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18th Nov 2020, 11:05 PM #1Member
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Check calibration of a hall effect led rpm indicator?
Gentlemen; Several weeks ago I bought one of the commonly available Hall Effect rpm LED indicator units via E--y. This was used to check the actual chuck spindle rpm's at various belt settings of the Sheraton lathe that I recently rebuilt. It worked fine.
All of $25 or so for the indicator, sensor unit & magnet. The data label says it's accurate within 0.1%.
Since using the indicator to check a couple of other machines, I have suspicions that it may be over-reading.
For instance, after trying it on my AL-300 lathe that I had previously checked some years ago using a known accurate calibrated strobe-type rpm counter, it shows + 2% on those previous numbers.
On the HM46 mill it shows + 6% on the data plate quill rpm's. I didn't expect great accuracy there anyway. I doubt the numbers on the data plates of Chinese machines. Their 230v vs our 240v supply perhaps?
Today I tried checking it against the rpm on the pulley of the brand new Techtop drive motor for the Sheraton whilst free-running.
It's data plate says 1420 rpm, whilst the led tacho shows 1496, which is about + 5%. How reliable would that 1420 number be? Or is that an average for when it is under load?
Any ideas how to otherwise confirm the accuracy of the new led indicator? There is no adjustment pot on the pcb of the unit; all tiny fixed value surface-mount stuff.
Thanks,
H614
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19th Nov 2020, 12:58 AM #2
Hi H614,
What you are seeing is quite normal ! +/- 10 % As long as the reading is consistent over time. ie the same in an hour, a day, next week.
If you want to accurately calibrate your counter The strobe disk is a good way. Just put the magnet on a record deck turntable and use the strobe markings to check it.
The motor speed of 1420 is purely a nominal one ! It will vary with bearing loading and motor windage.Best Regards:
Baron J.
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19th Nov 2020, 10:58 AM #3Senior Member
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"..... data plate says 1420 rpm, whilst the led tacho shows 1496, which is about + 5%"
The synchronous (theoretical) speed of a four pole motor on a 50 Hz supply is 1500 RPM. The common electric motors available can never normally reach this speed of rotation - because of "slip frequency." It means the rotating spindle can't reach the synchronous speed of the electrical supply. They can go above 1500 RPM if, say, the motor was under heavy load and the load was suddenly removed it would reach a speed greater than 1500 RPM very quickly! It would of course slow down again. Very big motors have "overspeed" protection for this reason.
The data plate of the motor is there to indicate (often roughly and at some unknown test conditions!) what the motor is capable of doing at full load - KW (Horse Power) output, RPM (1420 in this case), current, PF, maximum temperature, etc.
So 1496 RPM at no load is fairly close to 1500 RPM, which seems reasonable - if the supply frequency was exactly 50 Hz! It may not be. Australian Standards require the System Frequency to be within + / - 0.100 Hz of 50Hz for 99.99% of the time!
If the supply frequency was 50.10, the theoretical speed of a 1500 RPM motor would be 1503 RPM. Not much, but the differenc would affect your readings.
The only way to really test you tachometer would to have a standard meter to compare it to.
Also, the tachometer's error may change with the speed it is measuring, and the claimed accuracy may only be a a very small speed range. As I said above ... often roughly and at some unknown test conditions!
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19th Nov 2020, 04:55 PM #4Member: Blue and white apron brigade
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I'd say the 1420 rpm is a nominal RPM at rated level of load or HP.
Here is the HP output as a function of RPM for a 4pole 3HP motor at 50Hz.
The RPM is measured by a laser tacho while teh HP is measured using a deProny brake.
You can see it starts off when free running at close to 1500 rpm but as more load is applied the RPMs drops to 1440-1420 at the rated 3HP output.
This is normal.
3HPABBRPMvHP.jpg
I have a bunch of those cheap digital tachos on my machines (2 lathes, mill and drill press) and when I get around to it I will check their outputs against my laser tacho which when I measured it against a calibrated tacho from work, was spot on.
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19th Nov 2020, 05:57 PM #5Member: Blue and white apron brigade
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I just did a quick check of the Hall effect tacho readings on my WW lathe and drill press against the Laser Tacho.
At ~1300 rpm the tacho on the WW lathe reads 2 rpm lower than the laser tacho.
At ~650 rpm the DP tacho reads 4 rpm lower.
I'm pretty happy with that.
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19th Nov 2020, 08:14 PM #6Member
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rpm indicator
Thank you Baron J, AJ & Bob L for your replies.
All very interesting.
Since retiring, I don't have access to the instrument calibration equipment that I used to, or I would check it against those.
I note that Jaycar sell a laser lit strobe-type tacho for about $70. Maybe I'll consider one of them. There is a branch in Cairns....
Cheers
H614
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19th Nov 2020, 09:22 PM #7Member: Blue and white apron brigade
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20th Nov 2020, 01:02 AM #8
Hi Guys,
I bought a laser one a couple of years ago ! But I have checked it against against a reference and found that if you aim it square to the shaft or wheel hand held, it will pick up your heart beat and it causes the reading to fluctuate a little. Holding it rigid the reading remains steady.
I got mine from Banggood and paid about £8 for it. Though the battery only lasted about 18 months, not like the ten years of yours Bob. I don't leave the PP3 battery in it.Best Regards:
Baron J.
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20th Nov 2020, 10:28 PM #9Golden Member
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I can't offer any suggestions as to how a hall effect tacho could be checked for accuracy in the home shop but I can suggest a method, that I use to check a non contact hand held laser tachometer by simply pointing it at a fluorescent light tube operating at 50Hz and taking a reading, that reading should be very close to 3000rpm (50 cycles per second X 60 seconds)
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21st Nov 2020, 11:57 AM #10Golden Member
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To check your halls effect tachometer I can think of two ways. The first is to buy an old rpm counter meter on ebay. They often go for about 15 to 20 dollars. They are just a counter that counts each revolution. You hold it against the shaft for say a minute and if the counter has counted up to 1256, the rpm is 1256 rpm per minute.
The second way is with an adjustable strobe light. You dial in the strobe light until the chuck appears to stops rotating. That is your rpm.
The third and most crazy would be get a 50 meter roll of wool or string. Tape it to a shaft and run the lathe. Time how long it takes to use up the wool. By knowing the diameter of the shaft you can calculate the circumference and how many rotations it did in that time. Yes we all need some humor in our lives .
Steve
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22nd Nov 2020, 06:07 PM #11Member
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- Cairns, Qld.
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Check calibtn. of hall effect rpm display
Thanks guys for the suggestions, including the ball of string idea..! But isn't there a world shortage of string?
What was I thinking when I mentioned buying a laser strobe tacho at Jaycar for about $70, when as Bob L points out they are are a fraction of this on fleabay? I did however buy a $4.50 small plastic box from them yesterday to mount the indicator in. Looks good now.
I did subsequently re-test the unit on the Sheraton lathe drive motor, & as Bob L & AJ have said, the indicated rpm does drop a bit when a physical load is applied. So I'll probably lash out on a cheap strobe unit for about $25 as another toy & double-check.
We had an old 1940's strobe-type tacho test unit at in the avionics/instrument workshop where I played before retirement. It had a 240v elec. motor driving a variable output swash-plate type multi-piston hydraulic pump that via a belt drove the strobe wheel that was illuminated by the red glow of a thermionic valve. We could test most anything on that at 25 rpm strobe intervals (by winding the pump output hand-wheel) either directly or via external gearboxes & drive shafts, to a turntable if necessary.
Of course it was little used, being replaced by a modern electronically controlled dc unit that you just dialed in with a tiny pot. Where's the fun in that?
Cheers
Halifax
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23rd Nov 2020, 01:28 AM #12Senior Member
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23rd Nov 2020, 02:17 AM #13
Hi Sparksy,
Easy Pi X D 1000 / D X 50, 60 / D = rpm.
Pi = 22 / 7 times diameter to get length per turn, D
1000 mm divided by D. Or 50,000 for 50 mt., to get number of turns in 60 seconds.
The stuff that was embedded in your head at school ! And I still had to think about it.Best Regards:
Baron J.
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23rd Nov 2020, 12:38 PM #14Member
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Gosh, there's some clever people here!
H614
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25th Nov 2020, 11:32 AM #15Senior Member
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- melbourne
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There's an app for that...
I saw a guy on YouTube checking a turntable speed. Start the app, drop your phone on the turntable and read the speed. (the display rotates contra to the turntable to keep the view fixed..) may be easy enough to attach your phone to the rotating thing you want to measure.
Of course that begs the question how accurate is the phone but it's likely to be highly accurate.
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