Motors & electromagnetics

  • #26
Drakkith
Staff Emeritus
Science Advisor
21,275
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Blue Scallop, your questions are getting way off-topic.
 
  • #28
290
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What is the best kit available that one can learn about dynamics in motors and electromagnetism such as the Fleming right hand rule where you can perhaps rotate something to manually light a lamp.. this is available in science labs.. but where do they purchase them and what is the best model with the most flexibility? Thank you.
 
  • #29
825
195
What would happen if the pilot accidentally just pull the throttle to less than half and the airplane stalls and fall..
That has happened on final approach but the autopilot was to blame. (Faulty altimeter indicated plane was already on the ground.) The pilots were caught by surprise and results were catastrophic.
 
  • #30
375
21
Not quite. If you connect a motor to a volt meter and spin the shaft you will see it can generate a voltage. This voltage is called the motor Back EMF and it depends on how fast you spin it. When power is applied to such a motor the motor accelerates until the back EMF is roughly equal to the supply voltage. The back EMF also depends on the number of turns on the windings and the strength of the magnets. This relationship between voltage and speed is called the Motor velocity constant or back EMF constant and is given the symbol Kv. It has the units rpm/volt. If the voltage is increased the motor will spin faster.

The current drawn depends mainly on the load on the motor. If the motor was an ideal motor and had no load then once up to speed the current would fall to zero. Adding a load increases the current drawn by the motor. If the motor is an ideal motor the speed would remain constant despite increased load. Real world motors have losses caused by the resistance of the windings, friction and air resistance etc. These all conspire to mean the speed isn't constant and will vary with increasing load. As the load is increased the motor will slow down until at some point it cannot turn the load and it stalls. If you want the motor to maintain speed you will have to increase the voltage.
Not all motors will produce BEMF only those with perminant magnet rotors.or stators. Back EMF or eddy current losses can reduce positive torque of the motor and will only reach a speed where that equilbrium is reached.
you mentioned AC induction motors where voltage has nothing to do with speed, as the line voltage is at a frequenzy (usually 50/60Hz) the other type of two motors where increaseing voltage doest increase speed, are DC Brushless motors or AC servo motors. these get their "frequency " from a solid state controller that includes a frequency generator and transitors that rapidly control currenty curren direction through the motors windings which determines speed. the same thing basically happens in brushed DC motors, but thats a function of it being self switched via its commutator and brush combo. The speed limitation of the electronically switched windings, is more a fuction of winding inductance and resistance. the L/R time constant of each winding, determines how fast the current can rise and create the magnetic field in order to move the rotor. the faster the speed, the less percentage of the power capacity of the motor and thus motor. they solve that by overdiving the motor and chopping the overdrive voltage to lower the current rise time and create faster speeds and more torque at those speeds.
 

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