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Chapter 1
General Principles of Motor
Control
Objectives
• State the purpose and general principles
of motor control
• Discuss the differences between manual
and automatic motor control
• Discuss considerations when installing
motors or control equipment
Objectives (cont’d.)
• Discuss the basic functions of a control
system
• Discuss surge protection for control
systems
Introduction
• “Motor control” can mean anything from a
simple on/off toggle switch to a complex
system intended to control several motors
Installation of Motors and
Control Equipment
• Several factors should be considered
– Motor, machine, controls, etc.
• Some machines have the motor and
control equipment pre-mounted on the
machine
• Others require separately mounted motors
connected by belts, gears, or chains
Installation of Motors and
Control Equipment (cont’d.)
• Power source: considerations
– Single or three-phase
– Horsepower
– Inrush current
– Capability of existing power source
– Availability of power
Installation of Motors and
Control Equipment (cont’d.)
• Motor connections: considerations
– Horsepower
– Service factor (SF)
– Marked temperature rise
– Voltage
– Full load current rating
– (NEMA) Code letter
Installation of Motors and
Control Equipment (cont’d.)
• Motor type:
– Machines that employ gears
– Machines that require a long starting period
– Squirrel cage motors:
• Controlled by variable frequency drives or direct
current motors
• Used to power most of the machines throughout
industry
Installation of Motors and
Control Equipment (cont’d.)
• Controller type: motor starters are divided
into two classifications
– NEMA (National Electrical Manufacturers
Association)
– IEC (International Electrotechnical
Commission)
Installation of Motors and
Control Equipment (cont’d.)
• Consider environment in which the motor
and control system operate
– Housing of controls
– Hazardous areas
– Corrosives
– Extreme temperatures
Installation of Motors and
Control Equipment (cont’d.)
• Codes and standards:
– Occupational Safety and Health Act (OSHA)
– Underwriters Laboratories (UL)
– National Electrical Code (NEC®)
– NEMA
– IEC
Types of Control Systems
• Manual: operator must go to controller to
initiate any changes
• Semiautomatic: uses push buttons, limit
switches, etc., to control the operation
• Automatic: similar to semiautomatic but
the operator does not have to initiate
actions after control conditions are set
Functions of Motor Control
• Starting
– Examples: across-the-line, ramping
• Stopping
• Jogging and inching
– Methods employed to move a motor with
short jabs of power
Functions of Motor Control
(cont’d.)
• Speed control:
– Most common method:
• Variable frequency control for alternating current
motors
• Controlling voltage applied to the armature and
fields of a direct current motor
– Motor and Circuit Protection
Functions of Motor Control
(cont’d.)
• Surge protection:
– Protects against damage from spikes/surges
Metal oxide varistor (MOV) connected across
coils with alternating current
• Safety:
– Protection for anyone near the machine
Functions of Motor Control (cont’d.)