Motor Control EE-232

Overview of Motor Control

• Motor control has a broad meaning, ranging from simple to complex systems
• Motor control systems consider various factors, including power source, motor connection, motor type, environment, and code and standards

Basic Motor Principle

• A motor consists of two main parts: stator and rotor
• Stator: stationary electrical part with windings that change polarity when an AC is applied, creating a rotating magnetic field
• Rotor: moving part with conductive bars that interact with the stator's magnetic field, causing it to rotate

Motor Control Systems

• Three types of motor control systems:
  1. Manual
  2. Semi-automatic
  3. Automatic
• General purpose of motor control: to control motor output parameters, such as speed, position, acceleration, torque, and power
• Important functions of motor control:
  • Starting and stopping
  • Jogging and inching
  • Speed control
  • Motor and circuit protection
  • Surge protection
  • Safety

Symbols and Schematic Diagrams

• Motor control diagrams use various symbols, including:
  • NC (normally closed) and NO (normally open) contacts
  • Push-button switches
  • Sensing devices
  • Overload relays
  • Motor symbols
• Standardization bodies:
  • NEMA (National Electrical Manufacturers Association)
  • ANSI (American National Standards Institute)
  • IEC (International Electrotechnical Commission)
  • PEC (Philippine Electrical Code)

Motor Protections

• Common motor failures and faults:
  • Electrical stress: overload, insulation breakdown, locked rotor, and phase imbalance
  • Mechanical stress: bearing failure, running reverse, shaft misalignment, vibration, and overheating
• Overcurrent protection: protects against short circuits and excessive current
• Overload protection: protects against overheating due to sustained overcurrent
• Overvoltage protection: protects against excessive voltage

Overload Relays

• Types of overload relays:
  • Thermal: operates based on heat produced by motor current
  • Magnetic: operates based on the strength of the magnetic field produced by motor current
• Thermal overload relays:
  • Divide into solder melting type and bimetal strip type
  • Sensitive to ambient temperature
• Magnetic overload relays:
  • Not sensitive to ambient temperature
  • Generally used in areas with extreme temperature changes

Relays, Contactors, and Motor Starters

• Relays and contactors: electromechanical switches that operate on the solenoid principle
• Motor starters: contactors with an overload relay
• Motor control centers (MCCs): combinations of motor starters, circuit breakers, fuses, disconnects, and control transformers
• Equipment standards: NEMA and IEC

Control Transformers

• Control transformers: step down 240 or 480 volts to 120 volts for control systems
• Typically have two primary windings and one secondary winding
• Power rating: 250 volt-amperes

Timing Relays

• Time-delay relays:
  • On-delay relays (DOE): delay changing contacts when energized
  • Off-delay relays (DODE): delay changing contacts when de-energized
• Motor-driven timers: used in applications where a motor is run for a specific period

Pressure Switches and Sensors

• Pressure switches: sense pressure changes in pneumatic or hydraulic systems
• Available for sensitivities as low as 1 psi and pressures up to 15,000 psi
• Used in various industrial applications### Industrial Motor Control
• Pressure switches are used to turn off a motor and turn on a pilot warning light
• Pressure sensors produce an output voltage or current dependent on the amount of pressure sensed
  • Piezoresistive sensors are popular due to their small size, reliability, and accuracy
  • Available in ranges from 0 to 1 psi and 0 to 30 psi
• Float switches are used to start and stop a pump motor according to changes in liquid level in a tank or sump
• Phase failure switches detect single phasing in a three-phase motor, which can cause the motor to draw excessive current
• Operation of a motor at other than its rated voltage can affect its performance
  • NEMA-rated motors are designed to operate at plus or minus 10% of their rated voltage
  • Voltage variation affects full-load current and starting current

Determining Voltage Unbalance

• Unbalanced voltage can cause harm to motors
• NEMA recommends that unbalanced voltage not exceed plus or minus 1%
• Steps to determine percentage of voltage unbalance:
  1. Take voltage measurements between all phases
  2. Find the average voltage
  3. Subtract the average voltage from the voltage reading with the greatest difference
  4. Determine the percentage difference

Basic Control Circuit

• Control circuits can be divided into two-wire and three-wire control circuits
• Two-wire control circuits use a simple switch to make or break connection to a motor
• Three-wire control circuits use momentary contact devices such as push buttons
  • Require three wires to control a starter with momentary contact devices
  • Can be mounted in remote locations
  • Do not restart automatically after a power failure

Jogging and Inching

• Jogging or inching involves quickly repeated closure of a circuit to start a motor from rest
• Jogging starts a motor with short jabs of power at full voltage
• Inching starts a motor with short jabs of power at reduced voltage

Motor Starting Methods

• Various methods for starting motors are available, including:
  1. Direct-on-line starting
  2. Star-delta starting
  3. Auto-transformer starting
  4. Soft starting
  5. Frequency converter starting
• Starting methods are used to reduce the starting current
• Different starting methods have advantages and drawbacks

Direct On Line (DOL) Start

• Direct-on-line starting means connecting the motor directly to the supply at rated voltage
• Advantages:
  • Simplest, cheapest, and most common starting method
  • Gives the lowest temperature rise within the motor during start-up
• Drawbacks:
  • Can cause high inrush currents
  • Not suitable for motors that start and stop frequently

Star-Delta Motor Starting

• Star-delta starting reduces the starting current by connecting the current supply to the stator windings in star (Y) for starting
• Advantages:
  • Reduces the starting current to about one third of the DOL value
  • Suitable for high inertias
• Drawbacks:
  • Reduces the starting torque to about 33%
  • Can cause a current surge during the switch from star to delta connection

Autotransformer Motor Starting

• Autotransformer starters reduce the amount of inrush current by reducing the voltage applied to the motor during the starting period
• Advantages:
  • Reduces the starting current
  • Provides a low-voltage starting method
  • Can be adjusted to suit specific requirements
• Drawbacks:
  • Can cause a current pulse when the motor is switched to full voltage
  • Reduces the torque during the starting period

Soft Starter

• Soft starters are semiconductor-based devices that ensure a soft start of a motor

• Advantages:

  • Gradually increases the motor voltage during the starting process
  • Reduces motor torque during the starting period
  • Less expensive than frequency converters

• Drawbacks:

  • May inject harmonic currents into the system
  • May disrupt other processes### Industrial Motor Control

• Soft starters and frequency converters can be used to set run-up time and locked-rotor current (starting current).

Variable Frequency Drive

• Frequency converters are designed for continuous motor feeding but can also be used for start-up only.
• Advantages of frequency converters include:
  • Low starting current, as the motor can produce rated torque at rated current from zero to full speed.
  • Decreasing cost, making them increasingly used in applications where soft starters would previously have been used.
• Drawbacks of frequency converters include:
  • Higher cost compared to soft starters in most cases.
  • Injection of harmonic currents into the network.

Summary of All Starting Methods

• The main objective of motor starting methods is to match the torque characteristics to those of the mechanical load while ensuring peak current requirements do not exceed the supply capacity.
• Run-up times should not be too long to avoid unnecessary heating of the winding.
• Various starting methods are available, each with different characteristics.

Braking

• Methods of braking for motors include:
  • Mechanical brakes (drum and disk types).
  • Dynamic braking (used for both direct and alternating-current motors).
  • Plugging (used for direct-current motors and three-phase, squirrel-cage motors).
• Dynamic braking advantages:
  • No mechanical brake shoes to wear out.
• Dynamic braking disadvantages:
  • Cannot hold a suspended load.
• Plugging involves:
  • Reversing the motor connections to develop a counter torque that acts as a retarding force.
  • Reversing contactor is typically larger than the forward contactor due to increased plugging current.