240V from a 3 phase main ?

But, again, we're talking about shop tools, most of which are going to be hand fed their work and whose motors are going to be lightly loaded most of the time. If the voltage is less, the available power will be less so you have to feed the work to the tool a little bit more slowly. In other words, you won't be pushing the motor load to the limit and causing the smoke to start rising. Up to a point, you can even overload a motor for a brief period without causing any harm.

The available power will not be less. Remember, increasing slip decreases effective reactance in the motor, so at a given load, current will automatically increase to satisfy load demand when voltage decreases. Since P = I * E, power can remain the same when voltage is decreased. It is how electric motors work. So you will not have any sensible feedback telling you to slow down.

Now for some power tools, such as a table saw, you can consciously and deliberately reduce the load by decreasing feed rate so that power demanded decreases to match decreased voltage, and then current will not increase. But you can't do that by noting the motor is bogging, it won't. You'd have to continuously monitor motor current to stay within the safe area. The tool itself won't give you any feedback telling you to slow down, until you note the smoke coming from the motor.

For some motor operated loads, such as an air compressor, you have to change pulley ratios to reduce the load. While this will reduce running current, it may make the compressor hard to start, and possibly damage motor, contactors, or capacitors anyway.

In short, you can run a lightly loaded motor on reduced voltage, but you need some way to monitor current to ensure you really are loading it lightly enough to keep it from overheating. And you have to be wary of other factors which may come into play, such as excessive current draws required to come up to speed on reduced voltage.

Gary

P != I * E

You're saying the utilities cannot reduce their demand load by reducing voltage because the motor loads will just draw increased current?

I would think you'd reduce the size of the driving pulley so reduce the motor load, thus making it easier to start.

And, again, we're talking about voltages that are within the motors' voltage rating class.

Ok, to be a pedant, P = I * E * cos(theta)

For a non-zero theta, there will be circulating reactive currents as well as load currents. The former won't contribute to motor power, but will contribute to I^2 * R winding heating. So they make the situation worse than a simple P=I*E calculation would imply.

That's true for motors, it isn't true for resistive loads. Since the load on the grid is a mixture of motor and resistive loads, the utilities can decrease demand by decreasing voltage, but only the resistive loads will have reduced demand. The motors will just draw more current until they overheat and fail.

This is an unfortunate fact of life in some parts of the world where brownouts are common. Motors fail by overheating under low voltage conditions.

Gary

Power does = E * I for D.C. Power does = E * I * Cos(theta) for A.C. theta is the phase angle between E and I. Sometimes Cosine equals 1. :-) Those are the facts.

The swinging transformers move taps raising and lowing voltage. These are massive horz. transformers in substations.

They often drop voltages to shed load. Many motors and compressors don't start. Those that do will draw more but with the drop off's and the resistive load loss it is a win.

They often run this valley on between 68 and 93 volt not the normal 120-140v.

This is when storms take out a substation - they back-strap this valley from another.

I caught them one Sunday a.m. - TV worked and most things - computer UPS didn't like it one bit. I called it in - got service on the line - Naw that just can't be. I asked for a service person to verify the substation as I have checked my house from stem to stern. I gave the service person my number.

A super nice response engineer called from the substation. He was about to unlock, but though to call first. I told him I used my Beckman and my Tektronix true RMS voltmeters.

Once he heard the true RMS - he knew I knew something. We talked as he into the station - and found the main line and the back strap installed. That is when the fun came.

He had to undo a double hot backstop line voltage and heat. He asked me to stand by and call in for him if he didn't come back. I did and he did. I verified 120V was on the lines. We chatted as he locked up and off we both went. Him home, me computering.

This reminds me of a class we had in engineer school. The object was to calculate the horsepower required to move a quantity of dirt in a scraper over a certain soil at a certain grade and speed. You did the math and came up with the required horsepower. However, There you are out in the field, you load your scraper and it either goes up the hill or it doesn't. If it doesn't, you take off some of the load, or go a different way. You don't go to your Company Commander and ask for a bigger motor because your theta ain't cosigned with the delta max. Paul

Quite true. The subject line says 240V, 3-phase, so we have to presume AC is being discussed.

If they do this very often, you have grounds for some loud complaints to your state utility commission. Electric utilities don't guarantee much of anything, but they do have to stay within shouting distance of the nominal voltage of the service class. 68V isn't even close.