Larger VFD/Motors

[Steve PaveyParticipant @stevepavey65865]

Like most, I have a varied collection of induction motors in the workshop. Four of them are 3 phase, the rest single phase, and all variously rated at between 0.5 and 3 kW. The only one that pops fuses is the 0.75kW Brook Compton motor on the bandsaw. I reckon a fuse lasts around 6-12 months. This must be partly due to the design of the motor, as it must be drawing close to 13 amps at startup. It never blows the fuse when running, even taking a deep cut, only when starting. In contrast, the 3 kW motor on the spindle moulder has not blown a fuse in the six years I've owned it.

[Neil WyattModerator @neilwyatt]

I'd be surprised if my 400W inverter was only 90% efficient, 40 watts coming out of the heatsink would be very noticeable and would probably melt my swarf guard over the top!

[not done it yetParticipant @notdoneityet]

Yes of course, but your pressure washer has a single phase motor.

Precisely what difference does that make? 3HP (2240W) is being derived from the single phase mains, whether or not the motor is single phase or three. Just need a few more Watts to run the VFD for the 3 phase motor.

So a three phase 3HP motor will require marginally more at full power. Any overloaded motor should blow its line fuse (or it will burn out). If you do that with a VFD, you would likely jigger the VFD, so they have internal trips which is part of the setting up for the particular motor/VFD combination.

Soft start makes not a jot of difference to the current drawn at full load.

The point was that my single phase motor does not compromise the fuse link at start up. Running at full power will not, either. Unless it is a crappy motor, its efficiency will be above 90%. 3kW is 3kW single or thee phase.

[Mark RandParticipant @markrand96270]

Currently, I'm running all the 3ph machines in my shop from a single 23kVA inverter. That's a 2.8KVA J&S1400 surface grinder, a 2.5kVA Hardinge HLV lathe and a 1.8kVA Beaver mill. I don't run them all at one time, but do run two machines at a time if one of them is doing a prolonged cut with automatic stops at the end (flattening a large piece on the mill or SG).

At the moment, the inverter is fed from a 13A plug because I haven't got around to feeding it a dedicated single phase circuit, The 3ph wiring was enough work for that session and it works ok for now…

Anyway, what it comes down to is that I'm running a 23kVA inverter feeding potentially 4-5kVA of machines from a single 3kVA 13A plug.

I've popped the fuse once. I was running a cut on the milling machine and then started the lathe at high speed with a large-ish (8"-25lb) lump of steel in it. That was my own fault, since I normally start in low speed and switch to the high speed setting, a bit like star-delta starting.

When I get some relief from the jobs that the domestic manager keeps delegating to me, I'll be running a dedicated 32A circuit to the inverter. Then it'll run all the 3ph machines I've got at once, assuming I can get her to mind them while I'm having a beer…

[Michael BriggsParticipant @michaelbriggs82422]

Posted by Neil Wyatt on 20/07/2017 21:53:22:

I'd be surprised if my 400W inverter was only 90% efficient, 40 watts coming out of the heatsink would be very noticeable and would probably melt my swarf guard over the top!

A Schneider 0.37 kw 240v single phase supply drive dissipates 41 watts of power at full load. To melt your swarf guard you would have to run at full load for an extended period.

[Martin KyteParticipant @martinkyte99762]

There must be many set ups where 3HP motors are fitted but are never loaded to the max. So some of this is somewhat theoretical.

regards Martin

[KWILParticipant @kwil]

Mark,

If you are actually drawing 5KVa from your 13A plugs I would not be surprised to find the socket a little warm. Still if you do have a fire. only you will be to blame.

[SillyOldDufferModerator @sillyoldduffer]

Posted by not done it yet on 20/07/2017 23:58:23:

Yes of course, but your pressure washer has a single phase motor.

Precisely what difference does that make? 3HP (2240W) is being derived from the single phase mains, whether or not the motor is single phase or three. Just need a few more Watts to run the VFD for the 3 phase motor.

…

I see we're not quite in tune yet! Anyway you answer your own question 'Precisely what difference does that make?' with the comment 'Just need a few more Watts to run the VFD for the 3 phase motor.'

'A few more watts', but how efficient is a VFD? I conservatively suggested 80%, which I agree is too low. But it's most certainly not 100% efficient either. My 1.75kW unit comes with cooling fins, a fan, and instructions about adequately ventilating the box it's mounted in.

The other issue is that an electric motor is not a W=I²R resistor. The power output and inputs are time-limited steady-state ratings, not maxima.

When the motor is idling, the input current is much lower than rated. The current increases as the motor is physically loaded up to the rating. At that point the motor draws the expected current and I=W/V is a reasonable approximation.

But what happens if the load increases even more? Overloads aren't unusual on a mill or lathe. When an electric motor is overloaded the current demand may be much more than normal, particularly if it stalls. If the supply is already close to maximum, overloading the motor may take the circuit over the top.

In his Original Post Nick said: '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.'

I think Nick got his answer: yeah, but no, but yeah… It depends. A hobbyist running lightly loaded kit will be OK with 13A because his average load is low and he rarely hammers it. On the other hand, a hard working professional wouldn't take the risk because his average load is higher and – because time is money – he is more likely to push his machines. His electrics will be sized more carefully.

Dave

[Martin KyteParticipant @martinkyte99762]

Slightly off topic. Not all 13A sockets (and indeed plugs) are equal. I have certainly come across examples that have much less conducter material than others. I would suggest that the three critical points are the connection to the back of the socket (the better versions have duel screw connects and doubles work better than single sockets in ring mains), the contact between the plug and the socket itself and the cable connetion in the plug. Of the three probably the weakest (prone to the highest resistance) link would be the connection in the plug. I am of the opinion that this is the seat of failure in most cases of overheating with the heat then being conducted down the pin and affecting the socket. YOu then get more resistive connections and the temperature rises.

In my workshop I have two 2.5mm2 rings and one 4mm2 rings. All the heavy loads, such as they are, are on the 4mm cabling which gives a larger area of contact on the back of the sockets which have 'in and out' screw terminals. The main feed from the house is 10mm2 SWA.

I would freely admit it's way over engineered but I was only going to do it once and the extra cost was not huge.

I do have a 3HP motor on my table saw which I have been known to stall without ill effect.

One more comment on the VFD input current. For installations with long feeds from a house on small diameter cable the input voltage cannot just be assumed to be 240V. With VFD's the will work with lower voltages but will draw more current for the same output load.

regards Martin

[Nick Taylor 2Participant @nicktaylor2]

Posted by Martin Kyte on 21/07/2017 11:01:52:

In my workshop I have two 2.5mm2 rings and one 4mm2 rings. All the heavy loads, such as they are, are on the 4mm cabling which gives a larger area of contact on the back of the sockets which have 'in and out' screw terminals. The main feed from the house is 10mm2 SWA.

 

That's exactly the kind of setup I would like to go for, a 4mm2 circuit for the machines/VFDs then a 2.5mm2 ring for secondary single phase things like coolant pumps/radio etc all on a decent standalone feed. Once I have the money in order it will be on my list.

Cheers

Edited By Nick Taylor 2 on 21/07/2017 15:24:42

[JonParticipant @jon]

Not sure how to word this but something that's bugged me for 7 years even had a couple of customers round that was going to help for services rendered.

I have doubts on the only 2.2kw 3ph motor I have it takes enormous Amps to start up well exceeding the smaller caput 4kw inverters max output and a spare Transwave 5KW rotary. Without an ammeter I would estimate startup current is like trying to start in excess of a 9hp motor. Spindle only in fastest speed takes 9 secs from rest, wth just a small 5 1/4" chuck on its in excess of 12 seconds with a push start, way outside most inverters max overload at 150% for x amount of seconds.
Back then slightest cut above a cleanup, spindle slowed to a halt in 3 secs with Rotary 5.5hp convertor, whilst Inverter constantly ramping up and really struggled to maintain speed. Normal cuts forget it could easily slow spindle down with invertor, lasted 1hour 25 mins.

After the above debacle had to get another inverter quick and went for a 7.5kw and never looked back. Not much of a problem with 8" chuck on geared in mid range but will trip the RCCD 1 in every 3 startups. MCB is a C type 32A slightly low for the 35A 240V input but never trips. MCB did trip constantly on startup before swapping to a C type.

Consumer unit fed by 32A MCB using the old 40Amp shower cable to a 32Amp 1ph blue plug and socket then to invertor. Just swap over for welder which used to give grief on a 13A socket on low amps, now can use 170A mig.
Output 3ph to 2 hard wired sockets.

Previous 3hp 1ph compressors have tripped the 16A MCB, especially if pistons at almost TDC.
Previous lathe 1 1/2hp geared head never tripped RCCB or MCB 16A in 9 years with serious cuts up to 1/2" face off but always blew the 13A fuse 1 in every 3-5 startups, used to buy 13A fuses by the 100.

[Nick Taylor 2Participant @nicktaylor2]

Well the machine has been in the shop almost a week now, have been cleaning up, getting it mounted properly and have changed all the oils (variator, gearbox, headstock etc).

Re-wired the existing 3HP motor from star to delta, entered the motor settings (max Amps, poles and RPM etc) into the VFD and it fired up first time!

Nice and quiet and pulls 2.5A at full speed idle (running spindle and gearbox etc but with no cutting). I was enjoying being able to see the current draw on the front of the VFD so I found some EN8 1 inch bar, stuck it in the chuck and made some VERY blue chips for the first time on the new machine!

0.100 thou DOC, 8 thou/rev feed at about 1200RPM using a WNMG insert and the motor was pulling 4A, so still plenty of power to use! Doubt I'll ever need it.

[Anonymous]

Posted by Nick Taylor 2 on 28/07/2017 14:35:32:

0.100 thou DOC, 8 thou/rev feed at about 1200RPM using a WNMG insert and the motor was pulling 4A, so still plenty of power to use! Doubt I'll ever need it.

Assuming that the DOC was 0.1", and not 0.1 thou, I make that a removal rate of 3in³/min. The rule of thumb is 1in³/min per horsepower in low carbon steel. So if you're nearly doubling that in medium carbon steel you're doing pretty darn well!

May be I'd better look into the WNMG inserts.

Either way if you haven't stalled the lathe at some point you're not trying hard enough.

Andrew

[Nick HulmeParticipant @nickhulme30114]

Posted by SillyOldDuffer on 21/07/2017 09:56:08:

Posted by not done it yet on 20/07/2017 23:58:23:

But what happens if the load increases even more? Overloads aren't unusual on a mill or lathe. When an electric motor is overloaded the current demand may be much more than normal, particularly if it stalls. If the supply is already close to maximum, overloading the motor may take the circuit over the top.

If you have a decent VFD you simply set all the current parameters correctly and the VFD faults when the motor overloads saving your mains supply,

Simples!

[Nick Taylor 2Participant @nicktaylor2]

Posted by Andrew Johnston on 28/07/2017 16:20:05:

Posted by Nick Taylor 2 on 28/07/2017 14:35:32:

0.100 thou DOC, 8 thou/rev feed at about 1200RPM using a WNMG insert and the motor was pulling 4A, so still plenty of power to use! Doubt I'll ever need it.

Assuming that the DOC was 0.1", and not 0.1 thou, I make that a removal rate of 3in³/min. The rule of thumb is 1in³/min per horsepower in low carbon steel. So if you're nearly doubling that in medium carbon steel you're doing pretty darn well!

May be I'd better look into the WNMG inserts.

Either way if you haven't stalled the lathe at some point you're not trying hard enough.

Andrew

100 thou (0.2 inch from diameter) indeed, sorry that was badly written!

Been having a bit more of a play today and havent seen the motor pull more than 5 amps.

The WNMGs seem to generate very high forces, when I pushed the DOC to 125 thou I got a little chatter thanks to the bell mouthed jaws on the chuck I am using but I am sure they take a lot more from the machine than say a CCMT style neutal/positive insert.

The negative rake on the WNMG is massive and while they do give a nice finish and seem to be able to remove a lot of metal I think when I tool up this lathe I will stick with CCMT style inserts as the minimum DOC on the WNMGs is very high. I use 10mm CCMT tools on the myford so will probably buy some 16mm shank CCMT tools so I can use the same tips on both machines.

 

Edited By Nick Taylor 2 on 29/07/2017 18:08:02

[Carl FarringtonParticipant @carlfarrington53722]

Posted by not done it yet on 20/07/2017 23:58:23:

Yes of course, but your pressure washer has a single phase motor.

Precisely what difference does that make? 3HP (2240W) is being derived from the single phase mains, whether or not the motor is single phase or three. Just need a few more Watts to run the VFD for the 3 phase motor.

3kW is 3kW single or thee phase.

Is that all there is to it though?

Apologies for reviving this thread back from the dead, but I have been looking into this topic – curiously wondering why I need 22 amps for a 2.2kw (3hp motor).

As i understand it, a single phase input to a VFD will require 1.73x the output (sq. root of 3).

I have a WEG 2.2kw VFD which can be run on either 3 phase or single phase 230v input. (model CFW300B10P0B2DB20)

It's paired to a 2.2kw 230v motor.

The VFD states that it will output 10a 230v, (2.2ish kW), and to do that, will require 12a 230v 3 phase, or 22A, 230V single phase input.

So we have some VFD efficiency losses, but crucially, the single phase to 3 phase conversion increases the current draw by 1.732x

A Hitachi document (**LINK**&nbsp states:

[quote]

As background, for a given power (kW/hp) and voltage, the ratio of current for a singlephase circuit will be 3 (1.732) times that of a three-phase circuit.
This means that the input rectifier will see 1.732 times the current of the output devices. When powered by three-phase, these currents are nearly the same.
This higher current would destroy the input of the drive if an oversized inverter were not used.
Furthermore, full-wave rectified single-phase power has a much higher harmonic content than full-wave rectified threephase power. This would introduce large ripple into the DC bus of the inverter, potentially causing other malfunctions.
Larger size inverters have larger bus capacitors, thus more inherent filtering.
So upsizing the drive ameliorates the ripple problem as well. The rule of thumb Hitachi recommends is to start with the 3-phase motor’s nameplate full load amperage (FLA) rating and double it.
Then select an inverter with this doubled continuous current rating.
This will give adequate margin in the input rectifier bridge and bus capacitors to provide reliable performance.
NOTE: Fusing or Circuit Breakers should be sized to match the INVERTER input current rating, NOT the motor current rating! As shown in the figure below, single-phase power should be connected to the L1 (R) and L3 (T) terminals, and optionally, a jumper should be placed between terminals L2(S) and L3(T). This jumper prevents the inverter from detecting a loss-of-phase should that function be active. Otherwise, the L2 (S) terminal should remain unconnected.
Beyond the inverter considerations, be sure to size components upstream of the inverter to match the INVERTER’S current ratings, NOT the motor’s. This would include, but not be limited to wiring, fusing, circuit breakers, contactors, etc.

[/quote]

Now, it's entirely possible that the Huan Yang / eBay VFDs are actually stating their input current rather than output, which might mean you're running those 3hp motors at something like 1.75hp max.

The WEG VFD I have chosen is a sensorless vector control one, and they state that it should closely match the power rating of the motor for the sensorless feedback stuff to work.

I'm still trying to get a clear understanding of it all.

[Pete RimmerParticipant @peterimmer30576]

Your average 13a fuse can carry MUCH more than 13amps.

**LINK**

[JonParticipant @jon]

Posted by Pete Rimmer on 05/05/2019 23:13:04:

Your average 13a fuse can carry MUCH more than 13amps.

**LINK**

Previous lathe 1ph 1 1/2hp 240v used to blow 13A fuses 3 out of every 5 startups, no load fed from a 20A dedicated supply. Used to buy fuses by the hundred.
Likewise both 3hp compressors throw the 20A MCB.

[Carl FarringtonParticipant @carlfarrington53722]

Posted by Jon on 06/05/2019 02:21:46:

Posted by Pete Rimmer on 05/05/2019 23:13:04:

Your average 13a fuse can carry MUCH more than 13amps.

**LINK**

Previous lathe 1ph 1 1/2hp 240v used to blow 13A fuses 3 out of every 5 startups, no load fed from a 20A dedicated supply. Used to buy fuses by the hundred.
Likewise both 3hp compressors throw the 20A MCB.

 

Yes, my 3hp single-phase compressor would also blow fuses and MCBs. It now runs happily off a Type-C 16A MCB in the garage mini consumer unit. The Type-C is needed for the compressor.

The point of my post really was to clarify the actual current requirements of a VFD running a 3 phase motor from a single phase supply, since there seemed to be no mention of it here in this discussion, and I wasn't finding much else out on t'internet either. This for correctly/safely sizing other parts of the installation, switches, breakers, etc.

Look at the input current requirements on any of these 2.2kW 230V 1ph to 3ph VFDs, they are all between 20 and 24 amps. I think that's important to know.

**LINK**

**LINK** (I think that's an AliExpresss one. 24 amps it says!)

**LINK**

Anyway I'm using a 25A MCB and a 32A switch for mine, wired into the garage spur.

 

I'm wondering if the way to think of it, might be that instead of saying "3kW is 3kW", well, it's 3kW but at a higher frequency, so you could think of it as running 1.732 of those 3kW motors at the same time. You're powering those magnets on and off more frequently. That frequency being the square root of 3.

It still confuses me though, because surely 3 horsepower is 3 horsepower ??!? Whatever it is, those input current requirements are clear as day in the datasheets, and the clearest one is the one that states 12A when using 3 phase input, and 22a when using single phase input: https://inverterdrive.com/group/AC-Inverter-Drives-230V/CFW300B10P0B2DB20/

Plus that Hitachi document that I linked to before: http://www.hitachi-america.us/supportingdocs/forbus/inverters/Support/AN032404-1_Rev_A_Sizing_for_Single-Phase.pdf

Edited By Carl Farrington on 06/05/2019 10:21:14

[Pete RimmerParticipant @peterimmer30576]

Compressors are notoriously hard on startup loads but a 1-1/2hp motor blowing 13a fuses with no load is very unusual. I'd be checking the supply voltage under load, might begetting a significant drop.

[Ian ParkinParticipant @ianparkin39383]

This meter is showing the load on the single phase input on my 3hp 2.2 kw motor running via a vfd under no load and running the spindle with a cut on ,on a Colchester student

Edited By Ian Parkin on 06/05/2019 10:38:49

Edited By Ian Parkin on 06/05/2019 10:39:35

[Carl FarringtonParticipant @carlfarrington53722]

Posted by Ian Parkin on 06/05/2019 10:37:27:

This meter is showing the load on the single phase input on my 3hp 2.2 kw motor running via a vfd under no load and running the spindle with a cut on ,on a Colchester student

Edited By Ian Parkin on 06/05/2019 10:38:49

Edited By Ian Parkin on 06/05/2019 10:39:35

 

Interesting. Was that a hard cut, putting the motor under high load, or not? It's right at the upper limit for what you'd expect if the 1ph to 3ph conversion didn't increase current draw (despite the data sheets saying it does).

Or, if the VFD is 2.2kW in, and only putting out 1.25kW.

What VFD is it?

Edited By Carl Farrington on 06/05/2019 11:00:59

[Ian ParkinParticipant @ianparkin39383]

Its an Emerson commander control techniques one

its by far my preferred choice of vfd’s if i’m Buying one new

easy to configure and the instruction book is only 40 pages long and they seem to do everything i’ve Wanted i’ve Probably installed 40 of these on various machines in day job

[Carl FarringtonParticipant @carlfarrington53722]

Posted by Ian Parkin on 06/05/2019 11:10:43:

Its an Emerson commander control techniques one

its by far my preferred choice of vfd’s if i’m Buying one new

easy to configure and the instruction book is only 40 pages long and they seem to do everything i’ve Wanted i’ve Probably installed 40 of these on various machines in day job

There's a table at the bottom of page 8 here: **LINK**

It says the 2.2kw unit has a full load input current of 23.2 amps from a 1ph supply, or 11.9A from a 3ph supply.

[SillyOldDufferModerator @sillyoldduffer]

I waffled about this in another recent thread. Briefly, the maximum currents quoted in the specs are peak, and they mainly occur during start-up. Starting a motor from a dead stop is electrically brutal until the rotor moves.

As soon as the motor is turning the current drops to much lower levels. Ian's watt-meter pictures show his running lathe consuming 3A 'off-load' rising to 10A when he takes a cut. That type of watt-meter is to slow to register what happens in the first few seconds, likely a lot more than 10A!

Running off-load the lathe is doing enough work to exercise half a horse. That's due to friction in the machines bearings, belts, and gearbox etc. If the motor were uncoupled from the lathe it would likely turn on less than half an amp. Conversely, when the motor is asked to do more work by cutting metal, it draws much more current. A motor worked beyond its ratings can deliver far more power than the plate says but it will overheat, perhaps fatally. A VFD should detect this type of over-current and either limit or cut the power before the fuse sees anything unusual.

The fuse problem is how best to handle that initial start-up peak on the input side. It has to take a short but hefty wallop safely. Three choices:

1. A small ordinary fuse as used in a domestic plug; these stand short overloads fairly well but age prematurely and eventually die. It's safe but makes starting unreliable, perhaps unbearably if the motor happens to be a big fat pig. Single-phase induction motors are notorious.
2. A small slow-blow fuse; these are designed not to age but they react slowly to genuine faults. The motor will start and run reliably but you risk smoke in the event of a fault.
3. A big fast blow fuse sized to manage the peak; this is usually what's recommended by the makers. The idea is that the fuse won't blow during normal start-up conditions but it will react very quickly to a genuine fault. The problem with this type of fuse is the cost – they ain't cheap.

Circuit breakers can be had in similar 'ordinary', 'slow' and 'fast' variations.

I think it important a fuse rated for full peak current is NOT a slow-blow or ordinary type. The problem is the length of time they will pour power into a fault before tripping out. Better to rate these for the ordinary current consumed by the motor, not for the peak, and accept they might blow occasionally.

Running a big motor close to the capacity of a domestic circuit might not be smart; bit spendy but an electrician could be asked to upgrade the wiring.

I am not an expert! If a professional disagrees with any of the above listen to him.

Dave

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