Three common kinds of motor work in different ways and wear in different places. The 3D explorer shows only the three-phase induction motor. The diagrams below are separate, simplified drawings.
You can’t always tell by looking
Different kinds of motor can look very alike from the outside, and the same frame style is used for many designs. Start with the nameplate, the plate or label on the motor that lists its ratings. It states the supply (phases, voltage, AC or DC) and the rating. If there is no readable nameplate, a qualified technician can identify the type from its connections, internal construction and tests.
Three-phase induction motor
Power supply
Three-phase AC supply.
How it turns
Current in the stator windings (the fixed coils) creates a turning magnetic field. That field sets up current in the rotor bars, and the two fields together turn the rotor. The rotor runs slightly slower than the field. This difference is called slip, and it gets bigger as the load increases.
Starting
Starts on its own. Common starting methods are direct-on-line (switched straight onto the supply), star-delta and soft starters. Frequency converters (speed controllers) are also used.
Parts that set it apart
Three-phase stator winding
Squirrel-cage rotor
Terminal board with star or delta connection on many motors
Common wear points
Bearings
Winding insulation (heat, moisture, a missing phase, unbalanced voltage)
Cooling fan and airflow
Where you may find it
Pumps, fans, compressors, conveyors and other industrial machinery.
Also an induction motor with a squirrel-cage rotor, but it runs on a single-phase supply. It needs extra starting parts to get the rotor turning.
Starting
Designs vary. Some use a start capacitor and a run capacitor. A centrifugal switch, which works by the speed of the shaft, then cuts out the starting circuit once the motor is running.
Parts that set it apart
Main and starting windings
Start capacitor, run capacitor or both, on many designs
Centrifugal switch on some designs
Squirrel-cage rotor
Common wear points
Capacitors
Centrifugal switch
Bearings
Where you may find it
Smaller machines where only a single-phase supply is available.
General description. So far, only the parts named in the source below are matched to a manufacturer document.
DC supply, or AC converted to DC by a drive or rectifier.
How it turns
The turning part, called the armature, carries windings connected to a commutator (a segmented ring on the shaft). Carbon brushes press on the commutator to carry current into the armature.
Starting
Started and controlled through its supply or drive.
Parts that set it apart
Armature (rotor) with windings
Commutator
Carbon brushes and brush holders
Field windings or permanent magnets
Common wear points
Brushes, which wear and need periodic inspection and replacement
Commutator surface
Bearings
Where you may find it
Older variable-speed industrial drives and other DC-powered equipment.
General description. So far, only the parts named in the source below are matched to a manufacturer document.
The diagrams are simple cut-through drawings made for this site. They are not to scale and show only the parts that set each type apart. Sources: see the source list.
