electricians:How do electric motors and other high current devices use more current initially then die down?

Question

That is what internal mechanisms initially suck so much current out of the wire that other devices blink and then seemingly require very little energy once they have reched top speed? What about the machine changes when it tansitions from using 6x the operating current to only one times the operating current?

Is it simply a matter of the resistance decreasing as soon as the motor attains its desired speed?

Thanks a lot.

Answer 1

Motors and transformers have an initial magnetizing current when they are first energized.

Many types of equipment have a rectifier that changes alternating current (AC) to direct current (DC) and a capacitor that filters the DC. When the equipment is first turned on, the capacitor takes a lot of current as it charges to full voltage.

Large incandescent light bulbs take a high current initially and then less after they get hot and reach full brightness.

Electric motors produce a back EMF voltage that opposes the supply voltage. The back EMF is very low when he motor is running slowly and increases and reduces the current as the speed increases. In addition, energy is used to accelerate the inertia of the driven machine. Once full speed is reached, the motor needs only to do work and overcome friction; it no longer needs to accelerate inertia.

Answer 2

Great answer from Chas EE. Transformers and motors draw a very large amount of current at first. This is sometimes called the "inrush" current, or as Chas described, the "magnetizing" current. Because of the fact that you have a coil of wire with very low physical resistance, a large amount of current is drawn at first. However, whenever you pass an alternating current through a coil of wire, an alternating magnetic field is generated that opposes some of the current. This is known as "inductive reactance" and is measured as a form of resistance. In fact, motors and transformers are designed so that most of their resistance actually comes from inductive reactance.

The only thing I would add is that gas tubes, like florescent and neon tubes, have what's known as negative resistance. Once the tube is ignited or "strikes" and an electric current is flowing through the tube, the gas is ionized and the resistance drops. The more current that passes through the tube, the more the resistance drops and more current would continue to flow. In fact, this current flow has to be controlled otherwise it would quickly lead to an overload. Once a gas tube is ionized, it essentially becomes a short circuit. So florescent lights have to use a ballast, basically an external resistance, to prevent overloads. Neon signs use a high voltage transformer to energize the tube, and the transformers are current-limited with shunts in the core. Florescent and neon tubes need a high voltage to energize the tube, but then once the conductive pathway is formed, very little energy is needed to maintain it.

Answer 3

Current is proportonal to torque. That is a fundamental law of motor physics.

In order to get a motor spinning, especially with a load, you need torque, which requires current. Torque for the load, plus torque for increasing the angular momentum of the motor rotor.

Once the motor is spinning, you need only maintain the angular momentum plus the torque required by the load, which is lower than the initial torque, and the current which is lower than the initial current.

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Answer 4

the present is tormented via the resistance of the cord, however the form of the cord has little to do with it. the significant parameter is the circulate-sectional component of the cord; the smaller that's, the better the resistance. yet generally, the cord, or the song on the circuit board, only gets the present to the region it desires to bypass, and the resistance of the cord is so below the gadget employing the present that that may now not significant in any respect. A bend interior the cord, or a narrow place interior the circuit, will improve the resistance fairly because of the fact it reduces the section, however the electrons can substitute course as much as needed with out any concern.

Answer 5

it takes more "power" to get it to turn than maintaining it turning. think of it like a car, from a dead stop it takes a lot of power to get all that weight moving compared with all that weight already moving. that's why highway mileage is better than city mileage. the electrons in the conductor , in a VERY over simplified way, are similar to the car.