240/415v panel saw inverter drive size

[andythefinn]

Hi.
I have a 4kw panel saw and am going down the inverter drive route.
What i would like to find out for sure is that would a 4kw inverter be big enough, or would it be advisable to go a size bigger ? (say 5.5 or 7.5kw)
Although this is the largest 3 phase machine I use.
Also, will there be a noticeable difference in power and torque having used the machine previously where i had 3 phase supply and soon to be from a 1 phase supply input.
If a 4kw inverter is large enough, are there a big differences in cheap v's expensive models.
(cheap example attached)

Thanks in advance for any help.

Andy

 

[Myfordman]

Lots to be aware of here.

1) Inverters do not normally increase the voltage from input to output ie they are 240 in/ 240 out or 415 in /415 out
2) Normally inverters at this power level need a three phase input.
3) Very few standard inverters will take 240v single phase in and produce 415v 3 phase out.

4) There is at least one supplier who modifies inverters to do what you want but this usually means whilst the equipment is CE marked but in truth no longer complies to the makers spec and may not be truly CE approved. This may or may not concern you or your insurers.

5) The modification process usually needs to start with an inverter of twice the power compared to the required output so the price is higher. Sensorless vector inverters are the best type to choose.

6) Inverters do not produce a neutral as standard. Your saws control system might require a neutral as well as the three phases.

I strongly advise that you find a supplier and talk to him about the specifics of your requirements to makes sure all will work for you.

Once you get a correct installation inverters are very versatile and reliable. Get the wrong set up and you will have continual niggles.

MM

 

[andythefinn]

What I meant was since the motor is dual voltage 220/415, so I will get it swapped over to 240v 3phase. 240v 1 phase in, 240v 3 phase out.
Thanks.
Andy

 

[Myfordman]

It would have been helpful to have stated that in the first place.

Vector inverter would still be preferable type and all the control gear will need modifying so that the control buttons go to the logic inputs to the inverter and all the contactors are removed from circuit so that the motor connects directly to the inverter with no switches in the path.

The inverter will have to be programmed to accept your stop, start buttons (and any safety interlocks if you are employing anybody)
Programming should also include a sensible (not too fast) start up time and an appropriate braking regime to achieve the 10 sec stop and also the no volt release functionality. Although the latter is usually a default setting.

 

[andythefinn]

I have asked one inverter company about the size of inverter needed, and he said 'for a 4kw motor a 4kw inverter is big enough'. I was just curious to know if anybody in the same situation finds that not just on paper, but in the real world it is so. Is it the same power and torque, or would it be advisable to get a size larger.?
Also, you see the chinese specials on certain auction sites at £115 for a 4kw inverter - would it be worth the risk of buying such a cheapy to see just how long it would last etc. ?
Cheers.
Andy

 

[wizard]

The main problem may be setting it up. One of mine was just wire it up and go the other one needed programming and i had to be talked through it on the phone, it was not easy.

 

[Myfordman]

Try out your understanding of Chinglish on the manual for the cheapy inverter here
http://www.jinlantrade.com/ebay/invertermanual.pdf
You will have to understand enough to do the programming - it is not just a case of wiring up to the diagram

 

[andythefinn]

Thanks for the replies.
No matter what language it'd be in, anything to do with electrics is a foreign language to myself - can barely wire a plug.. But I have a good electrician. Maybe see if he could make head or tail of that.
So.. In your opinions would a 4kw inverter be big enough ? Or get the next size up ?
Cheers.
Andy

 

[CHJ]

Difficult to be specific regarding that question.
A lot depends upon the start up resistance/mechanical load of the machine.
Connected to a 3ph. mains source the machine may well take considerably more than its rated running consumption on start up.
Set likewise on a convertor it may well have problems starting if the convertor shuts down because of current overload. (this trip time is sometimes adjustable within small limits)#

Suitably programed the convertor may well be able to soft start* the machine (via frequency regulation current limitation etc.) to get the machine up to speed without the excessive current spike.

* one big advantage of electronic control, also most will have a programmable braking function available, usually requiring an additional resistor to absorb the generated current.

# The one on my lathe is set tight to shut down if I get a bad catch or forget and leave spindle lock engaged.

 

[Myfordman]

I strongly suggest you talk to your tame electrician and show him the manual and make absolutely sure he is confident with programming the inverter before you buy. Take heed of Wizards comments too.
In my experience most electricians tend to "know what they know" and having learned via the apprentice route are rarely up to speed with things like inverter programming. It is always a pleasant surprise to be proved wrong however!

Most inverters will allow short term overload/boost mode of 150% of programmed capacity during use.
If you use a more powerful inverter, it ought to still be programmed down to the power of the motor, otherwise the internal protection algorithms will not protect the motor properly which means you can't benefit from the extra capacity.

As far as I can tell from the ebay advert, the one you proposed does not offer sensorless vector mode unless you know better.

Caveat emptor!