Amp meter for 1Hp motor

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randy kohl
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Amp meter for 1Hp motor

Post by randy kohl » Mon Sep 24, 2007 11:30 am

I want to add an amp meter to my wood lathe using something like an amp clamp so I don't have high voltage up at my display. Anyone with ideas for a simple circuit would be appreciated. I'd like to use an analog display. Thanks

Randy

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haklesup
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Post by haklesup » Mon Sep 24, 2007 1:26 pm

Do you want to have a calibrated Amps display or would something indicating more or less current be good enough.

In its simplest form, you can just wrap about 30 to 50 turns of fine wire around the power lead and place a diode on one end of the wire and close the loop with an analog meter movement. An additional series resistor will limit current if the meter movement is sensitive or you can use more or less turns around the hot wire to the motor to scale the current you measure (sensitivity)

If you want something more calibrated, the recipe is similar but you can't just use an arbitrary number of turns or an uncalibrated meter movement. You will have to know the specs of the meter to design the coil and resistor network to show a calibrated current. Those small 3.5 digit DMM displays work well. They can be wired for current sensing and should have a datasheet documenting how to do that.

A current transformer which you can get at DigiKey is more predictable than just winding wire around the hot lead. Using one will make it more reliable and easier to design the passives network to calibrate it for your meter display (digital or analog).

You may find it easier to just dedicate a low cost DMM or the "Kill-A-Watt" meter to the task.

Robert Reed
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Post by Robert Reed » Mon Sep 24, 2007 4:23 pm

Try this one from Digikey : Cat. No. 237-1103-ND $2.89
Have used these many times and they are sweet performers.
They will output 100 MV/amp depending on burden. Depending on the meter you are driving, it may require additional circuitry.
An analog AC meter may need some amplification ahead of it to drive the meter correctly and for scaling to it's required full scale current. An analog DC meter will definately need amplification before rectification in order to overcome the diodes poor linearity at low AC voltages. The good news is that all the requirments for any analog meter can be satisfied by one properly configured Op-Amp stage. With a digital AC meter you could probably drive it directly with proper scaling resistors.

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jwax
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Post by jwax » Mon Sep 24, 2007 4:32 pm

Welcome Randy!
Could I ask why you want to measure the current?
Perhaps the Kill-A-Watt that haklesup suggested is "easiest". I bought one for $20, with shipping included. Nice unit!

Robert Reed
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Post by Robert Reed » Mon Sep 24, 2007 5:19 pm

Jwax
Where did you get your "Killowatt"? I have been meaning to buy one of these, but haven't seen any that cheap.

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haklesup
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Post by haklesup » Mon Sep 24, 2007 5:39 pm

lots of places but eBay is probably cheapest. I just got two, one for home and one for work.

Dean Huster
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Post by Dean Huster » Mon Sep 24, 2007 6:07 pm

The only time I've seen an ammeter permanently mounted on woodworking equipment was on a surface planer [9/29/07: That's not correct -- it was on one of those 15"-wide surface sanders] where the ammeter indicated if you were taking too large of a bite out of the material being planed [sanded]. The same machine had a regular magnetic motor starter involved, so had overload protection. I guess the ammeter was much like having that factory-installed tachometer in my Taurus.

Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

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jwax
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Post by jwax » Tue Sep 25, 2007 3:54 am

Robert- The guy has jacked the price a few bucks- it's at $22.95 shipped now-
http://cgi.ebay.com/New-Kill-A-Watt-Ele ... dZViewItem

ecerfoglio
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Post by ecerfoglio » Tue Sep 25, 2007 6:28 am

Dean Huster wrote:The only time I've seen an ammeter permanently mounted on woodworking equipment was on a surface planer where the ammeter indicated if you were taking too large of a bite out of the material being planed. The same machine had a regular magnetic motor starter involved, so had overload protection. I guess the ammeter was much like having that factory-installed tachometer in my Taurus.

Dean
It is a good idea to have an indication of the power that the motor is taking - If you are using too little power you can "bite" more and work faster, but if you are using too mutch you are heating up the cutting edge and dulling it (or perhaps it is allready dull)

If the lathe has an "universal" motor (one that looks like a DC motor, with a wound armature, a colector and brushes) an ammeter will give a good idea of the power that the motor is taking. (P=UxIxCos fi, in an universal motor cos fi is more or less constant)

But if it has an induction motor (one with a squirrel cage rotor) the current doesn't give agood indication of the power because the motor varies its power factor (cos fi) a lot as it is loaded.

The "right" way is to use a wattmeter.
E. Cerfoglio
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Dave Dixon
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Post by Dave Dixon » Tue Sep 25, 2007 8:02 am

Not necessarily a recommendation for this usage, but still on topic. One of my favorite cheap ways of measuring current is to insert a one ohm resistor in line. Then run leads from each side of the resistor up to a cheap voltmeter. Ohms law makes it real easy to convert the voltage drop to an amperage reading if you use a one, point one, or maybe ten ohm resistor.

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Post by Dean Huster » Tue Sep 25, 2007 6:42 pm

The concept isn't bad, Dave, but the value may be a bit high. A one ohm resistor with 15 amps running through it will drop 15 volts and that's stolen from a motor that really could use the full voltage! In addition, that resistor will be dissipating 15 watts, so you'll have to have a pretty big one in there -- in a sawdust environment where heat like that could end up starting a fire. If it must be a series resistor, you almost have to drop it to 0.1 ohm and use a meter with a lower range. Safer, but not as safe as a current transformer.

Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

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Dave Dixon
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Post by Dave Dixon » Fri Sep 28, 2007 1:22 pm

Hi Dean, I completely agree with you. I just thought I would throw that method out there for general info. In many (other) cases it can be a practical, safe, and simple way to get a reading. I should have added the details you mentioned concerning that particular circuit - to clarify. Regards,
Dave

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MrAl
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Post by MrAl » Fri Sep 28, 2007 10:13 pm

Hi there,

Just to add to the interesting replies...

You could also try a hall effect linear magnet field sensor.
These are about $4 roughly and have a linear output.
The idea is to place the sensor close to the power wire
so it picks up the magnetic field generated by the passing
current and use the output to drive a digital meter.
The devices run on 5 volts.
Magnetic field is proportional to current, and the devices
output is proportional to magnetic field, so the output
should be fairly linear.
LEDs vs Bulbs, LEDs are winning.

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