Help In Understanding Voltage Drop and Motors

[JFletcher]

Long before any electrical training at all, I was told to use larger extension cords with long cords/runs, say for instance, drilling out studs, that significant voltage drop would quickly kill any motor under a decent/high load. What I'm not understanding is this: dont variable speed power tools vary voltage to the tool? and if low voltage kills a motor, then why does higher voltage? They are both drawing excessive amperage, yes? or does high voltage simply short it out b/c the tool isnt insulated for that? examples:

A 13A 120V drill on a 100' 12ga cord = no problem
A 13A 120V drill on 400' of 12/14 ga cord = overheats, will probably fail if used continuously/heavily
A 13A 120V drill plugged into 240V = poof???

If you plugged in a 100W 120V bulb to the same cords/voltages, you'd get normal operation, a little dimmer, bright then poof, yes???

Any links or comments to how motors work re: different voltages would be much appreciated.

 

[Jraef]

Voltage relates to torque in the motor. When you lower the voltage, you lower the torque that the motor can produce. In a "Universal" (AC/DC) motor as used in most power tools, the lower voltage will also result in lower speed, that's what a variable speed drill is doing. So on the variable speed drill too, when you are going slow you have also lost torque. You can tell because you can stall it more easily. On a drill though, you need less torque at a slower speed (usually).

On something like a circular saw operating from too small of a cord, the voltage drop and loss of torque means the motor cannot do all of the work you are demanding of it. If you back off on what you want it to do, it can be fine. But does anyone do that on a circular saw? No, they push the envelope. Then also the saw blades are designed to operate at a particular speed. When you lose votlage/torque and speed, the blade efficiency drops and the motor needs to work HARDER to cut, right at the time it CAN'T. So when you push the envelope AND you make it weaker AND you make the blade work harder TOO, you run the risk of burning it up.

On something with a standard AC motor, like a compressor, the motor always tries to spin at the same speed; the speed is determined by the line frequency, not the voltage. So dropping voltage still makes it lose torque, but you DON'T lose speed, so the motor is putting out less HP and almost automatically getting overloaded. The load is what requires the torque, so unless the motor is seriously over sized*, the loss of torque ALLOWS the speed to drop under load because the motor can't keep it spinning. That increases the "slip" of the motor, which increases the current, which heats it up, but does LESS work with higher losses. The end result is tripping the protection device, or burning up the motor if you bypass that.

*On portable equipment, the motor is NEVER over sized...

 

[gar]

151007-2341 EDT

JFletcher:

You need a basic understanding of how various devices work, and then some analysis should help you understand how the devices can be used.

I will briefly touch on some basic concepts of a DC series motor. When this type of motor is used on AC there are some secondary effects, but the basic concept remains.

Read the website http://www.electrical4u.com/series-wound-dc-motor-or-dc-series-motor/ .

If we disreguard the need for cooling the motor, then the motor can be operated at any speed at the maximum continuous rated current of the motor. Because torque is proportional to current this means the same maximum torque can be produced at any practical speed. Note, at constant torque power is proportional to speed. High power at low speed is not feasible because of the high current required.

Universal motors in power tools usually have an internal fan for cooling. At constant current the internal power dissipation in the motor remains approximately constant at any speed. With less cooling at lower speeds this really means from a practical perspective you can not run at constant maximum current at low speeds.

A motor does not produce torque of some value, rather a motor supplies the torque required by the load at some speed if the motor can.

AC induction and synchronous motors are a different story.

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