NCCER Electrical Module 2 Level 2 Motors: Theory and Application Questions and Answers

How DC motors operate? Magnetic lines of force are continuous and always form closed loops; Magnetic lines of force do not cross. Armature: (Definition) The rotating windings of a DC motor. The ____ is a movable electromagnet located between the poles of another fixed permanent (field) magnet as shown. Armature A twisting force called ______ must be developed. _____ is produced by mounting a loop in a fixed magnetic field and as long as the fields of the magnet and armature interact. Torque Commentator: (Definition) A device used on electric motors or generators to maintain a unidirectional current. In the neutral plane, no ____ is produced and the rotation of the armature should stop Torque A _____ is produced by the fields poles, and a loop of wire in the armature turns and cuts this magnetic field. Magnetic field CEMF Counter electromotive force (CEMF) Horsepower: (Definition) The rated output capacity of the motor. It is based on breakdown torque, which is the maximum torque a motor will develop without an abrupt drop in speed. Horsepower: (Equation) Hp= Watts/746 Types of DC motors Shunt Series Compound Types of AC motors Synchronous and Induction (Single and Polyphase) Periodic duty: (Definition) Intermittent operation at a substantially constant load for a short and definitely specified time. Revolutions per minute (rpm): (Definition) The approximate full-load speed at the rated power line frequency. The speed of a motor is determined by the number of poles in the winding. Rotation: (Definition) For single-phase motors, the standard rotation, unless otherwise noted, is counterclockwise facing the lead or opposite shaft end. All motors can be reconnected at the terminal board for opposite rotation unless other wise indicated. Synchronous speed: (Definition) The speed of the revolving field of the stator, which is dependent on the supply frequency and number of poles in a stator winding. Field poles: (Definition) The stationary portion of a DC motor that produces the magnetic field. The name of the motor part that rotates during operation is the ____. Armature The principle reason for developing the brushless DC motor was to ___. eliminate commutator problems The electrical energy required to produce 1hp of mechanical energy is _____. 746W When there is a wide variation in both torque and speed requirements, such as a blower or hoist, use a _____. series DC motor The name of the stationary motor part that produces the magnetic field during operation is the ____. stator The rpm of a 60Hz motor with two poles is ___. 3,600rpm In three-phase motors, the rotor most commonly used is the _____. squirrel cage rotor