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wyn
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As Robert says 1200mm ?
Probably 120mm That could be where your "10X out" is coming from.
HasBean
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I've been thinking of making a digital rpm display for my lathe (for some considerable time 
) but as yet it hasn't left the back of the fag packet yet.
Now I know the difference between rpm and circumfrential speed (yes I know the spelling's wrong but Mr Strongbow is visiting
) but I think the ability to calculate the correct rpm by the device depends a lot on the reflective 'spot', ie most of the designs specify more than 1, if you have the space, as this increases accuracy as long as the counter can trigger fast enough.
I seem to remember if you take a disk of cardboard and mark it evenly with 50 err... marks then when rotated under a fluorescent light it should appear stationary at some multiple of 50 (due to the 50Hz flickering) so this should be a good indication of the meter working if it detects this.
If you're really bored read this lots of info
) but as yet it hasn't left the back of the fag packet yet.Now I know the difference between rpm and circumfrential speed (yes I know the spelling's wrong but Mr Strongbow is visiting
) but I think the ability to calculate the correct rpm by the device depends a lot on the reflective 'spot', ie most of the designs specify more than 1, if you have the space, as this increases accuracy as long as the counter can trigger fast enough.I seem to remember if you take a disk of cardboard and mark it evenly with 50 err... marks then when rotated under a fluorescent light it should appear stationary at some multiple of 50 (due to the 50Hz flickering) so this should be a good indication of the meter working if it detects this.
If you're really bored read this lots of info
bill_gj
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If the pulley on the chuck shaft was really 1200mm, that 1.2 metres or about 48" in old money - typing error?
The marked motor speeds are usually at either 50Hz or 60Hz, depending on whether it's a European or USA-made motor. I think the no-load shaft speed is dependent on the mains frequency.
So, if your motor is a European-made unit, running at 1450rpm, and the effective size of the motor pulley is 57mm, to get an output (chuck) speed of 480rpm, the chuck pulley effective diameter would be about 172mm, or about 6.8 inches.
I said 'effective' diameter because that depends on how worn your pulleys and vee-belts are.
If your pulley is actually 120mm, with 1450 motor rpm at 50Hz, 1450 x (57/120) = 689rpm approx.
If it's a USA motor intended for 60Hz then I think your chuck speed would be about 574rpm.
None of these calculations seem to align with what you actually measured, so I'm going to have a beer.
Shenion - Help !
The marked motor speeds are usually at either 50Hz or 60Hz, depending on whether it's a European or USA-made motor. I think the no-load shaft speed is dependent on the mains frequency.
So, if your motor is a European-made unit, running at 1450rpm, and the effective size of the motor pulley is 57mm, to get an output (chuck) speed of 480rpm, the chuck pulley effective diameter would be about 172mm, or about 6.8 inches.
I said 'effective' diameter because that depends on how worn your pulleys and vee-belts are.
If your pulley is actually 120mm, with 1450 motor rpm at 50Hz, 1450 x (57/120) = 689rpm approx.
If it's a USA motor intended for 60Hz then I think your chuck speed would be about 574rpm.
None of these calculations seem to align with what you actually measured, so I'm going to have a beer.
Shenion - Help !
HasBean
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I think the problem is everyone getting confused between RPM and circumfrential velocity 
Changing the diameter will alter the velocity but not the RPM
As bill_gj said under no load a 4 pole induction motor will run at 1450 ish rpm at 50Hz so this would be a good place to start to test.
Changing the diameter will alter the velocity but not the RPM
As bill_gj said under no load a 4 pole induction motor will run at 1450 ish rpm at 50Hz so this would be a good place to start to test.
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shenion
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Yes, motor speed is tied to mains frequency (50/60Hz).
Single pole will try to rotate once per AC cycle 3000/3600
Double Pole will try to rotate one half turn per AC cycle 1500/1800
Triple pole will try to rotate 1/3 turn per AC cycle 1000/1200
Quad pole will try to rotate 1/4 turn per A/C cycle 750/900
There is some slip in the core, usually 50-60 RPM.
Wanted to measure my drill but my RPM gauge only works outdoors. Inside it will always read 3600 RPM as it picks up the flicker of the lights. Or it will give odd readings.
Single pole will try to rotate once per AC cycle 3000/3600
Double Pole will try to rotate one half turn per AC cycle 1500/1800
Triple pole will try to rotate 1/3 turn per AC cycle 1000/1200
Quad pole will try to rotate 1/4 turn per A/C cycle 750/900
There is some slip in the core, usually 50-60 RPM.
Wanted to measure my drill but my RPM gauge only works outdoors. Inside it will always read 3600 RPM as it picks up the flicker of the lights. Or it will give odd readings.
HasBean
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Ooh Scott it's giving me a headache
Yup I know those as synchronous speeds but as 2,4,6,8 poles
peterd51
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Hi,
thanks for the input. I used a digital caliper to take the reading and it looks like I missed a decimal point...it must have been 120.5, or whatever. Having said that, the caliper's also new and 'untested', it was doing odd things, like zero it, open it, close it fully down and it now reads 50mm or something, so it may have actually read 1200 on the pulley. I'm not usually /that/ thick!
If I'd been working in inches then I'd have known it was wrong as I can see that the pulley's about 5" across...
The pillar drill is a Turner and Walker, made in New Jersey, USA, but I'm sure the plate says it's 50 Hz. It could have been made for export to Europe,
Regards
Peter
thanks for the input. I used a digital caliper to take the reading and it looks like I missed a decimal point...it must have been 120.5, or whatever. Having said that, the caliper's also new and 'untested', it was doing odd things, like zero it, open it, close it fully down and it now reads 50mm or something, so it may have actually read 1200 on the pulley. I'm not usually /that/ thick!
If I'd been working in inches then I'd have known it was wrong as I can see that the pulley's about 5" across...
The pillar drill is a Turner and Walker, made in New Jersey, USA, but I'm sure the plate says it's 50 Hz. It could have been made for export to Europe,
Regards
Peter
Pete.
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Are you quoting for 3-phase motors here? My 2-pole motor spins at ~2800rpm and my 4-pole at ~1450rpm. My milling machine was retro-fitted with a 2-pole instead of a 4-pole and all the speeds are doubled
Peter there's no way the diameter is going to make a difference. If something turns at 5rpm then a point on the circumference orbits the axis 5 times each minute. It makes not a toss of difference whether the thing that is turning is 1cm or 1m in diameter. The only way I can see the gadget you have becoming inaccurate is when the reflective strip passes by the detector too quickly for it to register either at all (in which case the machine would be seeing a constant blur and would likely either register a zero or infinite rpm) or reliably every time the reflector passes (in which case the rpm given would be an underestimate). This is slightly murkier water and almost guaranteed to confuse because this will depend on both the rpm and the diameter!
EDIT:
I somehow missed the last pages of this thread so I guess you've already worked it out! Sorry for repeating things!
EDIT:
I somehow missed the last pages of this thread so I guess you've already worked it out! Sorry for repeating things!
peterd51
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Hi,
Ok, getting closer now. The drill motor runs at 1490 rpm measured. The bigger pulley is 120mm measured with a 'standard' caliper and a ruler. Allowing for a bit of 'loss' though the belt and inaccurate pulley diameter readings, 700 rpm would be about right but the tester reads 480 so that looks a bit low to me.
I stuck a strip on the car bottom pulley started it up and the rev counter shows a touch under 900. I got a reading of 849. The Haynes manual states it should be 800 - 900 rpm.
My meter may be reading slightly low but it's close enough! I'll get a better 'feel' for it as I use it on different things over time.
Thanks for the suggestions.
Regards
Peter
Ok, getting closer now. The drill motor runs at 1490 rpm measured. The bigger pulley is 120mm measured with a 'standard' caliper and a ruler. Allowing for a bit of 'loss' though the belt and inaccurate pulley diameter readings, 700 rpm would be about right but the tester reads 480 so that looks a bit low to me.
I stuck a strip on the car bottom pulley started it up and the rev counter shows a touch under 900. I got a reading of 849. The Haynes manual states it should be 800 - 900 rpm.
My meter may be reading slightly low but it's close enough! I'll get a better 'feel' for it as I use it on different things over time.
Thanks for the suggestions.
Regards
Peter
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Do you know anyone with a calibrated tacho?, get them to read something then compare with your reading.
Drains
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Hi Folks!
Sorry, a bit late on this one, and I know the questions have been answered, but if I may i'd like to add some stuff...
Peter, your error is simply in confusing rpm with tip speed. The number of revolutions per minute are just that, eg, how many times a minute does the thing go round.
Tip speed is, well, just that - how fast is the outside edge of whatever is going round. If the circumference is 1", then at 1000 rpm, the travel speed is 1000" per minute. If the circumference is 2", then it's 2000" per minute. You notice the rpm is the same for both sizes.
Working out the rpm of a pulley driven drill (or similar machine, or gear driven one for that matter) is a matter of working out the pulley ratios (or for gears, tooth ratios). If the outside diameter of the driven pulley is 2", and the o/d of the driving pulley is 4", then the ratio is 2/4 (or more plainly, 1/2). Multiply that by the motor speed, in this case 1450 rpm, and you get a driven speed of 725.
Motor speeds relate directly to ac frequency. A 2-pole motor will run at a theoretical speed of 3000 rpm (50hz x 60 seconds, 2 poles per hz), a 4 pole at 1500 rpm (still 2 poles per hz, hence half the speed of a 2 pole motor), and a 6 pole at 1000 rpm.
Motors don't actually turn at those speeds cos they don't keep up with the frequency and are not 100% efficient, and the more power you absorb from the motor the slower it runs, hence the no-load speed of your motor is 1450, not 1500.
I hope that lot is some use...
Si
Edit - Now reading back over this thread, I see i'm just repeating other people. B*gger.
Sorry, a bit late on this one, and I know the questions have been answered, but if I may i'd like to add some stuff...
Peter, your error is simply in confusing rpm with tip speed. The number of revolutions per minute are just that, eg, how many times a minute does the thing go round.
Tip speed is, well, just that - how fast is the outside edge of whatever is going round. If the circumference is 1", then at 1000 rpm, the travel speed is 1000" per minute. If the circumference is 2", then it's 2000" per minute. You notice the rpm is the same for both sizes.
Working out the rpm of a pulley driven drill (or similar machine, or gear driven one for that matter) is a matter of working out the pulley ratios (or for gears, tooth ratios). If the outside diameter of the driven pulley is 2", and the o/d of the driving pulley is 4", then the ratio is 2/4 (or more plainly, 1/2). Multiply that by the motor speed, in this case 1450 rpm, and you get a driven speed of 725.
Motor speeds relate directly to ac frequency. A 2-pole motor will run at a theoretical speed of 3000 rpm (50hz x 60 seconds, 2 poles per hz), a 4 pole at 1500 rpm (still 2 poles per hz, hence half the speed of a 2 pole motor), and a 6 pole at 1000 rpm.
Motors don't actually turn at those speeds cos they don't keep up with the frequency and are not 100% efficient, and the more power you absorb from the motor the slower it runs, hence the no-load speed of your motor is 1450, not 1500.
I hope that lot is some use...
Si
Edit - Now reading back over this thread, I see i'm just repeating other people. B*gger.
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