Vfd advice please

[not done it yetParticipant @notdoneityet]

Cooling air flow is a function of the cube of the fan speed.

I hope I got it the right way round, but what I mean is that slowing the fan (motor) speed by a half reduces the air flow by a factor of eight.

That may not be too important if: a) the fan was generously sized for normal 50Hz operation, b) the motor is not laboured for anything more than for a short time and c) the motor has been designed with good overload capability.

Unfortunately, some motors are not even rated for continuous running and can sail close to the wind, even at 50Hz operation. The onus is on the operator to run the motor without ‘letting out the smoke’.

I smiled at the comment from Andrew “even with a fan running at normal speed” but entirely agree with his post. I was wondering at what speed a motor becomes over-loaded.

[Philip MajorParticipant @philipmajor52482]

Hi Guys, all very very interesting, I now have glazed eyes! However I have selected the inverter from inverter supermarket, in fact the one suggested.

Overall this has been a great and very helpful thread, many thanks

[old martParticipant @oldmart]

Good for you, Philip, there have been several people on this forum who have commented favourably about their after sales support, should you need it. I had a query, and emailed them, and got a swift sensible reply.

[DC31kParticipant @dc31k]

Lots of replies answering his question, but could I break the trend and suggest that you may face challenges if you use a 3/4hp motor for a 3" wide sanding belt?

Further, sanding and linishing (an abrasive machining process) generally requires a high surface speed to be effective. Thus, speed control downwards from a motor's synchronous speed, even if a two pole motor is used, may not be necessary.

Simply put, you want the belt to whizz along powerfully, not crawl along, slowing down as soon as you try to sharpen a pencil on it.

So you need to start with the surface speed you need, see what diameter drive pulley you will use and select a motor rpm to give that surface speed. Then look at what motor is required to produce the necessary torque at that rpm.

To me, the 3/4hp seems way underpowered.

[John HaineParticipant @johnhaine32865]

Well, Axminster sell a trade belt sander with a 750W induction motor; or a craft sander with the same belt size and a 370W motor. 3/4 HP looks about right.  Both take 100mm wide belts, the trade one says the belt speed is 840 m/minute.

Edited By John Haine on 14/02/2021 09:56:36

Edited By John Haine on 14/02/2021 09:58:23

[Philip MajorParticipant @philipmajor52482]

Hi again, food for thought in these posts… the reason I am using 3/4 hp 3 phase motor is because I have one. And again I have a 7" bar of nylon to use as drive. This should give a belt speed of about 820 m/min. However on YouTube ( always with pinch of salt) advice was when using thin steel there is a tendency to burn the burn the part with high speed belt, hence speed reduction. Of course the other demand is to convert single to 3 phase. . I could wire it into my static converter like my other machines, but I want to make this belt sander more mobile. I know there are many options to this, but its the onei have chosen.

Of course I could just spend my money on a new 1hp single phase motor!!

[bernard towersParticipant @bernardtowers37738]

The traverse direction dictates whether the table moves on the cut or return stroke!

[Nick WheelerParticipant @nickwheeler]

Posted by John Haine on 14/02/2021 09:56:02:

Well, Axminster sell a trade belt sander with a 750W induction motor; or a craft sander with the same belt size and a 370W motor. 3/4 HP looks about right. Both take 100mm wide belts, the trade one says the belt speed is 840 m/minute.

They're meant for sanding wood. If you intend using one to shape metal, as a belt-grinder, you'll quickly discover that they don't have enough power. Like trying to use an air-tool off a small compressor.

[old martParticipant @oldmart]

The power available to the belt could be maximised by making the drive gearing equal the 840m/min with the motor running at 80Hz, 1.6 times its standard speed. That would give greater power at the lower speeds which would be used more often.

[noel shelleyParticipant @noelshelley55608]

I think my axminster 6" belt sander has a 3/4 Hp motor 1Ph. I can see no reason to slow it down. It is used for metal so beware of metal particles getting into ANYTHING electrical, smoke and sparks will ensue. Noel

[DiogenesIIParticipant @diogenesii]

I have a 2" vertical linisher, running @ 2750 over a 6" pulley.. ..um, think that's about 1300m/m? ..it burns wood like a taxpayer-funded power station on double-subsidy, but haven't noticed a problem with steel, I think it depends to a large degree on belt material / spec. and (especially) the avoidance of excessive pressure when cutting – I use it for both grinding HSS toolbits (fine 'blue' zirconia or 'red' ceramic), or rousting-off mild steel for fabricating etc., using coarse Alox..

I think if you have the 3ph motor anyway, variable speed will be handy for mixed workshop 'jobbing'.. ..Noel's right about the particles, I might add that neither would I have mine in the same room as any 'precision' machinery..

..I built it out of scrap – there's some pics in an album..

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