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armature
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The part of a motor in which a current is induced by a magnetic field. The armature consists of a series of coils mounted on a shaft and rotates through the magnetic field.
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base speed
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The speed (in rpm) at which the motor runs with full-line voltage applied to the armature and the field.
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brushes
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Sliding electrical contacts used to provide a connection between the armature and the external circuit. The brushes are stationary and are contacted by the rotating commutator.
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chattering
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The occasional unwanted vibration between components. Chatter decreases productivity and can cause wear.
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commutator
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The rotating switch that contacts the brushes of a DC motor. The commutator maintains DC when the rotation of the armature switches the polarity of the conductor.
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compound motor
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A DC motor that uses both series and shunt field windings. Compound motors provide most of the positives of both types.
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constant speed
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A type of motor that maintains a steady rate of rpm from no load to full load. DC shunt motors are often referred to as constant speed because they run at constant speed.
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counter emf
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The voltage induced in the armature of a DC motor that opposes the applied voltage and limits armature current.
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DMM
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A device that can measure voltage, current, or resistance. A digital multimeter is the most versatile and common meter used today for electrical maintenance.
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electric motor
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A machine that converts electrical energy into mechanical motion. Electric motors use magnetic induction to produce torque.
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field windings
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The conducting wire connected to the armature that energize the pole pieces. Field windings are connected in series or parallel.
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frogleg wound armature
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Term used to describe a series-parallel combination armature winding. Froglegs are the most commonly used winding.
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grounded circuit
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A circuit where current strays from its intended path and flows through the frame of the motor. Grounded circuits are caused when conductor insultation breaks down or is damaged.
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horsepower
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A unit of power used to describe machine strength. One horsepower equals 33,000 ft-lbs of work per minute, or 746 watts.
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inrush current
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The initial surge of current into the windings. Inrush current can be up to ten times higher than the continuously needed current because there is low initial resistance.
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lap wound armature
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Term used to describe a parallel armature winding. Lap connections are used for high current, low voltage loads.
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lockout/tagout
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A method of protecting employees from accidental machine startup through proper locking and labeling of machines that are undergoing maintenance.
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lubricate
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To use a fluid to reduce the friction between components. Typically a lubricant reduces resistance, heat, and wear, but brushes cannot be lubricated.
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magnetic induction
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The use of magnets to cause voltage in a conductor. Magnetic induction occurs whenever a conductor passes through magnetic lines of flux.
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permanent magnet motor
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A type of motor that uses permanent magnets instead of field windings as a way to create torque. A permanent magnet is one that retains its magnetism after the magnetizing force has been removed.
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polarity
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Having two oppositely charged poles, one positive and one negative. Polarity determines the direction in which current tends to flow.
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pole piece
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Devices mounted on the inside of a motor housing. When connected to field windings, the pole pieces form the electromagnets that create lines of flux.
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rheostat
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A continuously variable electrical resistor used to regulate current.
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right-hand motor rule
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The relationship between the factors involved in determining the movement of a conductor in a magnetic field. The current flow is opposite in generators.
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self-excited shunt field
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Field windings connected in parallel with the armature that are supplied with current by the same power supply as the armature.
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separately excited shunt field
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Field windings connected in parallel with the armature that are supplied with current from a source other than the armature. A separately excited field helps maintain constant speed.
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series motor
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A method of connecting field windings in series with the armature. A DC series motor provides very high start-up torque but must never be run without a load.
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series-parallel
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A compound connection that uses both series and parallel connections.
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short circuit
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A circuit where current takes a shorter, unintended path between two conductors. Short circuits are caused when the insultation of two conductors is worn or damaged.
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shunt motor
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A method of connecting field windings in parallel with the armature. The shunt DC motor is commonly used because of its excellent speed regulation.
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speed
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The amount of distance an object travels in a given period of time. In motors, speed reflects rotational movement and is measured in revolutions per minute.
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speed control
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The external means of varying the speed of a motor under any type of load.
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speed regulation
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The ability of a motor to maintain its speed when a load is applied. A motor's speed regulation is fixed based on its design.
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torque
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A force that produces rotation. Torque is measured in pounds-feet in the English system and Newton meters in the metric system.
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wave wound armature
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Term used to describe a series armature winding. Wave windings are used for high voltage, low current loads.
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