Empowering Industries - Industrial Motor Protection Systems

Revealing the Key Benefits of SOLCON-IGEL’s Motor System & Protection Products

By Wassim Mousa

Introduction

In the examination of motor protection, it is essential to take into account that, during the initial phase, the machine draws a current approximately six times its rated value for a variable duration. This duration is dependent on the specific characteristics of the driven machine. In the case of a network short-circuit, the motor exhibits behavior akin to that of a generator in the initial moments. It supplies the fault with a current theoretically equivalent to the starting current. Both the startup conditions and the motor’s contribution to the short-circuit are significant operational scenarios. Therefore, the design of protection measures is crucial to prevent false trips under these circumstances.

Motor faults or abnormal operating conditions can be categorized and identified.

Motor Protection

The protection must be capable of recognizing the following issues:

• Overload
• Short-circuit
• Ground fault
• Prolonged starting
• Locked rotor during running
• Unbalanced load
• Excessive number of starts

Different protection functions can detect these operating conditions, functioning as backups for one another. For example, the identification of short circuits may involve the use of overcurrent protections and/or differential protections.

Essential protection functions recommended for a motor are as follows:

• ANSI 49: Thermal protection against overload
• ANSI 50: Overcurrent short-circuit protection (which may be in association with fuses)
• ANSI 87M: Differential protection
• ANSI 51G: Overcurrent ground fault protection
• ANSI 48: Overcurrent protection against prolonged starting
• ANSI 51LR: Overcurrent protection against locked rotor during running
• ANSI 46: Negative sequence overcurrent protection
• ANSI 66: Number of starts control protection.
• ANSI 27: Under voltage protection against motor disconnection
• ANSI 37: Undercurrent protection (typical for submerged pumps)
• ANSI 78: Protection against out-of-step for synchronous motors

Figure 1 – Solcon-IGEL Soft-Starter System

The provided image highlights the diverse range of protection solutions provided by Solcon-IGEL for various applications, designed to meet the specific requirements of clients.

The figure presents a variety of protection system configurations providing flexibility in selection based on factors such as machine power, connection unit, and neutral status.

Solcon-IGEL’s comprehensive range of protections underscores their commitment to delivering customized and effective solutions tailored to address the unique needs of their clientele across various applications.

Viewing the motor as a user terminal within the electric system, the protection system setting operates independently of other network protections. Differential protection holds particular significance for motors, especially in scenarios where short-circuit currents are low. This offers the advantage of faster trip times compared to standard overcurrent protection. However, implementing differential protection for motors can pose as a challenge, making the inclusion of star point side CTs necessary. At times, special versions may be required for the motor terminal box, or external cabinets may be needed for CT installation and star point formation.

ANSI 49 Thermal Overload Protection

The thermal overload protection element, mimicking the motor’s time constant, guarantees effective motor protection in cases of overload. In accordance with the thermal relay standard, the protection’s trip curve exhibits an exponential pattern.

In the protection setting process, particularly when the motor is designed for ‘HOT’ restarting, it becomes crucial to verify that the equivalent thermal curve allows the passage of the starting current for the necessary duration. In the thermal protection domain, it is standard practice to configure both the heating time constant and the cooling time constant to ensure optimal performance and effectively safeguard the motor.

(See my previous article “Optimizing Motor Protection: The Advantages of Coordination Tool for Thermal Element Setting in SEL-710 Relays“)

ANSI 46 Overcurrent Protection (Negative Sequence)

Balanced three-phase loads generate a synchronous rotating field in the stator that aligns with the rotor. However, in the presence of unbalanced loads, the negative sequence component in the stator current induces a rotor current with twice the frequency of the rated one. This induced current, flowing through the rotor winding, results in significant heating in the rotor.

This protection function is crucial for safeguarding the motor in scenarios involving one or two missing phases in the power supply, as well as for addressing unbalanced currents of minor amplitude. The protection mechanism is designed to activate over extended durations for small transient currents. Given its primary objective of preventing excessive heating in the rotor circuit, this protection feature necessitates a well-defined time characteristic.

ANSI 50 Overcurrent Protection

This protective function is dedicated to safeguarding the motor, especially when connected with a relative connection cable, from critical phase-to-phase or three-phase faults. In situations where outgoing feeders are equipped with both a contactor and a fuse, the short-circuit protection depends on the fuse. However, it is crucial to integrate short-circuit overcurrent protection to identify smaller faults that can be resolved by interrupting the contactor

In situations where a contactor and fuse are involved, the protection mechanism necessitates a delay to coordinate effectively with the opening characteristics of the contactor and the trip curve of the fuse. Conversely, when an outgoing feeder is equipped with a circuit-breaker, a delay in the trip time is invariably introduced to accommodate the passage of peak currents during motor starting. This meticulous approach ensures comprehensive protection against various fault scenarios in the motor system.

ANSI 51G/67G Ground Fault Overcurrent Protection

This functions as a homopolar overcurrent protection specifically designed to handle ground faults, with a crucial consideration being the evaluation of the necessity for directionality. Remarkably, directional protection is essential when the capacitive contribution of the motor and its associated cable surpasses the predefined threshold.

As a result, this protection mechanism operates as the initial stage of selectivity for detecting and addressing ground faults, emphasizing its pivotal role in the overall protective strategy for the motor system.

ANSI 48 Maximum Start Time

This protection function is exclusively activated during the motor’s startup phase, becoming inactive once the designated startup time elapses. Its primary objective is to verify that the motor initiates within the predetermined time frame. Exceeding the calculated starting time can result in overheating of the windings, posing a significant risk to the machine’s lifespan.

In the selection of the protection system for machines and plants, a crucial parameter for configuring this protection is the starting time. This parameter is intricately linked to the operational characteristics of the machine connected to the axle, underscoring the need for a tailored and precise approach in defining the protection settings.

ANSI 51LR Locked Rotor Overcurrent Protection

This protective function remains inactive during the initial starting phase and is engaged only after the startup process ends. It serves to inspect abnormal operating conditions in the motor resulting from a lock, often attributed to issues such as bearing failure or stall conditions.

ANSI 27 Under-Voltage Protection

An under-voltage relay employed for motor protection serves various purposes in different applications:

1. To prevent voltage drops in the network from reaching critical levels, which could potentially lead to motor stall conditions.
2. To confirm that the voltage is sufficiently high when a circuit breaker closes, ensuring proper conditions for motor startup.
3. To disconnect the motor in the absence of voltage in the network, preventing simultaneous restarts with other machines and the subsequent risk of network collapse.

The settings and operational commands of the protection mechanism vary based on the specific function it is intended to fulfill, highlighting the adaptability of under-voltage relays in addressing diverse motor protection requirements.

ANSI 87M Differential Protection

Differential motor protection employs two distinct techniques:

1. Compensated Differential Protection: This method employs a set of three line-side and three star-point-side current transformers (CTs). A challenge with this approach arises from the significant performance demand placed on the CT on the star-point side, which must effectively supply the connection cable between the CT and the relay, considering the distance between the switchgear and the motor.
2. Self-Balancing Protection: This alternative utilizes only one set of three star-point-side CTs. Each CT, covering a phase of the motor, measures the vectorial sum of the incoming and outgoing currents. Any non-zero value indicates a fault within the machine.

This protection function, operating as a simple overcurrent relay with an instantaneous threshold, offers the advantage of allowing the CT for differential protection to have a much lower rated primary current than the motor’s rated current. This characteristic enhances the sensitivity of the protection, surpassing even that of the traditional differential protection method.

Conclusion

The detailed exploration of motor protection considerations, as well as the extensive array of solutions presented by Solcon-IGEL, underscores the company’s commitment to innovation and client-centric solutions.

By addressing crucial aspects such as overload, short-circuit, ground fault, and various abnormal operating conditions, Solcon-IGEL demonstrates a thorough understanding of the diverse challenges faced by different applications and offer cutting-edge technology.

In essence, this article not only serves as a comprehensive guide to motor protection considerations but also highlights Solcon-IGEL position as a leading provider motor solutions and client-specific protection solutions in a diverse industrial applications.

Our commitment to addressing the unique needs of our clients positions us as a reliable partner in ensuring the optimal performance and longevity of motor systems across diverse sectors.