(still) confused on single-vs-3phase motor ratings. 3hp from 10 amp domestic?

[carl0s]

I am still confused on this single-phase vs 3-phase motor rating stuff.

Since buying a '3hp' compressor (SIP) that runs off a 13a outlet quite a few years ago, I've always assumed '3hp' to be somewhere within the realms of a 13a mains outlet. In fact, 3hp single-phase motors are rated at 2,200w - a little less than an electric kettle, and if it wasn't for start-up surges, inrush currents, whatever, then 10a is basically what they draw. but usually a 16a slow MCB is needed, however that's another thing.

When I started measuring up for my new KRV2000 mill, I was sort of surprised to discover that a 2.2kw 3-phase motor doesn't just need the same 2,220 watts from the wall (to the VFD) as a single-phase motor. Apparently the watts are 'per phase' (that information wasn't exactly forthcoming..), but then because not all 3 phases are pushing at same time, so to speak, it's 1.732 (square-root-of-3) times 2,200w, which means the motor needs 3,810 watts, plus whatever overheads the single-to-3-phase converter has. The VFDs say they need 20 - 24 amps which I just found really surprising. However I have accepted that and it's not a problem, I will size T&E accordingly!

What I still can't get my head around though, is how the horsepower ratings can be the same between a 1ph-3hp-2200kw (10A from the wall on 220v) motor and a 3ph-3hp-2200kw (17.32A from the wall plus VFD overheads).
I do now understand that 'watts are not watts, when you're comparing single-phase to 3-phase', but how can a machine mart 3hp compressor run off 10A while my 3hp mill motor plus this VFD (https://inverterdrive.com/group/AC-Inverter-Drives-230V/Delta-VFD022EL21A/ ) requires ~20 amps.

Are the HP ratings not mechanical shaft horsepower ? Are they simply stating the per-phase-electric ratings in horsepower instead of watts?

yeah, still a bit confused..

 

[Guineafowl]

Watts are not quoted ‘per phase’. Where did you read that? You don’t normally multiply the wattage by root3 either.

Your 3hp compressor is relying on the slow-blow nature of the plug fuse for the inrush, then settles out to 10A or so.

20A supply for the VFD/mill sounds like overkill.

 

[carl0s]

Watts are not quoted ‘per phase’. Where did you read that? You don’t normally multiply the wattage by root3 either.

Your 3hp compressor is relying on the slow-blow nature of the plug fuse for the inrush, then settles out to 10A or so.

20A supply for the VFD/mill sounds like overkill.

Where did I read that.. well, after lots and lots of confusion and reading and questioning, that is the conclusion I came to. I did say I was still confused. hmmm

The VFD manual states 24 amps (see link above)

The 2.2kw VFD for my lathe (WEG CFW300), also says 22 amps.

 

[carl0s]

look - "input current: 22A single phase 230v, or 12a three phase 230v" - that's the 2.2kw VFD that I have on the lathe.

WEG CFW300 IP20 2.2kW 230V 1ph to 3ph AC Inverter Drive, DBr, Unfiltered - AC Inverter Drives (230V)

Part No: CFW300B10P0B2DB20 (13059419), Brand: WEG, Model: CFW300 IP20 2.2kW 230V 1ph to 3ph AC Inverter Drive, DBr, Unfiltered

inverterdrive.com

 

[carbon]

Electric motors are not 100% efficient

Apparently the watts are 'per phase'

Usually you measure the current per phase and all three should be similar.


If you look on the data plate for the three phase motor it should show the current ratings for both voltages. The higher voltage 415 will have a lower current rating than the 240 rating.

 

[daleyd]

Whatever the rated input says you should be able to run a 2.2kW vfd from a 13 amp supply.

They tend to overstate the required current I guess to cover their asses and also to ensure it doesn’t trip, but I’ve run them from a normal domestic supply no bother.

 

[carl0s]

Here's a chat I had with ChatGPT back in September. I think it might highlight how confused I am..
I know ChatGPT is dumb especially when it comes to anything mathematical, but it sort of shows how lost I have been trying to understand this.
I also had a lengthy discussion on another forum back in 2019 which is where I ended up concluding that ratings were per phase, although in hindsight I think that was current being per phase, not watts.. it was a while ago.

Here's my "discussion" with ChatGPT:

ChatGPT

A conversational AI system that listens, learns, and challenges

chat.openai.com

And here's me still struggling to understand back in 2019: https://www.model-engineer.co.uk/forums/topic/larger-vfd-motors/page/2/#post-407805

 

[Guineafowl]

Here's a chat I had with ChatGPT back in September. I think it might highlight how confused I am..
I know ChatGPT is dumb especially when it comes to anything mathematical, but it sort of shows how lost I have been trying to understand this.
I also had a lengthy discussion on practical machinist which is where I ended up concluding that ratings were per phase, although in hindsight I think that was current being per phase, not watts.. it was a while ago.

Here's my "discussion" with ChatGPT:

ChatGPT

A conversational AI system that listens, learns, and challenges

chat.openai.com

Just scanning through, the bot factors in power factor for the 3ph motor, but not the 1ph.

Also, it takes 8.36 amps per phase, and multiplies by three to give ‘total current’ of 25-odd amps, which is bogus.

When you ask how many watts a single versus three phase motor uses, both are 3hp. Hp is just another way of saying watts. The answer is 2200 in each case. This is a nominal rating and varies according to usage, though.

Edit: Just read the rest. That bot is confused and confusing. Read up some wiki articles on three phase power - some of them are pretty good.

 

[carl0s]

I think Andrew Johnston's explanation about why the VFD required so much current, makes sense. He says they take power in short high current bursts.

"I would expect the basic efficiency of the output stage of a VFD to be on the order of 85-95%. So why the large input currents? Two basic reasons, one the appalling power factor as illustrated by Ian Parkin. Two, most small, and cheaper, VFDs do not have a power factor corrector as a first stage, but simply a rectifier. Once the DC link capacitors are charged the input current is anything but sinsoidal. Current only flows near the peak of the voltage waveform when the input voltage exceeds that of the DC link. So the current is taken in short bursts twice a cycle. During each burst the input current is much higher than one might calculate from simple V x I requirements. The bursts of current are also bad as they create harmonics that are reflected back into the power network. To prevent these most VFD manuals recommend using an external power line filter."

I'm glad I asked on here and revisited the subject. Thanks for the pointers so far.

 

[skotl]

Slightly off topic but ChatGPT will state a falsehood with absolute conviction, so it's very difficult to tell whether you're being lied to.
Don't get me wrong - love me some ChatGPT, but you kind of already need to be an expert in the topic you're asking it about to ensure you're not being fed incredibly plausible lies...

 

[m_c]

I think you're trying to read into it too much.
The main reason for the overrated input current, is partly for switch on surge, but also for the overrated output. Most VFDs will allow short duration overload, typically to 200%, which will reflected in the required input.

The big thing is switch on surge. Once that's been dealt with, the power consumption isn't likely to be an issue, unless you're really pushing the limits of the motor.
And on a manual milling machine, I'd very much doubt that you'll ever manage to make full use of 3hp

 

[minimutly]

The big thing should NOT be switch on surge - if the vfd is half decent it will limit the charging current ofnthe capaitors with a simple resistor and relay.
And when you start the motor its up to you to set the ramp up time.

 

[m_c]

Define half decent.
I've got a brand name one that has a higher switch on surge than any of the Chinese ones I've used.

 

[6ply]

Where did I read that.. well, after lots and lots of confusion and reading and questioning, that is the conclusion I came to. I did say I was still confused. hmmm

The VFD manual states 24 amps (see link above)

The 2.2kw VFD for my lathe (WEG CFW300), also says 22 amps.

If I remember correctly, I have that same VFD on my Harrison M300 lathe. Standard mains plug with 13A fuse. Never blown the fuse nor tripped a breaker. I have the diagnostics cable to go with the VFD, and one day I'll get round to using it.

 

[fizzy]

I am still confused on this single-phase vs 3-phase motor rating stuff.

Since buying a '3hp' compressor (SIP) that runs off a 13a outlet quite a few years ago, I've always assumed '3hp' to be somewhere within the realms of a 13a mains outlet. In fact, 3hp single-phase motors are rated at 2,200w - a little less than an electric kettle, and if it wasn't for start-up surges, inrush currents, whatever, then 10a is basically what they draw. but usually a 16a slow MCB is needed, however that's another thing.

When I started measuring up for my new KRV2000 mill, I was sort of surprised to discover that a 2.2kw 3-phase motor doesn't just need the same 2,220 watts from the wall (to the VFD) as a single-phase motor. Apparently the watts are 'per phase' (that information wasn't exactly forthcoming..), but then because not all 3 phases are pushing at same time, so to speak, it's 1.732 (square-root-of-3) times 2,200w, which means the motor needs 3,810 watts, plus whatever overheads the single-to-3-phase converter has. The VFDs say they need 20 - 24 amps which I just found really surprising. However I have accepted that and it's not a problem, I will size T&E accordingly!

What I still can't get my head around though, is how the horsepower ratings can be the same between a 1ph-3hp-2200kw (10A from the wall on 220v) motor and a 3ph-3hp-2200kw (17.32A from the wall plus VFD overheads).
I do now understand that 'watts are not watts, when you're comparing single-phase to 3-phase', but how can a machine mart 3hp compressor run off 10A while my 3hp mill motor plus this VFD (https://inverterdrive.com/group/AC-Inverter-Drives-230V/Delta-VFD022EL21A/ ) requires ~20 amps.

Are the HP ratings not mechanical shaft horsepower ? Are they simply stating the per-phase-electric ratings in horsepower instead of watts?

yeah, still a bit confused..

The spindle on this is 2.2kw - remember if it is 3 axis cnc then the motors will draw a fair bit on top of that. Even a manual machine will use power on the feed.

 

[carl0s]

Yes it is just the spindle motor that I'm looking at. That's all the VFD runs. The power feeds run off a 110v transformer and of course will increase the overall draw of the machine, but I am simply trying to understand the VFD and 3ph motor vs a single phase motor and the apparent power requirements of the VFD.

 

[Pete.]

Never blown a 13a fuse on anything I've had a 2.2kw drive on. Just had my mill running for 30mins doing a heat soak test and it's been fine.

I don't think this is something to worry about, it's just a blown fuse if anything happens. Give it a go and if it won't hold up fit a 16a commando plug and socket.

 

[carl0s]

Never blown a 13a fuse on anything I've had a 2.2kw drive on. Just had my mill running for 30mins doing a heat soak test and it's been fine.

I don't think this is something to worry about, it's just a blown fuse if anything happens. Give it a go and if it won't hold up fit a 16a commando plug and socket.

Cheers. I have bought commando plug and socket already, and there are no sockets on that side of the garage, so I have to throw some T&E up and through the garage-loft over to the consumer unit on other side.

 

[Martin Hocking]

Thanks for this @carl0s i’ve wondered this too and you’ve done a much better job putting it into words than I would have done.

My teco vfd claims it needs over 20amps for 3hp. When I thought about it I sort of assumed that if you set it up to start instantly under load that’s what it would draw. It seemed to me that if it drew 22 Amps to run a 3hp motor all the time they just wouldn’t be efficient enough to be a viable product and kind of left it there.

Reading the other posts in this thread has definitely helped to clear things up.

 

[GRW]

A 3kw motor is the same rating (3kw) regardless of whether its 1ph or 3ph.

The difference is that the current required to run is all on 1ph on 1ph, but spread over 3 phases on 3ph.
At 4.5A per kw (230vac ph-n) on 1ph its 13.5A while on 3ph its 4.5A per phase.

1 Hp is actually 746 Watts.... but its a nice rule of thumb to consider 1hp to equal 1kva, as this allows for the motor power factor (about 0.84) & efficiency (about 92%) in the mix without messing about.... so a 3hp motor is basically 3kva (so 13.5A running on 1ph, 4.5A per ph on 3ph).

The kicker on 1ph motors is starting multiple inrush demand.... maybe 7x the rated motor load (maybe 4-5x for a 3ph motor). With a VFD though that can drop to maybe 1.3-1.5 x the running load.

On that basis, the inrush current off a 1ph supply of a 3hp motor (depending on what the motor is driving) could be 13.5A × 1.5 = 20 Amps ramping up to start it.

That's what I think anyway . A clampmeter would answer the real demand.