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Motor overloads occur when motors are drawing too much current. The main issue with excess current is that it creates heat which degrades the insulation surrounding the conductors that creates stator magnetic field. Continued degradation of the winding insulation results in failure of the insulation and eventual motor failure.
It is important to recognize that an overload condition exists but just as important to determine and correct the cause of the overload before attempting to restart the tripped motor.
There are many reasons why motors draw excess current, but they can be classified as mechanical, electrical or load related.
Mechanical issues include (but are not limited to) mass unbalance, shaft or bearing misalignment, over-tighten or loose belts. These faults are the most common sources of vibration associated with rotating equipment. Approximately 30% of the time, when these sources are present, they will create a resonant condition. Resonance occurs when the frequency of an oscillating force is near the natural frequency of a spring system. Resonance is an enormous energy robber and will create an increased load to the motor.
Electrical issues can be a cause of incoming power, such as voltage mismatch, (over voltage or under-voltage), voltage unbalance or excessive harmonic content. Winding insulation degradation or breakdown can cause intermittent faults. Since electrical insulation has a negative temperature coefficient, these faults disappear after the motor stops and the insulation cools. Electrical Rotor issues such as static or dynamic eccentricity, cracked, broken rotor bars or casting voids cause the rotor to operate below rated speed which reduces the back emf created by the rotors turning action and will cause an increase in rotor current.
Process or load issues such as too much flow, cavitation, flow resonance will also cause the rotor to run below the rated speed which causes the motor’s rotor current to increase, creating an overload condition.
To protect against these failures, motor controllers have protective relays (overloads) that automatically remove power from the motor to prevent these failures from causing the motor to catastrophically fail. In most applications the motor trip is the first indication of a problem in the motor system.
When this occurs, operators are allowed to attempt to restart the motor 3 times before contacting maintenance. However, depending on the cause of the overload, these restarts may be exacerbating the issue resulting in further motor damage or catastrophic failure. Restarting the motor does not address the cause of the excess current.
ALL-TEST PRO 7™ provides an easy-to-use handheld instrument that can provide you with a complete and thorough examination of the motor system from the Motor Control Center (MCC) in less than 3 minutes. These tests ensure the motor is “safe” to restart. This instrument will quickly evaluate the condition of the ground-wall insulation, winding insulation, and any developing rotor issues and assesses the condition of the motor and displays its condition on the instrument screen in one of three conditions, “Good”, “Warn”, or “Bad”.
After the motor is restarted or even before the motor trips the ATPOL III™ can be used to evaluate the entire motor system from incoming power to the entire process itself. The ATPOL III™ uses the motor’s voltage and current to completely analyze the entire motor system while the motor is operating under load. The ATPOL III™ performs a simultaneous data capture of all three phases of voltage and current to quickly evaluate any power supply issues that can cause the motors current to increase. Additionally, it performs an A/D conversion of the motor’s voltage and current that is uploaded to the ESA Software to evaluate the electrical and mechanical condition of the motor as well as the mechanical condition of the driven machine.
After resetting the overloads, the operator will restart the motor. If the motor operates successfully, that is usually the end of the situation. However, the reason the motor tripped is still unknown and it could cause additional trips to occur in the future. Typically, each subsequent trip occurs at reduced intervals indicating further degradation of the motors condition. However, before restarting the motor, a few basic mechanical and electrical checks should be performed.
Recommended procedures to an unexpected motor trip or the most basic checks performed before attempting to restart tripped motors:
• Mechanical check is to rotate the shaft: Does the coupled shaft rotate freely?
If not, determine if it is the motor or the driven machine that is preventing the motor system shaft from rotating freely, by separating the coupling and turning each of the machine’s rotating element. If either shaft does not rotate, correct the fault prior to attempting a restart. If either of the machines does not rotate freely suspect the bearing.
• Electrical checks
Use the ALL-TEST PRO 7™ to perform all the static tests and Insulation Resistance to ground (IRG) tests from the MCC. If a fault is detected from the MCC, retest directly at the motor. At the motor perform the static test, IRG, dissipation factor (DF) and capacitance to ground. If the Test Value Static (TVS) deviates by more than 5% from Reference Value Static (RVS) perform a Dynamic test. If the TVS is <3% from average and DF & IRG are within recommended range the fault is in the cabling or the controller.
After restarting the motor, conduct an energized test using the ATPOL III™ to evaluate the mechanical and electrical condition of the entire motor system. These one-minute tests will determine the quality of the incoming power, the electrical and mechanical condition of the motor, the mechanical condition of the driven machines as well as any anomalies of the process such as cavitation, pump impeller clearance issues or flow resonance.
The ESA Software then automatically analyzes the uploaded test results to evaluate and report the condition of the entire motor system in an easy-to-understand multi-page report that provides the electrical and mechanical condition of the motor and the driven machine.
Electric motors convert electricity into mechanical energy and are a key to powering a variety of modern devices. All sorts of equipment and devices use electric motors, including vehicles, fans, blowers, cranes, toys and other electronic mechanisms. An overload happens when an excessive amount of electricity passes through the system to the motor, which can lead to motor failure. To prevent electric motor breakdowns, motor overload relays are used to protect motors from too much current, inadequate torque and general overheating, which is a prime cause of motor failure. Today, we dive into the question: “How does a motor overload work to prevent failures?”
An overload occurs when a motor draws too much current. An overload triggers a relay that causes the motor to shut down, which prevents damage due to overheating. Overload relays are a component within a motor starter, which monitors the current flowing through the circuit in order to protect the motor. When current goes above a predetermined limit for a specific amount of time, it causes the overload relay to trip.
This results from an auxiliary contact that interrupts the motor’s control circuit, which in turn stops electrical energy from flowing to the contactor. When power stops flowing to the motor, it cannot overheat, saving the motor from catastrophically breaking down or otherwise being damaged. These overload relays have the capability to be reset manually, though some reset automatically, allowing the motor to restart.
The basic principle on which overload relays work is simple. Most depend on a component called a bimetallic strip, which is actually two strips made from distinctive metals that, when heated, expand at different rates. This arrangement within the motor’s circuit uses this differentiation in temperature changes to bend the bimetallic strip when it’s heated.
Once the bimetallic strip heats up, it allows the contact to activate, breaking the power supply and stopping electricity from moving toward the contactor coil. This stops the flow of current toward the motor, deactivating it. An overload relay allows electricity to flow through it during normal operation, only tripping if excess current flows through it. Tripping gives an operator time to determine the cause of the overload without jeopardizing the motor or possibly causing injury.
Overload relays are made up of more than contacts and a bimetallic strip.
Most also feature the following:
Electric motors are not all the same, and motor overload relays differ too in how they work, depending on the application for which they’re designed. Once these differences are understood, the question “How does a motor overload relay work?” can then be answered more definitively. Yet all overload relays have this in common: they protect the motor by deactivating it when too much electricity passes through the system.
Types of motor overload relays, along with their applications, include:
These devices primarily just cut off power after exposure to prolonged and excessive electrical current, similar to how circuit breakers operate. The difference in how thermal overload relays work involves their secondary purpose, which is to measure the motor’s heating profile over time. When too much electricity passes through the motor’s circuit over a specified period, heating the motor excessively, it shuts the motor off by breaking the circuit.
These relays work by detecting the strength of the magnetic field that’s generated by electricity flowing to the motor, cutting off power not in response to high temperatures but rather because of changes to the motor’s core. It’s often designed with a coil that surrounds a variable magnetic core and which holds current going to the motor. The coil drags the core upwards and, once it expands far enough, it trips connections at the relay’s summit. Magnetic overload relays work well in places where there are extreme changes in ambient temperature.
These overload relays rely primarily on the heating properties of the bimetallic strip, relying either on direct or indirect heating. Directly heated, these overload relays supply all the electrical current that flows to the motor, and it relies on the heat from the current’s flow to trip. Indirectly heated relays use a conductor within the relay that comes in close contact with the strip. Electrical flow heats both the conductor and bimetallic strip as the current flows towards the motor. When the current heats too much, it bends the bimetallic strip and activates a mechanism that halts the flow of electricity to the motor.
Also called solid-state overload relays, these don’t have a bimetallic strip, resulting in less heat loss through the relay. Instead, they feature current transformers that measure the current continuously passing on to the motor during each phase. These current flows pass through the transformers and a circuit that evaluates them to identify an overload, tripping the circuit when overloads occur. Electronic overload relays are controlled by a microprocessor within a device designed to protect three-phase motors when they experience phase failure or when temperatures rise to a certain level, which can affect single phase motors as well. Some manufacturers include design features to protect against the motor stalling or providing an earth fault. Electronic overload relays work well in situations when motors must stop and start frequently.
This relay features a tube that surrounds an eutectic alloy, a winding heater and a mechanism that activates the tripping device. The term “eutectic” means “easily melted”, so an eutectic alloy refers to a blend of materials that goes directly from a solid to liquid state without passing through an intermediate stage. This alloy dissolves due to heat, enabling a spring-loaded ratchet wheel to activate the tripping device when an overload occurs. Once tripped, the relay can be manually reset with a reset button on the relay cover.
In refrigerators, the overload relay is positioned within the compressor circuit. Current passes through the overload relay to the compressor motor windings. The relay brings the start winding into the circuit, but only includes it until the compressor begins to run at full speed. If the overload relay senses overheating, it shuts the compressor down temporarily to let it cool.
Springer Controls has been a trusted electrical controls industry leader since 1996, supplying motor starters, power-switching devices, 22 mm & 30 mm pilot devices, hoist controls, signaling devices, and other control devices across a variety of industries. If you have questions about motor overload and associated products, contact the experts at Springer Controls today!
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