View Full Version : Load Meter For Lathe

03-25-2001, 12:08 PM
I'm in the process of making V-belt pulleys for my South Bend 10K lathe as shown in MW a couple months ago. Without belt slip, the motor will be working to its limit. I have seen on new CNC machining centers a gauge that tells the operator what percentage load capacity the motor is working at. I would like to have a similar feature on my lathe. Is it an amp meter? does anyone have an idea as to how this works? Thanks!!

Cliff Lawson
03-25-2001, 06:38 PM
I am not sure what you saw, but for measuring
amps, a simple technique is to use one of those meters that clamp over one of the lines feeding the motor. The one I have has a locking switch so the reading you have will be saved.

Also, the one I have has a interconnect that goes between a 110v outlet and a plug, so you can read that way (also with a 10 times) section to get precise amps for low readings.

Of course if you are using 220v, this won't work, but then the exposing of one lead is needed. I find this method is quite nice for other things, such as seeing what a welder takes under varying outputs.

Good luck, Cliff Lawson


04-04-2002, 09:14 PM
you can't determine the motor load by measuring the input amperes and voltage. You need a wattmeter and they are expensive. Probably the best way of measuring how loaded a motor is is to take its temperature when it has been run under constant load for a long time (long enough to stabilize its temp). For short term loads, measure the RPM (for fixed speed motors).

For intermittant use, if the motor can pull it, the motor isnt overloaded - but that doesnt mean it wont get hot and fail. Its hard to define what the HP rating of a motor is. You can pull lots of power for short times (duty cycle rating) or less power for longer times.
Running the motor unloaded and measuring the amps will give near the same figure from no load to full load until the motor begins to slow too much, at which time it will develope LESS horse power and more heat and draw more current.
This is all related to "Power Factor" and PF explains why you need a watt meter not an amp meter to measure AC power

04-04-2002, 10:19 PM
Docsteve66 is correct. Measuring current only work for purely resistive load. Motor is highly inductive so the phase relationship of the current and the voltage will largely determine the power consumption.


Bill Neufeld
04-05-2002, 11:27 PM
Load meters are generally driven by a circuit in a speed control for the motor.The speed control measures the current the motor draws and compares it against what it should be for a given speed. It is not just a straight current measurement of motor draw, although it is a good approximation.

04-06-2002, 01:32 AM
All you have to remember is if it starts to grunt or the belts start screaming or smoking - back off. Since you do not have to worry about production times a load meter is a waste of money.

However, some VFD units can drive a 0-100mA. meter from a special circuit or display the load on the front panel or programming unit. If you have a fully progammable VFD your might have that feature - but don't sweat it, you don't need it.

04-09-2002, 09:11 PM
Thanks all for the informative responses! After having it done shortly after my original post- check the date- I have to say that this is a great upgrade! Thrud- you are right to say I don't need a load meter. The 1/2 horse 110volt motor pulls well until it slows to the point that the starter switch engages. It is very easy to tell when it slows even a small amount as well. Don't worry, I rarely load it that far! But, I still worry about production times! Tomorrow's job is 100 aluminum bearing housings 2.5 inches in dia with the bearing fit to be within .00005! Thanks again for all the replies and all the great knowledge found in this board!!

04-10-2002, 06:17 AM
1/2 HP is a little light for that machine. Scrounge up a 1 or 1-1/2 HP, and you will really take advantage of the V-belt drive.