L&T Motor Starter Handbook

CONTENT

SECTION I 

SECTION II

Normal connections for4 Direct-on-line (DOL) starters

 SECTION III

Normal connections for19 Star-delta starters 

SECTION IV 

Normal connections for70 reversing Starters

 Normal connections for75 pole changing Starters 

SECTION V

Applications79 

SECTION VI

Chart of relay ranges90 & back up fuses

 SECTION VII

Information for Installation &107 Preventive Maintenance 

SECTION VIII

Trouble shooting chart111 for starters

 SECTION IX

Capacitor ratings for115 power factor improvement 

SECTION X 

Switchgear Standard Products Range118 Switchgear Training Centers * 

Section IV 

contains wiring diagrams of the starters which are not manufactured by L&T at present. Wiring diagram of such starters are given for reference purpose. General

SECTION I

General Relay setting 

Tripping of starters 

Back up fuses 

Coil Voltage 

 essential to keep the following points in mind :

 1) Relay setting The overload relay unit is the protection center of the motor starter. Relays come in a number of ranges. Selection of a relay for a starter depends upon motor hp and duty. A selection table indicating the proper range for different ratings is given in section VI. For all direct-on-line starters, relays should be set to the actual load current of the motor. This value should be equal to or lower than the full load current indicated on the motor name plate. Here is a simple procedure for setting the relay to the actual load current: Set the relay to about 80% of full load current. If it trips, increase the setting to 85% or more. The relay should never be set at more than the actual current drawn by the motor (The actual current drawn by a motor will be less than the full load current in most of the cases, as motors may not be loaded to capacity.) The relay operates on the phase currents in Star-Delta starters. It should, therefore, be set at 0.6 times actual load current. A procedure similar to the one described for setting relays in DOL starters can be followed for Star-Delta starters. 

2) Tripping of starters A starter may trip due to the following conditions; a) Low voltage or failure of power supply b) Persistent overload on the motor In the first instance, the tripping occurs through the coil which opens the contacts when voltage falls below a certain level. The starter can be restarted as soon as supply is back to normal. The relay trips the starter when there is an overload. It can be restarted only after the relay is reset. However, the cause of overload must be investigated before restarting, or else the starter will trip again. Persistent overloading can reduce the life of the motor. 23 L&T offers manual reset type thermal overload relays. These relays clearly discriminate between tripping caused by an overload and tripping due to failure of supply voltage. MN relays also give protection against single phasing and overload. MN relays are provided with Auto/Manual reset facility.When a starter trips because of an overload, it cannot be restarted by pressing the start button. The stop button has to be pressed first to manually reset the relay before restarted. The relays can be reset only after the bimetals have cooled sufficiently.

 Resetting must therefore be carried out after one or two minutes. This will also permit the motor to cool down. Back-up fuses Motor starters must never be used without back-up fuses. The sensitive thermal relay mechanism is designed and calibrated to provide effective protection against overload fuses preferably the HRC cartridge type, are essential for protecting the installation as well as the thermal overload relay of motor starter against short circuits. A reference table indicating fuse ratings for different ratings is given in Section VI. It is recommended that the use of semi-enclosed, rewirable tinned copper fuses may be avoided. Coil voltage Selection of coils depends on the actual supply voltage available. A wide variety of coil voltages like 24V, 42V, 110V, 220V, 230 / 250V, 360V, 400 / 440V ac are available as standard for L&T contactors and starters.

 

The L&T Motor Starter Handbook is a comprehensive guide developed by Larsen & Toubro (L&T) specifically for understanding and implementing motor starters in industrial and commercial applications. This handbook provides detailed information on various types of motor starters, including Direct-On-Line (DOL) starters, Star-Delta starters, and Soft Starters, as well as the protection systems and control mechanisms required to safely and efficiently operate motors.

Key Areas Covered in the Handbook

1. Motor Basics and Starter Types: The handbook begins with an overview of electric motors, describing different types (e.g., single-phase, three-phase) and their starting requirements. It explains the purpose of a motor starter—to safely start and stop a motor by controlling the inrush current, which is usually high when a motor first starts. The handbook then categorizes and explains the various types of starters, such as:

   • Direct-On-Line (DOL) Starters: These are used for motors that can tolerate high starting current and are generally suitable for low-power motors.
   • Star-Delta Starters: Suitable for larger motors, Star-Delta starters reduce the initial starting current by configuring the motor to start in a star configuration and then switch to a delta configuration.
   • Soft Starters: These use solid-state devices to gradually ramp up the voltage, thereby providing a smooth start and reducing mechanical stress.

2. Protection Systems: L&T emphasizes the importance of protection for motors and associated equipment. The handbook covers various types of protection, including:

   • Overload Protection: Protects the motor from excessive current due to overload conditions.
   • Short-Circuit Protection: Prevents damage to the motor and starter components in the event of a short circuit.
   • Phase Loss Protection: Ensures that the motor shuts down if there is a loss of one phase in a three-phase system, preventing damage from imbalanced loads.

3. Control Circuits and Wiring Diagrams: The handbook includes wiring diagrams and circuit designs that illustrate how to install and connect motor starters. It explains control circuit components, such as contactors, overload relays, and auxiliary contacts, and offers guidance on selecting the correct components based on the motor’s power requirements and application.

4. Selection Guidelines: To help users select the appropriate starter for their application, the handbook provides selection criteria based on motor type, load conditions, and operational requirements. L&T includes tables and charts with technical specifications, making it easier for engineers to determine the correct starter type, protection rating, and component compatibility.

5. Troubleshooting and Maintenance: Maintenance practices are essential for ensuring the longevity of motor starters, and the handbook provides guidance on regular inspections, testing procedures, and maintenance protocols. It also includes troubleshooting tips to help users identify common issues such as tripped starters, unusual noises, or overheating, along with step-by-step solutions.

6. Case Studies and Application Examples: L&T includes real-world applications and case studies to illustrate how their starters perform in different environments, such as manufacturing plants, HVAC systems, water treatment facilities, and more. These examples demonstrate the versatility of L&T motor starters and offer insights into best practices for installation and operation.

Who Can Benefit from the Handbook?

The L&T Motor Starter Handbook is designed for engineers, electricians, maintenance personnel, and students who need a practical understanding of motor starters. It serves as a valuable resource for both beginners and experienced professionals who want to expand their knowledge of motor starter applications, component selection, and system design.

Why the Handbook is Valuable

With the L&T Motor Starter Handbook, users gain access to a well-organized, reliable source of information backed by one of India’s most respected engineering companies. The detailed explanations, practical examples, and technical guidelines make this handbook an essential tool for anyone involved in motor installation and maintenance. By following the guidelines and recommendations, users can optimize motor performance, enhance safety, and reduce the risk of electrical faults.

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