Source for know RPM reading

[B2]

Many cheapo LED's have low Peak Inverse Voltage (PIV) rating - far under 110V. What I am saying is many LEDs (5050, 3030 white) will fail within 1/60 of a second, since their PIV was exceeded ( 3030 PIV is 5V), unless it was designed to be connected to line voltage. LED Christmas lights are a good choice since they are made for line voltage.

Thanks @WobblyHand ,

You are right about the reverse voltage rating. I was not thinking of this when I suggested the LED and resistor. One would need to do a resistor divider etc first. Another approach is to put several LED in series and so the voltage is divided between them so as to prevent reaching the reverse voltage limit, which is what I think the LED Christmas lights do. (This is what is commonly done for very high voltage power supply rectifiers, a number of 50 volt diodes in series) I don't think the LEDs in the Christmas lights are special in this regard. However, in the modern LED Christmas lights I think there is a shorting resistor wired in parallel with the LED so that when a diode burns out the whole string of lights do not stop working, but I am not positive about this. This is what is done with the strings of small incandescent bulbs any more. For this reason the strings come with a fixed number of bulbs of a given voltage drop. For example in one string I purchased each bulb is designed to drop about 2.5volts so there was about 120/2.5 = ~ 50 bulbs. They conduct about 170MA so the resistance is 2.5/.170 = ~15 Ohms. So each bulb is using about 0.42 watt (2.5*.170). These voltages and currents are RMS values.
There are other voltage bulbs available.

The fluorescent bulb rectifies the line and produces a spike of light on each half cycle. This is why it puts out 7200 pulses per minute and not 3600.

@yota Love the vintage Jagabi mechanical indicator! Do you know how they work ... inside mechanics? Is there a patent number on the back of the indicator that we could use to look it up? If I understand the instructions, it appears that it some hows counts vibrations for a fixed period of time?

Dave L.

 

[ChazzC]

@yota Love the vintage Jagabi mechanical indicator! Do you know how they work ... inside mechanics? Is there a patent number on the back of the indicator that we could use to look it up? If I understand the instructions, it appears that it some hows counts vibrations for a fixed period of time?

Dave L.

If it's anything like my Hasler-M,


it has a clockwork mechanism geared to the dial that counts down ~4 seconds and totalizes the number of revolutions; similar to the tachymeter dial on some fancy mechanical watches. See this post for more details and a link to a YT video:

https://www.hobby-machinist.com/threads/mrpete222-has-one.105205/post-1070675

 

[dabear3428]

Ha! That's kind of what sent me down the hole.
I figured the same thing. My understanding is the same that a generic AC motor for a machine - like me old SB 9a would to fine as a reference.

The motor plate claims RPM of 1725 - which I recall is very common. My RPM meter says it spins at 1793 RPM with no load (belt off, checking the output pulley).

No idea what's going on with that.

listed RPM is an estimate, all induction motors work on the slip speed which is a always less than sync rpm the difference between sync speed (1800 RPM in this case) and rotor speed is what creates the TQ, the more slip the more TQ. The rotor speed will approximate sync speed under ideal conditions but never be exactly sync speed.
by definition for an induction motor 0 slip === 0 tq == no rotation.

 

[dabear3428]

It's probably the motor is 4% off it's data plate.

Stu

negative, the motor is in spec, just lightly loaded.

 

[yota]

If it's anything like my Hasler-M,

View attachment 507570
it has a clockwork mechanism geared to the dial that counts down ~4 seconds and totalizes the number of revolutions; similar to the tachymeter dial on some fancy mechanical watches. See this post for more details and a link to a YT video:

https://www.hobby-machinist.com/threads/mrpete222-has-one.105205/post-1070675

correct. in the instruction sheet I posted, it says it is a clockwork mechanism.

 

[stupoty]

negative, the motor is in spec, just lightly loaded.

negative, the motor is in spec, just lightly loaded.

loaded vs no load rpm on the data plate, makes sense.

Stu

 

[yota]

@yota Love the vintage Jagabi mechanical indicator! Do you know how they work ... inside mechanics? Is there a patent number on the back of the indicator that we could use to look it up? If I understand the instructions, it appears that it some hows counts vibrations for a fixed period of time?

Dave L.

here you go.

 

[rwdenney]

Ha! That's kind of what sent me down the hole.
I figured the same thing. My understanding is the same that a generic AC motor for a machine - like me old SB 9a would to fine as a reference.

The motor plate claims RPM of 1725 - which I recall is very common. My RPM meter says it spins at 1793 RPM with no load (belt off, checking the output pulley).

No idea what's going on with that.

Machine motor speed ratings are for the rated power which requires a load and that will impose some slip.

1793 unloaded tells me the meter is accurate enough.

Rick “many machine motors rarely run fully unloaded and with zero slip, but synchronous clock motors do” Denney

 

[WobblyHand]

Sorry to be a broken record, but how accurate a reading is needed vs the stated accuracy of the instrument? Is it necessary to know RPM to 1, 2, 4, 5 or 10%?

I implemented an RPM meter on my ELS, and found it exceedingly annoying to read to a single RPM. I ended up filtering the display and not displaying the stuff next to the decimal point. It's useless information and distracting. It's like reading to a millionth of an inch when all you really care about is to a thousandth.

 

[ChazzC]

Sorry to be a broken record, but how accurate a reading is needed vs the stated accuracy of the instrument? Is it necessary to know RPM to 1, 2, 4, 5 or 10%?

I implemented an RPM meter on my ELS, and found it exceedingly annoying to read to a single RPM. I ended up filtering the display and not displaying the stuff next to the decimal point. It's useless information and distracting. It's like reading to a millionth of an inch when all you really care about is to a thousandth.

I fully agree with machine tachs: reading to the nearest 10 RPM is adequate. However, on DRO’s it’s good to see one significant figure below what you are shooting for as it makes it easier to sneak up on a dimension (like reading between graduations on a dial or mic).