Power options for a new bandsaw

[AndyMc]

Hi Folks



I’m in the market for a mid-range bandsaw and would appreciate some guidance on power options.

My workshop is around 60M from the main house and is powered from a single circuit with a 20A MCB from the house consumer unit. The cable is 6mm SWA, so according to the calculator (Voltage Drop Calculator) 20A is the upper limit, giving just under 4kW total power. In the workshop I’ve got 3 circuits, all on 16A MCBs. I can’t upgrade the cable, it’s all underground and would cost thousands (and possibly my marriage) to dig it up again.

I’ve been looking at various options for the bandsaw, the RP Sabre 450, the Laguna 14BX, the Axminster AP2029B. They’re generally single phase, but what worries me is the peak draw on start up.

I’ve read through Bob Minchin’s Induction Motor guide on “the other place” which was very insightful. He mentions that single phase motors can generate a significant load at start, several times what’s shown on the motor plate. I’m worried that if I take the plunge on one of these models I’ll end up with something that just trips the MCB in the house CU every time I try to start it. I’d also like to have my shop vac on, the lights on and perhaps even the radio, which doesn’t leave a lot from 3.9kW. I could turn all those on after the saw’s started, but I feel like I’m probably pushing it.

So now I’m wondering if I should consider a 3-phase motor along with an inverter/VFD? I get the impression that 3-phase motors are significantly more efficient than single-phase, so am I right in thinking the overall draw should be substantially lower, even after allowing for the inefficiencies in the VFD? If so, does anyone have any indication as to what sort of peak current a 3-phase setup, say for a 3HP motor would draw? Has anyone stuck an inline ammeter in front of a bandsaw/VFD and taken measurements?

Any other thoughts/advice?

Thanks

 

[Ttrees]

does anyone have any indication as to what sort of peak current a 3-phase setup, say for a 3HP motor would draw?

Hope you don't mind me asking some questions which would likely be of interest.

Starting I don't think would be problematic, and possibly still so with single phase..
That's a question for the knowledgeable folks, but to get an answer, you might need to state
what sort of MCB is feeding the sockets...
MCB type C is more suited to motors, for startup surge that is..
Type B is what the sparkie installed at me folks for the 240a stick welder,
it does "nuisance trip" occasionally, but I believe may be more suited to longer periods of
use.

Regarding the 3 phase route...
you can adjust the ramp up time, IIRC my 3hp saw is set to 7 seconds for 24" heavy wheels.
What your cutting might be the problem, should you be looking to cut, say long greenheart sleepers, or similar.

Can you take a break if cutting something like that?


Not sure about PUWER? if that is the body regulating the 10 second isolation rule in pro business, should it be applicable, I don't know if what I say is true, but I believe someone has mentioned
possibly not so straight forward but indeed do-able, some sorta relay I've seen before,
though not sure if this was done for those reasons, might have been for a complicated lathe
or something.


Another option what seems from someone totally knowledgeable on the subject, is soft start extension lead, what some use for easing the kick from router startup, i.e Bob posted that all that long ago, should shop vacs have a startup surge.
Not sure if that would work, and reckon everything is mentioned regarding your vac, it could well be over the rating.

Sorry I can't be of more help, only to mention my machine is seemingly easier on the juice than
my fairly average heat gun, or my pillar drill, which I think is 2hp.
I've only fixed it up properly recently, so my saw has never got a proper test at near full capacity, and only for say 4 or 5" rips.
Not noticed the 3HP, 12" TS, having any trouble at full capacity,
but I've prob never ran it constantly over 40mins.

You might as well mention the work you're doing, lots of things might not mean the be all and end all, for you.
Infeed tables can be made for smaller tables, motors can be swapped,
dust extraction holes can be cut if need be,
Need for changing blades often...etc

If yer goin cutting trees, you might appreciate a hefty tension system,
should you end up sharpening your own 1" blades,
John Teneyck's posts might be worth a look on Sawmill Creek,
if you want some ball park HP to depth of cut guidelines.
IIRC he reckons something like 1HP per 3" using the right blade, though worth noting he doesn't hang about babying his machine whatsoever, and reckons what I'd call adequate productive speed, he would suggest could be measured by calendar.

Tom

 

[AndyMc]

@Ttrees, many thanks for the comprehensive reply, lots to ponder there.

A couple of points that may help:

I'd planned to put in a new circuit on the workshop CU specifically for the bandsaw, so could easily drop in a type C MCB. I found quite a useful guide here: MCB (Miniature Circuit Breakers) Guide - Types, Sizes and Uses
...suggesting perhaps a type D for a welder, hope that's of use for your folks setup?

I'm planning to use the bandsaw in a general purpose sense, perhaps a bit of everything. I'll definitely want to resaw some logs, I've 2 felled apple trees sawn to logs that are winking at me right now, so would want to extract some small board from those. One of the criteria I was using to select a bandsaw was the maximum height of cut, although I wonder if the advertised maximum capacities (e.g. Axminster 2920 is apparently 300mm) might not be achievable with a wide blade or a glacial feed rate.
I'll look through your other suggestions tonight.

Many thanks!

 

[Spectric]

I can’t upgrade the cable, it’s all underground and would cost thousands (and possibly my marriage) to dig it up again.

A good reason to use cable ducts.

So now I’m wondering if I should consider a 3-phase motor along with an inverter/VFD? I get the impression that 3-phase motors are significantly more efficient than single-phase, so am I right in thinking the overall draw should be substantially lower

You cannot make power, the three phase is produced from the single phase and if you read about invertors they need to be oversized so stay with a single phase machine. What you have in your favour is that it is only you in the workshop and so you control what is in use, ie you cannot use two machines at the same time you cn treat each load as individual.

so could easily drop in a type C MCB

Providing the loop impedance is low enough to ensure that the fault current can actually trip the device, type C requires 5 to 10 times the rated current to disconnect compared to type B at 3 to 5 times.

 

[AndyMc]

A good reason to use cable ducts.

Indeed. I do have 32mm water pipe through which I'm running ethernet cable, however too small for 10mm SWA.

You cannot make power, the three phase is produced from the single phase and if you read about invertors they need to be oversized so stay with a single phase machine. What you have in your favour is that it is only you in the workshop and so you control what is in use, ie you cannot use two machines at the same time you cn treat each load as individual.

Yes, the question was more about efficiency, whether or not a 3-phase motor uses significantly less input power than a 1-phase motor for the same output. If I understand correctly, any efficiencies are lost with the inverter/VFD.

Providing the loop impedance is low enough to ensure that the fault current can actually trip the device, type C requires 5 to 10 times the rated current to disconnect compared to type B at 3 to 5 times.

This looks to be the path forward. Thanks!

 

[Ttrees]

I'm a Joe Soap, so you'll have to wait until someone knowledgeable can answer.
With that said, I can speculate on those queries you have mentioned.

Yes, the question was more about efficiency, whether or not a 3-phase motor uses significantly less input power than a 1-phase motor for the same output.

For about 2 seconds, or whatever on startup,
then I ignorantly suppose they are more efficient consumption wise, but likely not to see the difference on the leccy bill.

If I understand correctly, any efficiencies are lost with the inverter/VFD.

Regarding power, or losses by conversion..
Not sure, but at intended running speed at 50hz @ the motors rated RPM,
I don't get the impression of that.
It might matter with a lathe though, and there's VFD's which are suited for keeping the oomph factor running at slow speeds, not knowledgeable here, but I believe that's what's regarded as a Vector spec or something along those lines.

Just don't slow down the machine!
I can mention the cheapo 100 quid VFD's can fail if that happens, but that was my fault
since I was running a machine which really needed work done to it.
and they don't protect your motor either, mine got a bit hot for comfort,
which very well might be important if sawing into logs.

There's other means of conversion, like an RPC which has a slave motor,
so compared to VFD's, that's gonna be consumptive and noisy, don't know a thing about them really...
And then there's the old static phase converters also, which I've read, has that trade off of loosing some percentage loss of power for conversion.
Then there's digital phase converters, but I get the impression not a plug and play solution for someone like myself.
Those look sensible for folks who've got a workshop of their own.

Scold away knowledgeable folks, I've got no problem with being corrected.
Cheerio
Tom

 

[HappyHacker]

The problem I see is the 20A MCB at the house will not guarantee discrimination with the 16A MCBs in the garage. So if the startup current trips the garage 16A MCB it will probably trip the house 20A MCB as well.
Using a C type at the house end may improve the discrimination with a B type at the garage so only the garage end would trip but you would need to check the earth fault loop calcs to see if it is safe to do so.

What are you using for lighting in the garage? If from the same supply you should use the 3% for the VD calculation rather than the 5% for power.

 

[AndyMc]

The problem I see is the 20A MCB at the house will not guarantee discrimination with the 16A MCBs in the garage. So if the startup current trips the garage 16A MCB it will probably trip the house 20A MCB as well.
Using a C type at the house end may improve the discrimination with a B type at the garage so only the garage end would trip but you would need to check the earth fault loop calcs to see if it is safe to do so.

What are you using for lighting in the garage? If from the same supply you should use the 3% for the VD calculation rather than the 5% for power.

Yup, that's the problem. I figured maybe a 20A MCB type C in the house, with a new 16A type C just for the bandsaw in the workshop. Any faults on the other circuits in the workshop should be caught by the existing 16A workshop type B MCBs. I'd have thought the house MCB would need to be type C as well as the workshop otherwise it'd trip with the startup load from the bandsaw.

Lighting in the workshop is on the same circuit, it's LED strip lights - very low draw but very sensitive to voltage drop.

Anyhow, I'll be consulting a local sparks and will get the earth fault loop checked.

I think I also found the answer to my question about efficiency - appendix D of Bob Minchin's guide. It seems 3 phase is slightly more efficient, 74% for 1-phase, 83% for 3-phase, but that's for older motors. Efficiency may increase in modern motors.

Thanks to all for the replies so far.

 

[Spectric]

I'd have thought the house MCB would need to be type C as well as the workshop otherwise it'd trip with the startup load from the bandsaw.

Again the 20A MCB in the house is there to protect the supply cable and without you measuring the impedances you cannot know if they are suitable for a type C. The other info you have not provided is the number of cores in the 6mm SWA , the type of supply to the property and the earthing arrangements for your outbuilding.

The two advantages of having three phase are 1) Motors are self starting due to the rotating magnetic field and 2) increased power due to effectively three supplies. The bandsaw is not really an issue, my BS400 is on a std 13 amp supply and has never tripped, if you were looking at a P/T then you will have issues as my PT107 had to go onto a dedicated 16 amp supply but still a type B RCBO.

 

[AndyMc]

Again the 20A MCB in the house is there to protect the supply cable and without you measuring the impedances you cannot know if they are suitable for a type C. The other info you have not provided is the number of cores in the 6mm SWA , the type of supply to the property and the earthing arrangements for your outbuilding.

The SWA is 3-core, which also supplies the earth. I don't know how to categorise the domestic supply - standard? I don't have a 3-phase, for what it's worth.

The two advantages of having three phase are 1) Motors are self starting due to the rotating magnetic field and 2) increased power due to effectively three supplies. The bandsaw is not really an issue, my BS400 is on a std 13 amp supply and has never tripped, if you were looking at a P/T then you will have issues as my PT107 had to go onto a dedicated 16 amp supply but still a type B RCBO.

Partly the reason for my original post was that I'd contacted Felder for a quote on an N2-35. Their 1.5kW motor requires a 16A type C. Some of the larger Axminster saws also have this as a recommendation/requirement. But as you say, it all depends on the testing.

 

[Myfordman]

Its nice to know that folk are still getting use out of my motor paper written a good while back!

You will have no problems all day long starting a 3hp 3phase motor via an inverter. If the motor is dual voltage then a basic vector inverter is the way to go, If the motor is single voltage 440v then use a voltage converting vector inverter.
No need for type C breakers.

For one of my associates I set up an inverter programmed especially to start and run a 5hp motor and run from a 13amp fused plug. Not recommended for the unwary but it can be done and he re-sells them round the world.

HTH
Bob

 

[Ttrees]

For one of my associates I set up an inverter programmed especially to start and run a 5hp motor and run from a 13amp fused plug. Not recommended for the unwary but it can be done and he re-sells them round the world.

HTH
Bob

I run my machines from the 13 a plug, and it was mentioned to me before, that there must be adequate supply, i.e...
"As for the supply to the VFD that needs to be 20 amps and not the motors FLA"

Something seemingly related to the lifespan of the IGBT's, or input rectifier gizmo's
I guess... which for the cheapie 100 quid units I assume, more fragile in that sense.

Not that I can afford the fancy units, or indeed getting a 16a socket installed, so make do with what's not recommended,
and be aware of the limitations of the supply, and the cheapo VFD.
Being a hobbyist, the amount of work the machine needs to do could be classed at the bottom
or possibly "trade" at best, but not industrial use.
I can deal with that.
It seems the vacuum cleaner can be as bad, especially if the blooming thing
starts coiling up the lead on it's own accord, (looped cables causes a magnetic field which heats up rapidly)
thus making the lead even more flexible, exasperating the issue further,
and with no good way of clamping the lead, the thing could be dangerous to the un-aware.

I keep a feel of those leads and never noticed warming from my TS or bandsaw,
though I understand I won't be feeling the line what's going into the old fusebox in the shed, nor CU in the house.
Inspection of the plugs is another thing worth looking at, and obvious I never seen signs of anything like that, otherwise getting the 16a would've been a priority.

Here's a good website page with some stuff which might give hints of that, worth a little read through.
https://toughleads.co.uk/pages/why-does-my-plug-get-warm
I must make note that I don't use anything else but the machines on their own though,
as I wouldn't know if I was pushing the limits or not.
I always switch off everything before leaving.
For instance there was a little mouse pitter pattering on the felt yesterday,
and I've got an old pillar drill which the wire is getting perished, which I'm gonna sort soon.
Loads of reasons to be wary of anything electrical, and not just cuz I've got electrickery in a tabernacle.

Hopefully these cheapie units might keep me goin until maybe one day,
I have the option to get the supply, and go for a digital phase converter .
Until then, I'll be a cautious hobbyist, with a bigger framed bandsaw, (think undersized impeller)
and large table which is certainly cleaner than the machine I had before,
this one looks a dead ringer for it below, and the way I see things, better to get something which has momentum from the wheels, as I guess there is efficiency there should the machine not struggle, shouldn't be using up that much more juice than when running stationary.
Must have a look one of the days and see exactly how much juice it's using cutting, as
it was only using the same amount as me auld laptop, (though admittedly old and overheating)
We've got one of those smart meters now, so would guess it'd give me a better idea than before.

All the best
Tom

 

[thikone]

I’ve been looking at various options for the bandsaw, the RP Sabre 450, the Laguna 14BX, the Axminster AP2029B. They’re generally single phase, but what worries me is the peak draw on start up.

Yes, the peak draw on start up is high but it is only for a very short period. Don't confuse maximum rating with peak rating. Your cable can withstand a few thousands A for a short period.

I’d also like to have my shop vac on, the lights on and perhaps even the radio, which doesn’t leave a lot from 3.9kW. I could turn all those on after the saw’s started, but I feel like I’m probably pushing it.

They should be fine. There might be a short time voltage drop but usually other devices are tolerable to that. In worst case you might see some flicker.

So now I’m wondering if I should consider a 3-phase motor along with an inverter/VFD? I get the impression that 3-phase motors are significantly more efficient than single-phase, so am I right in thinking the overall draw should be substantially lower, even after allowing for the inefficiencies in the VFD? If so, does anyone have any indication as to what sort of peak current a 3-phase setup, say for a 3HP motor would draw? Has anyone stuck an inline ammeter in front of a bandsaw/VFD and taken measurements?

While VFD is a nice device (programmable, variable speed and such) and 3 phase motor is indeed more powerful and efficient, even 1 phase band saw of 3 kW will work just fine. You have to make sure that workshop cable is connected in your house to MCB of type C (it has slower reaction time and can tolerate like 10 times higher short time current). And that MCB has to be connected to a RCD (Residual Current Device or Residual Current Circuit Breaker) directly. And that RCD should be like 40 A at least but it usually is.

In my setup I have exactly this but 3-phase. From RCB wires go to MCB of type C, 16A. From MCB I have 20 m of cable, 5x4 mm² to a room in the basement. There I run Hammer A3-41 with 4 kW 3-phase motor and a chip extractor with 1,5 kW that starts 3 seconds after (with the help of additional circuit). No problem with starting and no flickering in the apartment nor in other apartments of my neighbors. I can even use chinese spot welder that usually triggers B 16A MCB but has no problem with C 16A MCB. But they both, type B and type C, will trigger if continues current is more than 16A for a longer time (operating time, like 0.04 to 5 sec).

So, don't be scared of peak current. Your cable can withstand a few kA for a short period. And start up time is a short period. Basically, the current can be as high and for as short time as to not heat the wires over maximum temperature. Since they have some thermal mass it takes some time, maybe even some seconds.

What you see as 20 A limit is for continues operation. And that depends basically on how much resistance wires have (that results in heat), then how good they are dissipating that heat and what maximum temperature they can withstand. While round wires grouped together in a cable have difficulty dissipating the heat, something like thin and wide traces on a PCB (printed circuit board) dissipate very well and have lie ten times bigger current rating. Or exposed cables have better dissipation than hidden in the wall. On the other hand, wires with a PTFE (Teflon) isolation can withstand like 250°C and therefore have higher maximum current rating.

Also, even if motor is 4 kW it doesn't really mean that it consumes 4 kW non-stop all the time. This depends on the load and without load it can be like 10-30% of that. If it trips the MCB while you cut then it was too much in total. In that case you might start thinking about turning off other devices.

 

[Jackie H]

Hi,
I just picked up on this thread. I may have missed something however the first thing that struck me was 'why is the 6mm swa on a 20 A MCB'? I have a similar setup with 6 mm swa cable running to my garden workshop which is 30 m away. This is fed from a BG MCB B40 at the main consumer unit to a garage type consumer unit in the workshop. This has 2 x 16 A MCB circuits (13 A power sockets) plus a lighting circuit. I have a Charnwood TS 13A plug 2200W and I need to run this on one power circuit when I am using the dust extractor (RP CamVac twin motor) on the other power circuit. If I try to run both on the same circuit it trips the workshop CU but not the main consumer 40A MCB. I seems a big drop from 40A (mine) to 20A (yours). It may be worth asking your local sparky to check out the calcs and that it is not a main CU capacity issue. I recently had my incoming supply uprated to 100 A from 60 A and a new main consumer unit so no issues with capacity.

 

[AndyMc]

Hi,
I just picked up on this thread. I may have missed something however the first thing that struck me was 'why is the 6mm swa on a 20 A MCB'? I have a similar setup with 6 mm swa cable running to my garden workshop which is 30 m away. This is fed from a BG MCB B40 at the main consumer unit to a garage type consumer unit in the workshop. This has 2 x 16 A MCB circuits (13 A power sockets) plus a lighting circuit. I have a Charnwood TS 13A plug 2200W and I need to run this on one power circuit when I am using the dust extractor (RP CamVac twin motor) on the other power circuit. If I try to run both on the same circuit it trips the workshop CU but not the main consumer 40A MCB. I seems a big drop from 40A (mine) to 20A (yours). It may be worth asking your local sparky to check out the calcs and that it is not a main CU capacity issue. I recently had my incoming supply uprated to 100 A from 60 A and a new main consumer unit so no issues with capacity.

Unfortunately that's the maximum current given the length of the cable. This is consistent with the voltage drop calculator (Voltage Drop Calculator)
- 3-core SWA
- 60M long
- Running a lighting circuit
- Underground
- Ambient temp 20 deg C
- Touching layout
- 1 circuit

If anything, 40A for 30M seems high - you might want to check your own setup too.

 

[Jackie H]

Hi,
Yes, thanks and interesting. I am not an electrician and my sparky did all the original calcs. The installation is certified so at least I am covered on insurance LOL.
Looking at your calculator it does seem that the extra length makes all the difference.
Out of curiosity I checked on another calculator. With my set up, using a design load of 35A (MCB = 40), 240 as the voltage & 30d temp etc a 6 mm cable gives a max of 36 m and all the parameters are OK per below.

This circuit design load is 35 Amps
Chosen Circuit Breaker OK
Chosen cable capacity in this circuit is 41 Amps (Iz)
Your chosen cable size for this load is OK
Volt drop is 7.665 Volts
Cable size OK for volt drop
Maximum Volt drop is 9.2 Volts ie 3% for lighting and 5% for other circuits Here we have used a 4% average.
Maximum length of cable for this circuit is 36.01 Mtrs you have selected 30Mtrs.

Keeping the parameters the same and specifying 60m gives.

This circuit design load is 35 Amps
Chosen Circuit Breaker OK
Chosen cable capacity in this circuit is 41 Amps (Iz)
Your chosen cable size for this load is OK
Volt drop is 15.33 Volts
Cable size too SMALL for volt drop
Maximum Volt drop is 9.6 Volts ie 3% for lighting and 5% for other circuits Here we have used a 4% average.
Maximum length of cable for this circuit is 37.57 Mtrs you have selected 60Mtrs

It seems it is all down to the voltage drop over the distance as opposed to the loading.

 

[Spectric]

I just picked up on this thread. I may have missed something however the first thing that struck me was 'why is the 6mm swa on a 20 A MCB'?

A lot of people just think of cable size equals x current but there is a lot more to it. Distance = voltage drop and current capacity is also decided by how the cable is installed, ie on a tray in air, clipped to a wall or burried will all decide the outcome.

The best way to get to grips with this is not to think of the cable as just a length of wire but rather as resistors, with no load Vout will equal Vin and adding the load gives the volt drop, more load more volt drop.