Motor Protection Techniques: Overload, Short Circuit & Phase Failure

Aug 20, 2025

Technical Articles

Motors operate under continuous stress during regular use and maintenance activities. Without proper protection measures, they are vulnerable to excessive damage, which can significantly reduce their lifespan and efficiency. Developing possible risks and implementing appropriate protection devices at an early stage helps avoid unplanned outages and the associated repair costs. Various technologies and motor protection devices are essential in ensuring that they become reliable and efficient in numerous applications within industries and commercial premises.

This blog explores the key problems motors face, the importance of using reliable devices for motor protection, and how to choose the right components.

Motor Protection Concepts Explained

AC Motors consume almost 70% of industrial power. While DC motors allow for precise speed control, AC motors dominate the industry due to the easy availability of AC power. With the advent of Variable Frequency Drives (VFDs), AC motors now offer efficient, flexible speed control, further minimizing the use of DC motors in many applications.

Given their critical role in industrial operations, knowing how to protect them is essential for ensuring reliability and long-term performance.

What are the reasons for motor failure?

Motors can break down for many reasons, often because of electrical or mechanical problems. If these problems aren’t fixed in time, they can cause the motor to overheat, damage its insulation, and eventually result in irreversible failure.

Overload: This occurs when a motor is forced to carry a current higher than its rated full load current for an extended period. This causes a temperature rise in the motor winding, and if it persists, it results in the breakdown of the winding insulation.
Heavy Starting: Motors are built to handle high starting currents for a limited time only. However, when a motor starts under a heavy load, it takes longer to reach full speed. during this extended start-up, the motor draws high current for a prolonged period, which can put stress on the insulation and eventually cause it to fail..
Locked Rotor: If the motor's rotor becomes locked or stalled, it will draw the starting current indefinitely, leading to rapid and severe overheating.
Voltage Unbalance: When one phase of the power supply has a higher or lower voltage than the others, the motor draws negative sequence currents. These produce a reverse rotating magnetic field, causing excessive heating in the stator and rotor, as well as torque pulsations that lead to vibrations and noise.
Excessive Starts: If a motor exceeds its specified number of starts per hour, it can overheat. The winding temperature rises with each start, and frequent starting prevents the motor from dissipating this heat, causing the temperature to exceed its withstand limit and leading to insulation failure.
Single Phasing: This fault occurs when one of the three supply lines to the motor is lost (e.g., due to a blown fuse or loose connection). The motor will attempt to run on the remaining two phases, causing a significant increase in stator current and rapidly heating the stator and rotor windings.
Mechanical Problems: When components like bearings fail or the motor shaft becomes bent, the motor draws extra current than normal condition. This extra strain generates excess heat, which can eventually cause serious damage.
Cooling issues : Motors rely on proper cooling to handle the heat they produce during operation. If airflow is blocked—whether by clogged passages, a broken cooling fan, or similar problems—the motor won’t be able to release heat effectively and will start to overheat.
High Ambient Temperature: If the surrounding environment is too hot, the motor struggles to get rid of heat. Over time, this inability to cool down can lead to significant damage.

What are the disadvantages of allowing overcurrent through the motor?

Excessive current generates intense heat. This can melt the motor's windings and burn the crucial insulation that protects them. Insulation materials are classified based on their tolerance to temperature, as defined by the IEC 60085 standard.

What are the different classes of insulation as per IEC 60085?

Class of Insulation

Maximum Attainable Temperature

°C as per IEC 60085

105

120

130

155

180

>180

Traditionally, motors were built with Class ‘B’ insulation. Today, many modern motors use Class ‘F’ insulation for added safety. However, they are often designed to operate within Class ‘B’ temperature limits to maximize reliability and extend service life

What are the different protection philosophies for motor protection?

There are three main philosophies for protecting a motor:

Temperature-based: Motor windings and their insulation have a specific thermal limit according to their insulation class. Exceeding this temperature dramatically reduces the insulation's life. Temperature-based devices detect and trigger an alarm or trip the motor before the winding or bearing temperature crosses a safe limit.
Current-based: Current sensors monitor the motor's current draw to detect overloads, short circuits, and other current-based faults. When an abnormal current is detected, these devices trip the motor to prevent overheating and insulation or mechanical failure.
Voltage-based: This approach uses devices connected across the power lines to provide protection against conditions like single phasing, undervoltage, and overvoltage. It is often a more cost-effective solution than current sensing.

What are the devices available for motor protection?

Here are the common devices, categorized by the protection philosophy they use:

1. TEMPERATURE SENSING DEVICES

a) Thermostat: A simple temperature-operated switch that trips the power supply when a set temperature is reached.

Advantage: Economical.
Limitation: Low accuracy.
Use: Areas where coarse temperature sensing is acceptable.

b) Thermistor: A resistor whose resistance changes significantly with temperature. PTC (Positive Temperature Coefficient) thermistors are embedded in the motor windings during manufacturing. A control unit reads their signal and trips the motor if the temperature gets too high.

Advantage (PTC): Provides highly accurate temperature protection.
Limitation (PTC): More costly and can only be embedded at the time of manufacturing.

2. CURRENT SENSING DEVICES

a) Electro-mechanical Overcurrent Relay: Operates using magnetic or mechanical force when the circuit current exceeds a preset value.

Advantages: Simple, rugged, and low cost.
Disadvantages: Low accuracy, slow operation, and no "thermal memory." It does not account for accumulated heat, meaning a restart soon after a trip could still put an overheated motor at risk.

b) Thermal Overload Relay: Uses a bimetallic strip made of two different metals with different thermal expansion rates. As current heats the strip, it bends and actuates a tripping mechanism.

Advantages: Has thermal memory, is compact, accurate, offers a long life, can be ambient temperature compensated, and is low cost.
Disadvantages: Relatively slow operation and limited trip characteristics.

c) Fuses and Circuit Breakers (MCBs/MCCBs): These devices are crucial for short-circuit protection. They react instantly to the extremely high currents of a short circuit, disconnecting the motor in milliseconds to prevent catastrophic failure, fire, and damage to other components.

d) Electronic/Numerical Motor Protection Relay: A microprocessor-based relay that monitors current, voltage, temperature, and other motor parameters to provide accurate and flexible protection.

Advantages:

High accuracy and adjustable trip settings for precision.
Features thermal memory to protect against frequent starts.
Faster fault detection and response.
Supports self-checks and provides predictive maintenance information.
Integrates with SCADA/PLC systems for communication and remote monitoring.

Disadvantages:

More expensive and requires skilled personnel for commissioning.

3. VOLTAGE SENSING DEVICES

a) Undervoltage/Overvoltage Relays: This relay operates when the feeder voltage drops below or exceeds a set threshold, protecting the motor from harmful voltage fluctuations.

b) Phase Loss Relays: Detects the loss of one of the three phases (due to an open circuit, blown fuse, etc.). Since a motor running on two phases (single-phasing) overheats very quickly, this relay should trip the motor immediately.

c) Phase Reversal Relays: Checks the phase sequence of the power supply. It trips or blocks the motor from starting if the sequence is wrong, which is critical for equipment like pumps, compressors, and conveyors where direction of rotation is important.

Advantages: Protects against damaging voltage fluctuations and single-phasing conditions.
Disadvantages: Offers no selectivity between upstream and downstream breakers.

How to Choose the Right Motor Protection

With several options on the market, choosing the right solution depends on various factors. Here are some practical tips:

Know Your Application: Understanding where and how the motor operates is the first step. Is it for industrial automation, a domestic water pump, or an agricultural irrigation system? For instance, motors in remote setups benefit from a submersible pump motor starter with integrated protections.
Check Motor Ratings and Load Type: Always look at the motor's rated current, operating voltage, and the type of load it drives. Constant load applications need different protection than those with frequent starting or high-torque requirements.
Choose the Right Device: For critical operations, using a motor protection relay with multi-fault detection is preferred for its reliability. For basic protection, a thermal overload relay combined with an MCCB may be sufficient.
Look for Built-In Features: Many motor starters come with built-in trip features. Especially in submersible pump motor starters, integrated logic can protect against dry runs, overloads, and phase loss, all in a single compact unit.
Evaluate Long-term Reliability: Motor protection isn't just about preventing immediate damage; it's about increasing the motor's overall life and performance. Devices that record fault history or offer remote monitoring allow for better maintenance planning and reduce long-term operational risk.

Conclusion

Overloads, short circuits, and phase failures are common causes of motor damage. Protecting motors against such occurrences is necessary to prevent operational losses and costly breakdowns. The right protection not only helps prevent failures but also ensures the motor's long-term efficiency and performance.

At Lauritz Knudsen Electrical & Automation, we have a comprehensive range of motor protection products, including protection relays, submersible pump motor starters, and other essential devices. Backed by decades of trusted expertise and product quality, we empower every customer with knowledge, reliability, and nationwide support to enhance operational performance and reduce downtime.

Explore our motor protection solutions today and connect with our experts to get started.