Larger VFD/Motors

[Nick Taylor 2Participant @nicktaylor2]

Morning all, it's been a couple of hours since I asked my last stupid question so I thought I better think up another one so people don't think I've been abducted or something.

So… VFD's on 13amp sockets. Running 1/2hp at the moment for the myford with no problems what so ever. I only have a 13amp spur to feed my workshop, not usually a problem as I only use one low power machine at a time, however;

Now I am looking at larger machines that have 2hp and 3hp motors I obviously have a problem as a lot of 2hp or higher VFDs quote a max input current of 16+ amps. Now on other forums you hear of people claiming to run a 3hp machine though a VFD on a standard ring main for years and it has never blown a fuse etc.

But I would like to hear your real world experiences of running larger motors on domestic electrics. I know there is the option of single phase but I think that may be my last option. Just after actually spending some cash and getting the garage rewired. That option is just after winning the lottery.

Many thanks,

Nick

[Nick Taylor 2Participant @nicktaylor2]

What HP motor do you run?

[Steve PaveyParticipant @stevepavey65865]

My largest motor is a 2.2 kW running on a Huan Yang vfd. My workshop is supplied by a 12m length of 4mm² cable run from the house consumer unit on it's own dedicated 32a mcb (type B). This supply feeds into the workshop consumer unit. The feed to the motor in question is via a 16A mcb and a 16A commando socket. No problems at all with this set-up.

One machine I occasionally have problems with is my Startrite bandsaw, a 1 hp single phase motor running from a 13a socket on the ring main, which occasionally blows a fuse in the plug. While a 2hp motor only draws the same as about half an electric kettle when it is running, there is a greater start-up current (which is why my bandsaw blows its fuse now and then). I wouldn't feel happy running the workshop with a spur off an existing ring main.

I would suggest that you run a larger cable direct from your house consumer unit to your workshop, rather than a spur off the ring main. It shouldn't cost very much – you could run the cable and stick a consumer unit on the wall, then get an electrician in to do the connecting if you don't want to do it yourself.

One vaguely related point is that some Vfd's seem to trip a domestic RCCB because they have an inherent earth leakage which exceeds the rating of standard domestic units (usually 30 or 100 mA). It might be worth investigating which makes/models are prone to this and which aren't.

[Nigel BennettParticipant @nigelbennett69913]

My Boxford 280 motor is rated at 4.9A for 3-phase 220/240V. It's run from an IMO Jaguar Cub 5A inverter, and I've no problem at all feeding it from a 13A socket. One of the beauties of an inverter is that it ramps up the motor speed relatively slowly – you can adjust it – so that there's no massive inrush current like you get with single phase motors.

Suggest you speak to Pete Moss at Transwave. Very helpful and knowledgeable bloke.

[Nick Taylor 2Participant @nicktaylor2]

Hi Steve, great info – thanks. I was actually looking at the Huan Yang VFDs on ebay, they seem to have a lot more sellers with EU/UK stock now. At under £100 for the 3HP version I am very tempted. Have you had any problems with yours?

Running a better feed to the garage is in the plan, but budget wise it's probably not going to happen until some time next year.

[Ian PParticipant @ianp]

+1 for the Huanyang. Documentation is poor and setting it up for the first time was not as easy as it should be but it has no problem feeding the 1.5Kw (I think on my M250. Plenty videos etc nowadays, of how to do it though.

As Nigel states using a VFD negates the massive start up current of a large single phase motor, apart from the fact that it can be set to have a gentle start, its charged internal capacitors act as a buffer, reservoir, power bank or whatever you want to call it, so they provide the initial backup.

I have my inverters connected via standard 13A plugs and sockets. Bear in mind too that most of the time its unlikely that you will be running the lathe or inverter at anything like full power so the current drawn will be reduced.

Ian P

[Andrew TinsleyParticipant @andrewtinsley63637]

The ring main should be capable of running 16 amps, most have 32 amp breakers. The real no no is the 13amp plugs and sockets which will overheat eventually. Use a 16 amp plug and socket in your ring main. Can't remember what the max current is for a spur.

Andrew.

[Steve PaveyParticipant @stevepavey65865]

Nick – my Huan Yang has been no trouble at all. I can echo Ian's comments about the documentation being poor – the information is all there but it seems to be scattered throughout rather than in the sequence you need to set it up. Programming the settings is fairly straightforward, and there only you few that you need to change (depending on your motor of course). Mine is supplied via an NVR starter, though a normal islolater would be fine. I have never had a problem with it tripping the RCCB, nor have I with my other vfd (a Lenze). A Siemens model I tried the other day tripped the RCCB every time I switched it on, with or without the load connected, and a bit of internet research suggested I would need a 300mA RCCB to run it – clearly not practical in a domestic setting.

A domestic ring main usually has 2.5 mm² cable, two of them of course as it is a ring. Depending on the length of the ring you can expect a cable of this size to be able to carry approx 30 amps. The 32A mcb is there to protect the cable (overheating is usually the biggest risk). Everything that is plugged into it will be protected by a 13a fuse, so in theory you only need to plug in a couple of kettles and a heater and you're up to the limit. The IEE Regs allow for diversification, i.e. while you may potentially plug in loads which far exceed the 32a rating of the mcb, in practice this is unlikely to happen. A spur off the ring can feed up to 2 sockets, i.e. 26 amps, but again, diversification means that this is unlikely to happen in practice.

Of course, the IEE Regs don't really cater for home workshops equipped with ex-industry machine tools. That is why I say that running a workshop from a spur is not a good idea.

It is becoming much more common for ring mains to be run in 4mm² cable in new installations, and also for radial circuits rather then rings to be used for specific loads such as cookers and ovens. In a workshop situation radial circuits for 16A sockets are a good idea – one socket per cable automatically limits the maximum load, and if each one is protected by a 16a mcb there is no danger of overloading the cable. And, as Andrew points out, 13a plugs and sockets have their shortcomings, and can overheat if the springy brass and copper contacts lose their springiness.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Nick Taylor 2 on 12/07/2017 11:27:18:

…

So… VFD's on 13amp sockets. Running 1/2hp at the moment for the myford with no problems what so ever. I only have a 13amp spur to feed my workshop …

Now I am looking at larger machines that have 2hp and 3hp motors I obviously have a problem as a lot of 2hp or higher VFDs quote a max input current of 16+ amps.

…

Nick

Tricky one if you are asking 'what's the maximum load I can put on a 13A spur without burning the house down?" It depends on lots of stuff we don't know like how long the spur is, and how efficient the VFD and motor are.

When it comes to electricity I'm risk averse, especially when doing stuff that might destroy the house and invalidate the insurance! So my answer would be the simple one, ie 'anything that can be powered by a VFD having a specified input of less than 13Amps.' Very roughly that suggests an upper limit on a 3-phase motor of about 2.5HP, with various 'if' and buts' that might reduce that number.

I know money is always short, but if I wanted a 3HP machine in my workshop, I'd have the wiring uprated. It's not rocket science; I think my pretty ordinary electric shower is wired to get 40A from the consumer unit.

Dave

[Rick Kirkland 1Participant @rickkirkland1]

It’s not a ring main. It’s a ring final circuit.

[Nick Taylor 2Participant @nicktaylor2]

Hi all, thanks for all the replies!

I'm no idiot and won't be trying to run a pair of 3hp motors off a single socket or anything like that!

Judging from people's experiences and reading as many VFD data sheets as I can it seems 1.5kw output is a sensible limit. Sure, a 2.2kw motor may start and run off load, but I bet it would blow a fuse or two with a heavy cut on.

The spur was installed by the previous owners and seems to be in good safe order, the cable is 4mm and protected by a 13amp fuse at the spur on the ring then each circuit in the garage is fused individually.

[Neil WyattModerator @neilwyatt]

1/2hp is ~400W, not a lot for a 13A socket which can take up to 3.3kW which is 4.4hp.

I would uncomfortable running a 4hp motor of a 13A socket for extended periods, but a 3hp motor at 2.2kW is not a vastly heavier load than a 2kW fan heater.

Bear in kind the vfd won't be 100% efficient so it will use a modest amount more than 2.2kW to run a 3hp motor, but then the motor will rarely be using anywhere near full power – my lathe with a 400W motor actually uses about 40% of that when running lightly loaded. (I know as I have a meter rigged up as a power read out on it).

Neil

[Nick Taylor 2Participant @nicktaylor2]

Hi Neil,

That's exactly my point – when we talk about a 2.2kw motor we are obviously talking about the three phase power output of the VFD, which bares little relation to the single phase input current requirment. My question in this thread was to see people's real world experience with running larger motors.

A few of the 2.2kw output inverters I've looked at today quote an input requirement of 40+ amps, granted that is peak, but still not something I would want to run off a bit of flex from a 13amp socket!

Still very tempted to follow Steve and go for a 2.2kw Huan Yang – the cost is hard to ignore!

Cheers,

[john fletcher 1Participant @johnfletcher1]

I have a friend with a Harrison lathe, I think 160 or maybe 180, bigger than a Colchester Bantam anyway. I/we altered the 3 HP motor from Star to Delta, a 10 minute job. Then fitted a Huanyang 2.2 KW inverter, bought off ebay for £90.65, it runs via a 13 amp socket outlet without any problems. The shed wiring is a spur, wired using 4mm PVCSWA cable. I don't know how often John uses maximum cut, but nothing over heats or trips out, so John is happy man making swarf. I forgot, we also altered the coolant pump into Delta and fitted a 2 micro farad capacitor so it now run from 240 volts. As the program instruction in the booklet were to us a bit confusing, I had a look on YouTube and followed what I saw. Once the machine was up and running, I wrote the program out, kept s copy and have given others a copy as well. So should you need a copy Nick send me a PM. John

[SillyOldDufferModerator @sillyoldduffer]

Just a few thoughts, feel free to comment:

• Rather more than 13A is needed to rupture a 13A fuse, so it's possible to overload it for a period of time
• If 13A is drawn continuously through a 13 Amp fuse the fuse runs hot and has a reduced life (months rather than years)
• A 13A fuse will handle short severe overloads provided it has time to cool off between
• The wiring that the fuse is protecting is heavy enough to deal with a continual 'nearly but not quite' fuse blowing current. But the insulation will eventually be damaged if the wire gets too warm too often
• The electric cable may get hotter than expected if it happens to be thermally insulated by the building, for example by being sandwiched between plasterboard and fibreglass
• It's not the law, but Horse-power is really only meaningful as a measure of power output. Assuming a 3HP motor is 80% efficient, it will draw about 2.8kW under full load, usually less on average on a machine tool, but much more if the motor is overloaded or stalled. A VFD designed for a 3HP motor will have a correspondingly beefy input requirement to cope with the odd overload,
• Assuming the VFD is also 80% efficient, it will need at least 3.5kW to run the 3HP motor (about 14.5 Amps)

So I think this lot is a nice balancing act. I think a lightly loaded 3HP motor would run for ages, but the same motor run hard would often blow fuses. Continually replacing fuses might damage the wiring. Even more dangerous if someone replaced the 13A cartridge fuse with 15A fuse wire. (Or a paper clip!)

Dave

[Anonymous]

Posted by SillyOldDuffer on 12/07/2017 22:34:55:

Just a few thoughts, feel free to comment:

• Rather more than 13A is needed to rupture a 13A fuse, so it's possible to overload it for a period of time

Fuses are essentially I²t devices, ie, how quickly they blow depends on the heat put in, which heats, and eventually melts, the element. That accounts for the current squared. You're correct that fuses will handle overloads – even the fast blow 13A mains fuse will take 100ms to blow at 2 times overload. At 1.5 times overload the line goes off the top of the graph. So don't bet the house on it blowing at 13A in any sort of hurry. To rupture quickly you need many times the nominal current capacity.

I think you're being rather pessemistic about motor and VFD efficiencies. If nothing else if they're only 80% efficient that's a lot of heat to get rid of, not likely with an air cooled heatsink on the back of the VFD.

I've run a 3hp 3-phase motor off a single phase supply without a problem. We did slightly overspeed the motor (ahem) to 24000rpm in order to test a racing car alternator but it survived. Although by gum it was noisy.

Andrew

[John OlsenParticipant @johnolsen79199]

Andrew, was that really 24000 at the motor? Back when I was a student, Canterbury University tested the locally available three phase motors and concluded that they were good for about 10,000 rpm maximum. That was partly the limit of the bearings, and partly the state of balance of the rotor. If you balanced the rotor more carefully and used high speed bearings you might get a bit more safely. The power available goes up linearly with the speed, provided that you can keep raising the voltage linearly as well.

Here in NZ our ordinary socket is only supposed to be 10 A. I have routinely run a 2.2 kW inverter from that with no drama, never blown a fuse or popped a breaker. The vfd itself will have a bit of an inrush current when connected as it charges its capacitors, but this seems to be much less than the inrush of an equivalent motor starting.

John

[Anonymous]

Posted by John Olsen on 14/07/2017 00:01:11:

Andrew, was that really 24000 at the motor? Back when I was a student, Canterbury University tested the locally available three phase motors and concluded that they were good for about 10,000 rpm maximum.

Yes it was. The motor was designed for use in an industrial wood router with a rated speed of 22000rpm. We took a chance and went to 24000rpm, ie, 400Hz from the VFD.

The high speed spindle motor on my CNC mill will run at 24000rpm, albeit at lower power, ½hp I think. It seems happy running for hours at a time. These gears took about 4 hours each:

Andrew

[Chris Evans 6Participant @chrisevans6]

Another happy Huan Yang user here. Been powering my 2HP Bridgeport for a good few years now. A friend set it up for me, he has 5 of them running his various machines and understands the instructions which he refers to as Chinglish. Best £90 I ever spent on the workshop.

[Nick Taylor 2Participant @nicktaylor2]

Well I took the plunge and my 2.2kw Huan Yang turned up today! I've tried it quickly with the 1/2hp motor on my super7 and I'm impressed, very low electrical noise compared to my older Mitsubishi which I usually use on the Myford.

The manual isn't as bad as I feared actually, however I do deal with translated data sheets etc at work so maybe I'm immune! What I like about it from the looks of the manual is that the hardware input functions can be changed so you're not tied to the factory setup.

Anyway the machine it is meant for arrives over the weekend (1970s Chipmaster), so I'll be able to get stuck into the config then. I hope to use the lathe's existing Rev/stop/fwd lever l (including the light up handle) as well as the e-stop peddle!

[MuzzerParticipant @muzzer]

Posted by Andrew Johnston on 13/07/2017 20:57:53:

I think you're being rather pessemistic about motor and VFD efficiencies. If nothing else if they're only 80% efficient that's a lot of heat to get rid of, not likely with an air cooled heatsink on the back of the VFD.

Indeed, you should expect something around 95-97% peak efficiency, depending on model, load, input voltage etc. Obviously with zero rpm on the motor you should expect zero efficiency from the VFD+motor, and even if you were to measure the AC-to-DC efficiency of the VFD alone (if you have a suitable power analyser) it would be pretty poor at that load point – consider the voltage drops around the circuit.

At full load you may see something like 3-4V drop across the rectifier bridge, 3-4V in the IGBT bridge, another few volts in the filters etc and before you know it you have dropped the best part of 10V in total, with a DC bus voltage of around 340Vdc. So no matter what you do, you won't do much better than 96-97%. That's before switching losses and snubber losses are added.

To run a 3HP motor, you might need 3kW or so (80% motor, 95% VFD) but apart from the plug rating, who cares? If you are bothered, fit a 16A or 32A version instead. The ring circuit will be rated around the 30A mark.

Murray

Edited By Muzzer on 20/07/2017 19:19:55

[not done it yetParticipant @notdoneityet]

My 3HP pressure washer runs happily through a 13 amp plug. It was supplied that way. It has never blown a fuse in the several years of ownership.

 

3HP starting on load will certainly drain the mains for a short period, mind.

Edited By not done it yet on 20/07/2017 19:44:43

[MuzzerParticipant @muzzer]

Posted by not done it yet on 20/07/2017 19:44:01:

3HP starting on load will certainly drain the mains for a short period, mind.

Interesting – what does that look like?

[SillyOldDufferModerator @sillyoldduffer]

Posted by not done it yet on 20/07/2017 19:44:01:

My 3HP pressure washer runs happily through a 13 amp plug. It was supplied that way. It has never blown a fuse in the several years of ownership.

…

Edited By not done it yet on 20/07/2017 19:44:43

Yes of course, but your pressure washer has a single phase motor. Assuming the motor is 80% efficient, the washer would draw about 11.7 Amps – comfortably inside the rating of a 13A plug.

We're discussing a rather different question: what's the biggest 3-phase motor that can driven by a VFD on a 13A plug? ( A 3-phase motor is desirable on a lathe or milling machine because it is smoother and has better torque characteristics.)

There's a conversion loss in the VFD; I suggested 80% efficiency. Andrew and Muzzer both confirm my estimated conversion efficiency is too pessimistic: I agree. Nonetheless, I stand by my comment that a 3-phase 3HP motor should be OK, but that the same motor overloaded is likely to blow 13A fuses.

Dave

[Nick Taylor 2Participant @nicktaylor2]

Out of interest while I had the VFD in work (that's where I got it delivered) I took advantage of an empty EMC lab and powered it up on our conducted emmisions kit. Obviously I had no motor to use on it but I wanted to get an idea of the power and earth leakage overheads.

Initial power up rush current was lowish (less than an amp), however it did pulse approx 15mA down the earth, so on an already busy ring that maybe an issue. Standby current was just under half an amp, so our idea of 90% efficiency might not to too far off.

 

 

Edited By Nick Taylor 2 on 20/07/2017 21:42:01

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