NCCER Electrical Module 2 Level 2 Motors Theory and Application exam 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 The starting torque characteristics of a wound-rotor motor are defined by _____. rotor resistance A three-phase synchronous motor is started with a(n) _____. amortisseur winding The speed of an induction motor depends on the power supply frequency and the ___. numbers of poles The speed on a multiple-winding motor is normally changed by ____. switching contacts in the motor controller A consequent-pole motor has _____. two speeds The method of braking in which DC is applied to the motor windings after AC is removed is known as ____. injection braking Reconnecting a running motor to act as a generator is known as ____. dynamic braking The two primary types of motor enclosures classified by NEMA are ____. open and totally enclosed An open AC squirrel cage motor that makes use of​ form-wound coils and an insulation system is known as​ a(n) _____. sealed windings(type) The NEMA frame designation for gasoline pump motors is ____. G The letters FLA on a motor nameplate stand for ____. full-load amps To determine the horsepower rating of a motor that is already installed and in place, ____. check the nameplate data Most industrial motors are NEMA Design ____. B The multiplier to the nameplate horsepower rating is called the _____. service factor An insulation resistance test can be used to determine _____. whether a motor is damp The code requirements for motor disconnects are covered in ___. 430.102 All motors must have a thermal protector if their voltage rating is greater than​ _____. 600V-1,000V To protect the motor from overloads and starting failures, use a ____. thermal protector When studying the operation of electric motors, it is important to remember that magnetic lines of force ____. do not cross A motor with the field coils connected in parallel with the rotor winding is a _____. shunt motor If a motor has 4 poles and is operating at a frequency of 60Hz, its synchronous speed is _____. 120(60Hz)/4= 1,800 rpm 1,800 rpm The type of motor most likely to be used in a location with flammable dust and torque fluctuations is a ____. squirrel cage induction motor A type of motor used in precision applications that require constant speed with zero slip is a _____. synchronous motor A heating system is most likely to use a _____. single-phase induction motor With a constant-torque load, the horsepower ____. varies in direct proportion to the speed