Pulley calculation un relable websites and actual speeds

I have been using Pulley and RPM Calculator at www.culvermotor.com/Engineering-Formulous/Pulley-and-RPM-Calculator.html

I used my rpm final speed of 2820rpm with Diameter1 of 1 and Diameter2 as 7 representing pulley sizes in inches. This gave 402.86rpm .My final pulleys are same size. Now I would expect my inverter at 50% to run half this and 100% run 402.86rpm . I bought a simple light rpm detector and amazingly at 50% the rpm is 800rpm. Can anyone help or explain what is wrong here please?

M

Edited By Martin Newbold on 30/08/2017 18:36:29

What is the actual measured input rpm when the output rpm is 600?

Edited By Bikepete on 30/08/2017 18:32:39

the inverter shows 50hz at 800rpm but my HS2234 digital tachometer is temperamental it usually gives you one acurate measurement before it gets confussed a battery reset removal usually works . 542 rpm is shown on inverter at 1 amp motor rpm is set and  moror is rated at 5.5 / 3.0  amp

M

Edited By Martin Newbold on 30/08/2017 19:43:31

Ah - if the inverter shows 50 Hz not 50 %, then I think I know what's going on.

50 Hz is 100% of normal mains frequency, so it will turn a two pole motor at around 2820 rpm like it says on the motor nameplate. Which with a 1:7 pulley reduction would give 403 rpm.

So when you had the inverter set at 100 Hz, thinking it was 100% of 'normal speed', your motor was actually whizzing round at approx twice normal speed i.e. 5640 rpm! Giving you the 800ish rpm output, after the 1:7 reduction.

That is quite possibly dangerously fast for a standard motor. Might be an idea to look at configuring your inverter so it can't go above say 60 Hz.

Perhaps someone else can double check my workings, though. I'm no longer quite sure what you measured - any chance you could specify when you quote rpm figures as above whether they are at the motor (i.e. input) or at the output (i.e. the output shaft after the pulley reduction)? And can you confirm if your inverter figures are Hz or percent?

Edit again - if the inverter shows 442 rpm (i.e. motor shaft input), and you are getting 800 rpm output after your gearing, then your pulleys are stepping up the speed 2:1! This is very strange if the pulley sizes are 1 inch and 7 inches. And what I wrote above is probably not the case.

Don't suppose you could post a picture of the set-up in case that clarifies anything? 

Edited By Bikepete on 30/08/2017 19:07:24

Edited By Bikepete on 30/08/2017 19:23:16

Edited By Bikepete on 30/08/2017 19:30:26

If the motor being used is the one shown in your "new 3ph motor" album, it is indeed a two pole motor that should run at 2820 rpm on 50 Hz.

Perhaps disconnect the belts, set the inverter at 50 Hz and measure the motor rpm, so as to have a known starting point?

Why does the inverter go 100hz. i presume it is driving it twice as fast then, and over driving it perhaps there is a limiter in inverter to stop this i will look.

I have a 7" pulley d2 and a four way pulley on lowest pulley is driving the 7" picador pulley. This rotates a shaft with a 5" pulley on it driving the lathes 5" pulley

aha it sinks in the cone pulley drive is RDGTOOLS 4 STEP VEE PULLEY 24MM BORE 2" 3" 4" 5" DIAMETER A SECTION PULLEY , which is 2" not 1"

This is why its driving 805.71 lol

OK THANKS EVERONE

Somtimes you need to look twice have found the limiter an set it to 50hz. Am still quetioning why its only drawing 1 amp .Is this more to do with the load my bearings etc. The motor plate states 5.5/3 does this not equate to real world value?

Thanks again

M

Hi Martin, nameplate current is usually the maximum i.e. at full load. So it will be much less normally. 1 amp sounds fine. Cheers Pete

Induction motors draw a relatively low current under no-load conditions. Only when the motor is fully loaded will it draw the current shown on the rating plate (so in the case of a lathe, when it is taking a substantial cut, enough to make the motor drop slightly in speed).

Ah , thanks Steve, Am going to have to redrill my old picador pulley to get back to 1" as cant find a 24mm bore pulley which is 1" anywhere. That is interesting information thank you

Here is a video **LINK**

Edited By Martin Newbold on 30/08/2017 22:39:10

What size belt are you planning to run on your 1" pulley?

It's far too small to get any sort of sensible power transmission from a Vee belt. For example the generally quoted official minimum diameter for an A section belt to transmit its rated power is around 4 or 5 inches diameter. Exact figure is pretty much academic as it depends on how the belt maker specifies things, belt speed, pulley centre distance and the drive ratio. Two pulleys of the same diameter giving better drive than a vastly dissimilar pair.

1" diameter is poly-vee or flat belt territory.

Multi-step Vee pulleys like the RDG tools one mentioned above having seriously undersize sheeves at the small end are generally intended for bench drills and the like. In that sort of application serious drive inefficiencies at the high and low speed ends of the speed range is probably a good safety feature as the drive will stall out and slip on a serious hang up rather than throwing bits across the shop.

Given the slap-dash way far too many folk approach the ordinary bench drill the idea of completely efficient power transmission and toque multiplication on the bottom speed range is "somewhat worrisome". I know my big, all gear drive, Pollard 15AY has caused me a frisson or two when I've been a bit un-cautious in "hurry-up'n get'er done" mode. And I've been used to effective drives on drills for maybe 35 years as my previous one, also by Pollard, had poly-vee drive.

Clive.

Edited By Clive Foster on 30/08/2017 23:06:03

.

Well said, Clive

... reminds me of similar advice offered in 2016

**LINK**

http://www.model-engineer.co.uk/forums/postings.asp?th=118632&p=3

MichaelG.

Martin, am i reading your post correctly - " ...going to have to redrill my old picador pulley to get back to 1" as can't find a 24mm bore pulley which is 1" anywhere. "

Does this mean you want a 1-inch diameter pulley to fit on a 24mm shaft? That leaves precious little metal between the shaft and the belt groove - 0.7mm to be exact. Won't be very strong and could split if belt tension too high?

The motor on my lathe is 6 pole, Nominal speed at 50 hz is a few percent off 1000 rpm as a result. This means I do not need to have big pulley ratios to get sensible chuck rpm values. The middle cone pulley position of the three positions is close to 1:1 and the chuck rpm is 800. Highest speed position is 1300. With back gear the lowest speed is 66 rpm. My inverter only gets changed from 50hz occasionally.

Is you motor a replacement, if so you should have got one to match the nominal rpm or the original.

Martin C