single phase motor-calculating rpm

Hi, my hobby is making mirrors for telescopes.

I am planning on making a mirror grinding machine. A 3 phase motor plus inverter from Machine Mart would come to over £400, which is too much! So although it would give variable speed? (I am not sure of this?) I plan to get a single phase motor I can use without an inverter. Probably 1/2 hp or 3/4 hp. 4 pole.

I am planning to buy a washing machine pulley, around 14 inches in diameter. I can buy smaller pulleys from machine mart. The rpm of the motor will be 1500. I need my grinding table to rotate at around 40 rpm. How do I calculate the size of the second (and possibly third if necessary?) pulley and the length of belt to get the rpm reduced 40 rpm?

Also, can I just plug this kind of motor in to the mains or is there anything else I need to buy for it to work safely?

David

Hi David,

If you don't want to do the calculations yourself there are online Pulley Calculators, (or here). You want to reduce the motor speed down to 40 rpm, this I think would need a countershaft so the reduction is made in two stages. Timing belts will give a more compact unit than using Vee belts. I would recommend that you get a no volt release switch for the motor.

Thor

David. Have a look for a Parvalux motor on the bay. They often have reduction gearboxes of the ratios you need.

David

How much power do you need

Remember that gearing down your motor from 1400 to 40 rpm increase power by 35 x

I have just bought some nice 3 phase British made motors only .4 kW so 1/2 hp or so for £15 each

And suitable inverters for £30 each.......use pulleys to gear down to a nominal 40 rpm and then use the variable speed to fine tune...using the inverter really to give you a 3 phase supply.

You will easily find a package like I have described for under 50 £

Ian

Hi Ian,
Gearing a motor up or down does not change the power. (Other than losses in the gearing.) Gearing a motor down will increase the torque and gearing it up will decrease the torque.

Les.

I made a polisher from an old wiper motor and box with a pwm board for speed control. Worked fine on the 8" but struggled with the 10" mirror (F6.4).

Pulleys work in simple ratios according to diameter. A 15" pulley driven by a 3" one will turn 15/3 = 5 times slower.

So 1500 to 40rpm means reducing the speed roughly 40 times. a 2" pulley driving a 14" one will give you a reduction of 7 times, to just over 200 rpm. Your next ratio could be lower at about 5:1, say 2" to 10" to give you about 40 rpm.

An alternative is to use a tumble drier motor and poly-v belt and make a large pulley from wood. As the motor pulley will be about 3/4" diameter the pulley (which can just be flat sided) would need to be about 2' diameter.

Neil

Les

I was trying to keep it simple for david

explaining the difference between torque and power would be overkill for the application he was wanting the motor for

Ian

You would probably be ok with a 1/6th hp shaded pole motor. You sometimes come across nice little reduction gear boxes, this would be more compact.

Ian S C

Hi, thanks for all the replies.

Ian, where did you buy the motors and inverter? The Machine Mart inverter costs £274! Although it does go up to 2 hp.

I would like to get a 1/2hp motor 4 pole electric motor, as then I should defintely have enough power. A 20 inch mirror is pretty heavy!

I would prefer something I could just plug in. The manual for the Machine Mart inverter states 'All models must be connected via a separate isolator, and must be protected by a fuse or circuit breaker,' I could probably figure this out. But single phase motors seem a lot simpler, as I presume I can just plug them in to the mains?

The only thing I am slightly worried about is starting the machine at 40 rpm staight away, but I don't see why this should be a problem, as the mirror will be secured to the turntable.

David

The next thing is where to get a 'counter shaft' and timing belts? I presume I can connect the first smaller pulley directly to the shaft on the motor? i will need to wait until I have the washing machine pulley to see the size of the shaft I will need to fit the hole (bore?).

David

Would something like this be too small? **LINK**

David

http://www.ebay.co.uk/itm/Brook-crompton-electric-motor-220-240v-3-phase-0-25kW-1380rpm-50Hz-/271822769149?pt=LH_DefaultDomain_3&hash=item3f49e64bfd

http://www.ebay.co.uk/itm/Jaguar-IMO-VXSM40-1-1PH-200-240V-3PH-0-4KW-3A-Inverter-0-2-400Hz-/291410578600?pt=LH_DefaultDomain_3&hash=item43d96c8ca8

If you want top-notch advice on inverters, speak to Transwave or Newton Tesla. Smaller ones are much less that £274.

Neither are as cheap as Ebay but you can be sure the three-year warranty on IMO inverters will be honoured by them...

On the subject of telescope mirrors, this is a subject I have pondered on since I was in school. At the risk of pandering to my own interests, an article on how telescope mirrors can be ground at home,including making a grinding machine and other tools, wouldn't seem tout of place in MEW.

Let me know if you are interested!

Neil

I've bought most of my machine drive belts from Halfords or other motor factors but there are specialist suppliers out there.

Edited By Vic on 04/04/2015 19:23:37

The mind boggles; for the life of me I can't see how incorrect information helps anybody, beginner or otherwise.

Andrew

PS: We really need some more imaginative smileys; have a look at the 'practicalmachinist' forum for some good ones!

I don't think Les gave any incorrect information, also no smileys would suite me.

As to the OP's question, I think the motor is only a minor part of the project. A 20" mirror sounds a heavy item so designing and building the whole machine must be a major task. I imagine that the end result is something like a potters wheel?

The small geared motors mentioned wont take kindly to having a large heavy mass hung on their output shaft especially at start up.

I admit I know nothing of mirror grinders or polishers but wouldn't a friction drive say the motor spindle direct on edge or underside face of a large faceplate be suitable? Alternatively a large diameter pulley from a scrap washing machine with its polyvee to a 1440 rpm motor with a tiny pulley would put the speed in the right ballpark.

Ian P

It wasn't the correct information given by Les that I was commenting on.................

I use Parvalux 12 volt dc motor ,max amps 6,watts output 46,rpm 3000,rated continuous which has a gearbox fitted which gears it down to 50rpm,nm 5.35. This runs my home made machine hacksaw photos of which are in my photo album. It is compact with plenty of oomph, I don't know a great deal of elec theory but this works for me maybe it would work for you

Mick H

The nm converts to 3.9 odd ft lbs don't know if thats enough torque you would need but it pushes my hacksaw through a lump of 2" steel no problem

• Mick H