Principles of Electric Motor Protection


Motors are used in a wide range of industries and applications, from manufacturing and construction to transportation and energy production. Here are some specific examples of motor applications accross industries: 

  1. Fans, Blowers: Motors are used in fans and blowers in HVAC systems, as well as in industrial applications such as ventilation and air circulation. These motors must be efficient and able to operate at different speeds to adjust airflow as needed. 
  2. Pumps, Compressors: Motors are used in pumps and compressors for applications such as water treatment, oil and gas processing, and refrigeration. These motors must be able to provide high torque and operate continuously without overheating.
  3. Grinders, Chippers: Motors are used in grinders and chippers in the wood processing industry. These motors must be able to provide high torque and operate at different speeds to handle different materials and product sizes.
  4. Conveyors, Shredders: Motors are used in conveyors and shredders in material handling applications, such as in recycling facilities and food processing plants. These motors must be able to provide high torque and operate at different speeds to handle different materials and product sizes.
  5. Crushers, Mixers: Motors are used in crushers and mixers in the mining and construction industries, as well as in food and beverage processing. These motors must be able to operate at high speeds and provide high torque to handle tough materials and mixtures.
  6. Cranes, Extruders: Motors are used in cranes and extruders in various industries such as construction, manufacturing, and plastics processing. These motors must be able to provide precise positioning and control, as well as high torque and speed output.
  7. Refiners, Chillers: Motors are used in refiners and chillers in the chemical and pharmaceutical industries, as well as in food and beverage processing. These motors must be able to operate continuously and provide precise temperature control and cooling capacity.

These are just a few examples of the diverse range of motor applications in various industries. Motors are essential for many industrial processes and activities, and their performance and reliability are critical for efficient and safe operations.


Motor Failure Rates and Cost

Electric motors are crucial components in a wide range of applications, from industrial machinery to household appliances. However, like any mechanical or electrical system, electric motors can experience failure over time. The failure rate and causes of electric motors can vary depending on various factors such as the type of motor, operating conditions, and maintenance practices. Understanding the common causes of electric motor failure can help prevent unexpected downtime and costly repairs, while also improving the lifespan and performance of the motor. 

  1. Motor failure rate is conservatively estimated as 3-5% per year. In Mining, Pulp and Paper industry, motor failure rate can be as high as 12%.
  2. Motor failures divided in 3 groups i.e. Electrical (33%), Mechanical (31%), Environmental, Maintenance, & Other (36%). 
  3. Motor failure cost contributors i.e. Repair or Replacement, Removal and Installation, and Loss of Production.


Figure 1. Statistics of Motor Failure Rate


Thermal Stress

Thermal stress is a significant cause of electric motor failure. When an electric motor is in operation, it generates heat due to the flow of electrical current and the friction of moving parts. Excessive heat can cause thermal stress, which can damage the motor's components and reduce its lifespan.


The effects of thermal stress on an electric motor can be numerous. For example, it can cause insulation breakdown, which can lead to short circuits, overheating, and ultimately motor failure. Thermal stress can also cause expansion and contraction of the motor's components, leading to misalignment, vibration, and bearing damage. Overheating due to thermal stress can also reduce the efficiency of the motor, increasing energy consumption and operating costs.


Preventing thermal stress is crucial to ensure the long-term performance and reliability of an electric motor. Proper ventilation, cooling systems, and regular maintenance can help manage heat buildup and reduce the risk of thermal stress-related failures.



Related Article: Control, Monitoring and Protection of High Voltage Motors 



Stator Windings Insulation Degradation (for stator limited motors)

Overheating of an electric motor is a serious concern, as it can lead to various risks that can affect the motor's performance and reliability. First and foremost, an overheated motor can suffer significant damage to its insulation, which can result in short circuits and other electrical issues. This can not only damage the motor but also pose a safety risk to the equipment and people around it.


Another risk associated with overheating is the potential for mechanical failure. As the motor's components heat up, they can expand and contract, leading to misalignment, bearing damage, and increased vibration. These issues can cause additional stress on the motor, leading to premature failure, increased maintenance costs, and even safety hazards.


Insulation lifetime decreases by half if motor operating temperature exceeds thermal limit by 10ยบC for any period of time. 


Figure 2. Risk for an Overheated Motor


Overheating can also have an impact on the motor's efficiency, leading to increased energy consumption and operating costs. As the motor works harder to overcome the additional heat generated, it can draw more power from the electrical supply, increasing energy bills and potentially causing power quality issues.


To prevent these risks, it is essential to monitor the temperature of the motor regularly and take appropriate steps to manage heat buildup. Proper ventilation, cooling systems, and regular maintenance can help mitigate the risks associated with an overheated motor, ensuring reliable and safe operation over the long term.


Motor Electrical Protections

Motor electrical protection is critical for ensuring the safe and reliable operation of electric motors. Electrical protection systems are designed to detect and respond to abnormal operating conditions, such as overloading, voltage spikes, and short circuits, which can damage the motor and cause safety hazards. There are various types of electrical protection systems available, including thermal overload protection, current protection, and voltage protection. These systems work by monitoring the motor's electrical parameters and triggering protective measures, such as tripping the motor or reducing the voltage, when abnormal conditions are detected.


Proper electrical protection is essential for preventing damage to the motor and extending its lifespan. Without adequate protection, an electric motor is at risk of suffering from insulation breakdown, overheating, and other issues that can lead to premature failure and safety hazards. To ensure the proper electrical protection of an electric motor, it is essential to choose the appropriate protection system for the motor's application, size, and operating conditions. Regular maintenance and inspection can also help identify potential issues and ensure that the protection systems are functioning correctly.


Related Article: Electric Motor Protection in Case of Voltage Unbalance and Single Phasing


Overload Protection - Thermal Model

The thermal overload protection system works by monitoring the temperature of the motor and triggering protective measures when the temperature exceeds a set threshold. The thermal model used in the protection system is based on the thermal time constant of the motor, which is a measure of how quickly the motor's temperature changes in response to changes in load and other operating conditions.


Figure 3. Motor Thermal Limit Curves

Motor thermal limit curves are graphical representations of the maximum temperature that an electric motor can withstand without suffering permanent damage. These curves provide a visual reference for determining the safe operating range of the motor and for selecting appropriate thermal overload protection systems.


The thermal limit curves are typically based on the motor's insulation class and its maximum allowable operating temperature. The curves illustrate the maximum permissible temperature for a given duration of operation, with longer durations corresponding to lower permissible temperatures.


Thermal Overload Pickup

Thermal overload pickup is a parameter used in thermal overload protection systems to set the threshold temperature at which the protection system will be triggered. The thermal overload pickup is typically set based on the motor's insulation class and maximum allowable operating temperature. The thermal overload pickup is used in conjunction with the thermal time constant of the motor to calculate the temperature rise of the motor for a given load and operating condition. When the calculated temperature rise exceeds the thermal overload pickup value, the thermal overload protection system is triggered, and the motor is disconnected from the power supply.


Figure 4. Sample Motor Nameplate 


The steps below can be a guide on how to set the thermal overload pickup setting. 

  1. Set to the maximum allowed by the service factor of the motor. 
  2. Set slightly above the motor service factor by 8-10% to account for measuring errors. 
  3. If RTD Biasing of Thermal Model is used, thermal overload setting can be set higher. 
  4. Note that motor feeder cables are normally sized at 1.25 times motor’s full load current rating, which would limit the motor overload pickup setting to a maximum of 125%. 

The thermal overload pickup value can be adjusted to provide different levels of protection for the motor. A lower pickup value provides greater protection against thermal overload, but may also result in more frequent tripping of the protection system, leading to increased downtime and maintenance costs. Conversely, a higher pickup value may provide less protection but can result in longer operating times and fewer false trips.

In addition to the thermal overload pickup, thermal overload protection systems may also incorporate other parameters, such as time delay and reset time, to provide additional protection and flexibility. Time delay parameters can be used to delay the activation of the protection system for a set period, allowing for temporary overloads or starting current surges. Reset time parameters specify the time required for the motor to cool down before the protection system can be reset.


Motor Over and Undervoltage Protection

Motor over and under voltage protection is an essential component of the electrical protection system for electric motors. These protection systems are designed to prevent the motor from operating outside of the safe voltage range, which can cause damage to the motor and pose safety hazards. Overvoltage protection systems are designed to detect and respond to voltage spikes or other abnormal high voltage conditions that can cause the motor to overheat and suffer from insulation breakdown, winding damage, and other issues. These protection systems typically use voltage sensors to monitor the motor's voltage and trigger protective measures, such as reducing the voltage or disconnecting the motor from the power supply, when the voltage exceeds a set threshold.



Figure 5. Three phase voltage monitoring with phase sequence and phase failure relay



Undervoltage protection systems are designed to detect and respond to low voltage conditions that can cause the motor to stall or operate inefficiently. These protection systems use voltage sensors to monitor the motor's voltage and trigger protective measures, such as reducing the load or disconnecting the motor from the power supply, when the voltage falls below a set threshold.

Over and under voltage protection systems can be implemented using a variety of methods, including circuit breakers, relays, and electronic control systems. The selection of the appropriate protection system depends on the motor's application, size, and operating conditions.

Proper over and under voltage protection is crucial for ensuring the safe and reliable operation of electric motors. Without adequate protection, motors can suffer from insulation breakdown, winding damage, and other issues that can lead to premature failure and safety hazards. By implementing appropriate protection systems and regularly monitoring voltage levels, the risk of damage to the motor can be minimized, ensuring reliable and safe operation over the long term.


Motor Unbalance Load Protection

Motor unbalance protection is an electrical protection system designed to detect and respond to unbalanced voltages in three-phase electric motors. Unbalanced voltages can cause the motor to vibrate, overheat, and suffer from bearing damage, reducing its lifespan and performance. There are various causes of motor unbalance, including faulty electrical connections, damaged or worn motor components, and uneven distribution of loads across the phases. By implementing a motor unbalance protection system, potential issues can be identified early, allowing for corrective action to be taken before significant damage occurs.

Depending on the application, the device shown in Figure 5 (Three phase voltage monitoring with phase sequence and phase failure relay) can also be used to protect the motor against this problem.



Motor Ground Fault Protection

Motor ground fault protection is an electrical protection system designed to detect and respond to ground faults in electric motors. Ground faults occur when the current flows from the motor windings to the motor's frame or ground conductor, potentially causing damage to the motor and posing safety hazards.

Figure 6. Motor having phase to ground fault. 



The motor ground fault protection system typically uses sensors to measure the current in the motor's windings and ground conductor. If a ground fault is detected, the protection system triggers protective measures, such as disconnecting the motor from the power supply or reducing the load, to prevent further damage to the motor and to protect people and equipment from electrical hazards. Ground faults can occur due to a variety of reasons, such as insulation breakdown, moisture or contamination, or physical damage to the motor. By implementing a motor ground fault protection system, potential issues can be identified early, allowing for corrective action to be taken before significant damage occurs.

For small motors, a simple RCD can be used to detect ground faults but for bigger motors, a differential protection is needed. 


Figure 7. Differential Protection for motors



There are different methods to detect and protect motors against ground fault. Depending on the type of application. Thus, the proper motor ground fault protection is crucial for ensuring the safe and reliable operation of electric motors. Ground faults can cause significant damage to the motor and pose safety hazards to people and equipment. By monitoring the motor's current and detecting ground faults early, potential damage can be minimized, ensuring reliable and safe operation over the long term.


Motor Short Circuit Protection


Motor short circuit protection is an essential component of the electrical protection system for electric motors. Short circuits can occur due to a variety of reasons, such as insulation breakdown, physical damage, or electrical faults, and can cause significant damage to the motor and pose safety hazards. The motor short circuit protection system typically uses a circuit breaker or fuse to protect the motor against short circuits. The circuit breaker or fuse is installed in the motor's power supply and is designed to interrupt the electrical current when a short circuit is detected.


Figure 8. Circuit Breaker


Circuit breakers and fuses operate by monitoring the electrical current flowing through the motor and triggering protective measures when the current exceeds a set threshold. When a short circuit occurs, the current flowing through the motor increases significantly, triggering the circuit breaker or fuse to interrupt the current flow and prevent further damage to the motor. Proper motor short circuit protection is crucial for ensuring the safe and reliable operation of electric motors. Short circuits can cause significant damage to the motor and pose safety hazards to people and equipment. By implementing a circuit breaker or fuse, potential damage can be minimized, ensuring reliable and safe operation over the long term.

It is important to note that the selection of the appropriate circuit breaker or fuse depends on the motor's application, size, and operating conditions. Improper selection or installation of the protection system can result in inadequate protection or false trips, leading to increased downtime and maintenance costs.


Stator RTD Protection

Stator RTD (Resistance Temperature Detector) protection is an electrical protection system designed to detect and respond to temperature changes in the stator windings of electric motors. RTDs are temperature sensors that measure the resistance of the stator windings and provide an output signal that is proportional to the temperature. The stator RTD protection system typically uses sensors to measure the resistance of the stator windings and compares them to determine if there is any deviation from the normal temperature range. If an abnormal temperature increase is detected, the protection system triggers protective measures, such as reducing the load or disconnecting the motor from the power supply, to prevent further damage to the motor.


Figure 9. Temperature Sensors in a motor stator


Actually, there are also other temperature that can be placed to detect temperature rise aside from RTD. In addition to RTDs, there are other types of temperature sensors that can be used for stator temperature monitoring and protection in electric motors. These include:

  1. Thermocouples - Thermocouples are temperature sensors that measure the voltage difference between two wires made of different metals. The voltage difference is proportional to the temperature difference between the two wires, which can be used to measure the temperature of the stator windings. Thermocouples are fast and accurate but have a limited temperature range and can be affected by electrical interference. 
  2. Thermistors - Thermistors are temperature sensors that measure the resistance of a material, which varies with temperature. Thermistors can be used to measure the temperature of the stator windings and are less expensive than RTDs. However, they have a limited temperature range and can be affected by self-heating and aging. 
  3. Infrared sensors - Infrared sensors measure the temperature of the stator windings by detecting the infrared radiation emitted by the motor. Infrared sensors are non-contact and can measure the temperature of the motor's surface. However, they are affected by the motor's surface properties and are not as accurate as other types of sensors.

The selection of the appropriate temperature sensor for stator temperature monitoring and protection depends on the motor's application, size, and operating conditions. RTDs are commonly used due to their high accuracy and stability, but other types of sensors may be more suitable for certain applications.

In conclusion, motor protection is essential for ensuring the safe and reliable operation of electric motors. Proper selection and implementation of motor protection systems can minimize the risk of damage to the motor and improve its lifespan and performance. 

The selection of appropriate protection systems depends on the motor's application, size, and operating conditions. Various protection systems, such as thermal overload protection, over and under voltage protection, motor unbalance protection, motor ground fault protection, and stator RTD protection, can be used to monitor the motor's operating conditions and trigger protective measures when necessary. By implementing adequate motor protection systems, potential damage to the motor can be minimized, ensuring reliable and safe operation over the long term.


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  • Title: Motor Protection Principles
  • Source: General Electric

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