3 times the price of a VFD which does essentially the same thing with extra capabilities. Basically no market for Mighty Big Company.

That said I am surprised that no one has attempted to do a 220 – 415 step up version of the Eaton DE-1 series of "headless" VFD boxes. Those things are sold as a "drop in" replacement for old style contactor controls giving soft start run up and the other VFD advantages without the front end complexity. Actually a full blown VFD inside but the most complicated the installation sheet gets is three speeds. Not stupid expensive and, being intended to sit inside of the machine most of Roberts objections to Drives Direct products just go away.

Has to be said that the primary market for a bigger converter to run a small workshop is the "have multiple machines, run one or two at time" one which is largely folk like us. Tiny one. For any commercial, semi commercial or paying hobby situation the extra cost of individual VFD boxes is not relevant. Especially if you factor in proper installation with a breaker / MCB panel between box and loads. At our power levels, typically up to 3 hp, the extra cost of an inverter is mostly not a significant extra to the machine cost when got at the same time.

Clive

[JellyParticipant @jelly]

Posted by Clive Steer on 10/10/2022 09:39:25:

Jelly

As you say a replacement 11kw motor will be difficult to come by. However I'm puzzled as to how you achieved the boost from 240V to 440V. Did you use a transformer or by using the motor windings?

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.

[JellyParticipant @jelly]

Posted by Andrew Johnston on 10/10/2022 09:10:58:

Posted by Jelly on 09/10/2022 23:03:26:

I don't mean a "VFD" as I've always taken those to be a specific type of motor drive which is intended for PLC control to adjust motor speed within acceptable ranges.

That is incorrect, a VFD is just a variable frequency drive. The output frequency can be varied by several means. Could be a PLC, but I expect most people on here just twiddle a knob on the unit or use a remote potentiometer to provide an analogue voltage input. Alternatively a VFD can just be used to provide a 50Hz 3-phase output.

If the OP is planning to move then I would agree that installing 3-phase isn’t sensible.

Andrew

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.

​

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.

​

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

[SillyOldDufferModerator @sillyoldduffer]

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

[Mark RandParticipant @markrand96270]

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

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