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Questions and Answers
What type of overload relays are generally used in areas with extreme changes in ambient temperature?
Magnetic-type
What is the formula to calculate the full load current (IFL) for a motor?
(P x 1000) / sqrt(3 x V x eff)
How are relay contacts constructed in a clapper type relay?
Motor starters are contactors with the addition of an overload relay.
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What interlocking function does the normally closed R contact provide in the circuit?
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What is an advantage of using an autotransformer in motor starting?
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Frequency converters are generally more expensive than soft starters.
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The ______ gradually increases the motor voltage during the starting process, thereby allowing the motor to accelerate the load to rated speed without causing high torque or current peaks.
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Match the braking method with its description:
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What is the voltage unbalance percentage calculated to be?
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What is the percentage of heat rise in the motor caused by the voltage unbalance?
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Which type of control circuit requires three wires and uses momentary contact devices like push buttons?
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Star-delta starting method is used to reduce the starting current of induction motors.
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What percentage of the normal voltage is typically connected to the motor during the starting period in autotransformer starters?
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What is the purpose of a motor control system?
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What is the most important function of a motor control system?
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What are some key functions employed by motor control systems?
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What are the two types of stress that motors are subject to?
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Overload protection is a protection against overheat due to the flow of overcurrent in the circuit for specific _______.
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Match the following motor protection terms to their definitions:
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Study Notes
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:
- Manual
- Semi-automatic
- 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:
- Take voltage measurements between all phases
- Find the average voltage
- Subtract the average voltage from the voltage reading with the greatest difference
- 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:
- Direct-on-line starting
- Star-delta starting
- Auto-transformer starting
- Soft starting
- 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
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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.
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Description
Quiz on basic motor principles, motor control principles, and motor protection devices. Covers topics such as overload relays, contactors, and timing relays.
