I'm new to electrical engineering, and probably asking the wrong question. Not sure how else to phrase it.
For the question, I'm going to use an example layout of simply a 9v battery attached to a toy DC motor.
From my understanding of my previous question, the voltage coming from the battery through the motor will be around 9v. The current will be from 0amps to however much the battery can supply without frying. What decides how much current goes through the motor?
Answer
Some non-math beginner concepts:
All motors are generators. Suppose your permanent-magnet-type DC motor generates 9V when the shaft is spun at, say, 100Hz or 6000RPM.
This means that, when the motor is connected to a 9V battery, it will speed up to 6000RPM, then remain at that speed. When it's up to 6000RPM, it draws zero current (ideally.) That's assuming that bearing- and brush-friction is insignificant, and the rotor is in vacuum (so, no air is heated, with no wind blowing out of the holes in the case.) At max speed the current falls to zero because the motor(generator) is supplying 9V, and the battery is also supplying 9V of the same polarity. The two opposing voltages subtract to zero. The rotor keeps spinning at a constant RPM. And, other than friction effects, it doesn't need any energy from the power supply. For a real motor, the entire supply current is caused by unwanted friction (and also of course by any mechanical-load wattage being drawn from objects connected to the spinning shaft.)
In other words, it's not easy to calculate the current a real motor will draw from its power supply. The current depends on internal friction of bearings and brushes and wind/fan effects, as well as depending on the amount of work (wattage) the motor shaft is performing.
With DC motors, the RPM of the shaft is proportional to the drive voltage and the generated voltage. If a motor runs at 6000RPM when connected to 9V, it will run at 1/3 the speed if powered at 1/3 the volts (so, 2000RPM for a 3V power supply.) There really isn't any such thing as a 9V motor or a 24V motor, etc. Those are just the operating voltages for a minimum bearing-wear and maximum brushes-lifetime. Ideally, any DC motor can be run at much lower voltage and RPM than its spec-sheet voltage. And, any DC motor can be connected as a generator, and used as an RPM-sensor: the output voltage is proportional to rotor speed.
The rotor of any motor has mass, and can't speed up instantly. So, whenever you connect your DC motor to a 9V battery, it won't draw infinite current, and won't instantly jump to 6000RPM speed. It will however accelerate, and momentarily draw a large current while doing so.
All motors are flywheels. When first connected to a DC power supply, the motor speed increases, and kinetic energy is stored in its rotor. When hooked up and running with constant RPM, we can disconnect the supply, and the RPM will stay the same, and not instantly stop. (The rotor is an energy-storage device.) Then, depending on friction and work-wattage, the motor RPM will smoothly decrease, just like any flywheel which is performing work and losing its kinetic energy.
If we want to rapidly slow down a DC motor, we can disconnect it from its power supply, then connect it to a resistor. The motor acts as a generator (as it always does,) and drives a current in the resistor. The motor slows down, and the "braking resistor" gets hot.
In all, DC motors resemble capacitors, where the "charge" is the kinetic energy of the rotor-flywheel. DC motors draw a big current when first connected to a DC voltage. Then, the current decreases to nearly zero. Disconnect a spinning DC motor, and the power-supply voltage still remains on its terminals. What if we short those terminals? POW! A huge current appears briefly, and the rotor jerks to a sudden halt. We've discharged the stored mechanical energy, and the conductors end up at higher temperature.
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