Journeyman Electrician: A Simple Guide To Motors

Construction, Purpose, and Operation of Motors
Motors convert electrical energy to rotational mechanical energy, which is the opposite of generators which convert mechanical to electrical energy.  The design and construction of motors is in fact very similar to that of generators.  The stationary part of a motor is known as the stator and the moving portion is the rotor.  These contain the field windings, which set up one magnetic field, and the armature windings which produce a second field.
The principle of operation for a motor is attraction and repulsion created by sets of magnetic fields in the field and armature windings of the motor.
Motors are classified according to the type of voltage and current on which they run: alternating current (AC), direct current (DC), and universal types that can run on either AC or DC.
The efficiency of a motor is a measure of how well it converts electrical energy to mechanical energy, and it equals the mechanical power output (usually in horsepower) divided by the electrical power input (usually given in watts, then converted to horsepower). 

Motor efficiency is generally in the 80-90% range.

Safety Features Incorporated in Motors
The frame of a motor is always grounded to prevent against electrical shock from stray voltages.  Overload circuits are often used with motors.  For single phase motors, an overload device may be placed in the ungrounded conductor.  The overload device is used to protect the motor from sustained overcurrent and overheating due to excess mechanical loading.  An overload is defined as operation beyond the normal full load rating or ampacity of the motor.
Thermal overload devices operate in any of a number of ways. 
A temperature control relay may respond directly to the temperature of a probe in the motor windings.  Other devices react to heat generated within them by the current drawn by the motor.  Solid state relays may be used to electronically monitor the current and open the circuit when the current and time limits have been exceeded.
A motor enclosure that totally contains sparks and flashes is referred to as explosion proof and is used in hazardous locations.

DC Motor
Motors rotate to align the magnetic fields from their armatures and fields.  In a DC motor, the direction of the torque must constantly be reversed as the motor rotates in order to keep it moving.  A commutator and brushes are used to reverse the current applied to the armature.  In a DC motor, the rotor is the armature and the stator contains the field coils.
DC motors have a wide speed range that is controlled by the current to the field windings.  To change the direction of rotation of DC motors, the polarity of the voltage is reversed.
DC motors have a high starting torque.  They have very low armature resistance so starting current must be limited to prevent damage to the motor.

AC Motor
Most common alternating current (AC) motors fall into two types: synchronous motors and induction motors.  Field coils are contained in the rotor and armature coils are in the stator.
Synchronous motors operate at a fixed speed regardless of the load or applied voltage.  Synchronous motors will stall if the applied current is not sufficient to provide the torque necessary to maintain synchronous operation.  The synchronous speed is determined by the applied power line frequency and the number of magnetic pole pairs (sets of N and S magnetic poles).  Slip rings or brushes are used to provide the field current to the rotor.
Induction motors contain no brushes or slip rings; the current in their rotors is induced from current in the stators.  Similar to synchronous motors, induction motors are near synchronous; that is, they operate slightly below synchronous speed.

Some motors have separate windings used only for starting. 
These are contained in the motor’s armature.  The auxiliary windings are connected in series with a centrifugal switch that disconnects the starting winding at some predetermined speed.  If this switch fails and the starting winding is left in the circuit, the motor will draw excessive current and overheat.

Series Motor and Repulsion Motor
In a series an AC motor, also called a series wound motor, the armature coil is connected in series with the field coil and receives current through a commutator.  Series motors were once used extensively where variable speed was required before the advent of modern electronic speed controls.  They have very high starting torques and must be loaded during start up to prevent excessive speed that could damage them.
The repulsion motor is a special case of an induction motor.
There are brushes and a commutator connected to the armature but the brushes are shorted together and the armature receives power by induction.  There is also a repulsion start motor that starts as a series motor, but at a set speed the commutator segments are short circuited and it begins operating as an induction motor.

Induction Motor
Induction motors have no commutator.  Power to the armature is received by induction from the field windings.  The armature may be constructed of wound coils or it may be made of solid bars in the case of the squirrel cage motor.  Induction motors usually run slightly below synchronous speed.  If an induction motor runs slow, it may be because of worn bearings.
Squirrel cage induction motors have low starting torque and can draw large currents on start up.  Large squirrel cage motors are started at lower voltages to avoid this excessive current.
Common designs for wound rotor induction motors are split phase and shaded pole.  Split phase motors have a secondary starting field winding that is displaced in phase.  After start up, at about 75% speed, this winding is disconnected by a centrifugal switch and the primary winding is used.  Split phase motors generate considerable torque for use in appliances like washing machines.
Shaded pole motors are used for low torque applications like electric fans.  A shading coil produces a moving magnetic field that causes rotation.