How to check motor overload


  • Electric Motor Tripping Reasons and How to fix them, Star Delta Motors
  • Basic Motor Starter Troubleshooting
  • Bimetallic Strip
  • Take a step-by-step approach to analyze motor starting problems
  • Quick Answer: How Do You Check Motor Overload 2
  • Inrush Current
  • Electric Motor Tripping Reasons and How to fix them, Star Delta Motors

    What are thermal overload relays and what motion components do they protect? October 21, By Danielle Collins Leave a Comment Heat is a major factor in the performance and life of a motor, and one of the primary sources of motor heating is current running through the motor windings.

    In a previous post, we described several types of sensors that can measure the temperature of motor windings directly. But in some cases — particularly for AC induction motors — motor heating can be measured indirectly by thermal overload relays, which determine motor temperature by monitoring the amount of current being delivered to the motor. Thermal overload relays are wired in series with the motor, so the current flowing to the motor also flows through the overload relay.

    When the current reaches or exceeds a predetermined limit for a certain amount of time, the relay activates a mechanism that opens one or more contacts to interrupt current flow to the motor. Thermal overload relays are rated by their trip class, which defines the amount of time for which the overload can occur before the relay responds, or trips.

    Common trip classes are 5, 10, 20, and 30 seconds. Taking time, as well as current, into account is important for AC induction motors because they draw significantly more than their full rated current often percent or more during startup. So if the relay tripped immediately when the overload current was exceeded, the motor would have difficulty starting.

    There are three types of thermal overload relays — bimetallic, eutectic, and electronic. Bimetallic thermal overload relays sometimes referred to as heater elements are made of two metals, with different coefficients of thermal expansion, that are fastened or bonded together.

    A winding, wrapped around or placed near the bimetallic strip, carries current. In a bimetallic thermal overload relay, heating due to current flow causes the bimetallic strip to bend to one side, activating a trip mechanism. Image credit: Siemens As the current running through the relay and, therefore, through the motor heats the bimetallic strip, the two metals expand at different rates, causing the strip to bend toward the side with the lower coefficient of thermal expansion.

    When the strip bends, it actuates a normally closed NC contactor, causing it to open and stopping the current flow to the motor. Once the bimetallic relay has cooled and the metal strips have reverted to their normal state, the circuit is automatically reset and the motor can be restarted.

    Eutectic thermal overload relays use a eutectic alloy a combination of metals that melts and solidifies at a defined temperature , housed in a tube and connected to a heater winding. The supply current to the motor flows through the heater winding and heats the alloy. When the alloy reaches a sufficient temperature, it transforms rapidly to a liquid.

    In a eutectic thermal overload relay, heating due to current flow causes a eutectic alloy to rapidly liquify, activating a mechanical device that trips the relay.

    Image credit: Rockwell Automation As a solid, the alloy holds a mechanical device, such as a spring or ratchet, in place. But when the alloy melts, the mechanical device releases, opening the overload contacts.

    Like the bimetallic design, a eutectic thermal overload relay cannot be reset until the alloy has sufficiently cooled and returned to its original, solid state. Electronic thermal overload relays are more accurate and reliable than heater designs, and they can provide data for diagnostics and preventive maintenance. Image credit: ABB Electronic thermal overload relays measure current electronically, rather than relying on a heater mechanism, and so are insensitive to changes in ambient temperature.

    Electronic designs can also protect motors against phase loss also referred to as phase failure , which occurs when one phase of current equals zero amps, often due to a short circuit or blown fuse. This causes the motor to draw excessive current on the remaining two phases and leads to significant motor heating. Thermal overload relays are typically part of the motor starter , which includes the overload relay plus contacts.

    Basic Motor Starter Troubleshooting

    What causes a relay to fail? How do you check motor overload? Overload Protection Test: Measure the normal motor running current i motor. Turn off the motor and let it cool for about 10 minutes. Set the overload to its minimum FLA and turn on the motor. Wait for the overload to trip. How do you troubleshoot a motor overload?

    Check the motor starter contactor, the fuses, and the overload relay. Start the motor and check the running motor amps and compare to rated FLA and overload heater size using a clamp-on ammeter. What are 2 types of overload relays? There are two major types of overload relays: thermal and magnetic. What would cause a motor to overload? Motor overloading can be caused by an increase in the load being driven by the motor, bearing failure in the motor or the driven load, or an electrical problem such as a low input voltage or single phasing.

    What causes a motor overload to trip? Causes may include a large change in load e. Power problems e. How do you know if a relay is defective?

    The quickest and easiest way to test a relay is to swap it out with a known good replacement. Grab a multimeter and set it to Ohms. Leave the multimeter on ohms or continuity. Energize the electromagnet coil with a V battery across the pins. How do I reset my overload relay? On the other hand, when set in A or AUTO position, the overload relay will reset automatically after a tripping event. How do I know if my overload refrigerator relay is bad?

    The Shake and Rattle Test Unplug the start relay from the compressor and give it a shake. If you can hear rattling on the inside of the start relay, then the part is bad and will have to be replaced. Is the most common cause of motor failure?

    Low Resistance. The most common cause of motor failure, and arguably the most difficult to overcome, is low resistance. Low resistance is caused by the degradation of the insulation of the windings due to conditions such as overheating, corrosion, or physical damage. What causes motor failure? Common causes of winding failure include high temperature, too many starts, contamination, too much current, excessive voltage and physical damage.

    Shaft failure is the least common cause of motor failure. Physical damage, corrosion, improper installation and excessive loads are often the root cause. How do you set a motor overload? What are the three types of overload? There are three types of thermal overload relays — bimetallic, eutectic, and electronic. Can you overload crit Genshin? Overloaded damage is only affected by the level and Elemental Mastery of the character causing the reaction and enemy elemental resistance.

    What is the symbol for a thermal overload? The thermal overload relay, whose symbol is shown in ill. The magnetic overload symbol is the same as the symbol for a relay with one normally closed contact.

    The current flow is relayed to the overload coil. What must be done before a motor overload can be reset? The overload relay is reset by first turning the starter all the way off and then turning it back on. What happens when you overload an electric motor? Motor overload occurs when a motor is under excessive load. Excessive motor heat is a major cause of motor failure. In the case of an overloaded motor individual motor components including bearings, motor windings, and other components may be working fine, but the motor will continue to run hot.

    How do you tell if an electric motor is burned out? When a motor burns out, a short circuit occurs in the windings and the motor ceases to turn. How do I know a motor is bad? With a multimeter set to low ohms usually , test between each winding terminal and the metal casing of the motor. If there is any reading on any of these then the motor is bad, do not use it. You may find that when it runs ungrounded that the casing becomes live at up to supply voltage. What is the cause of overload?

    Overuse of extension cords and multiple plug adapters on the same circuit are typical causes of an electrical overload — by placing too much current demand on the circuit.

    Circuit overloading is common around the holidays, when more electricity is used around the home for electric decorations. Can Autozone test a relay? A relay can be checked with a jump cable, a voltimeter, an ohmimeter or a test light.

    If the terminals are accessible and the relay is not controlled by a computer, the fastest method will be a jump cable and a test light. What does a bad relay sound like? A faulty relay often produces an audible clicking sound when you turn your car. Contact a mechanic immediately if your car fails to start and you notice this sound. In fact, the life of a relay is essentially determined by the life of its contacts. Degradation of contacts is caused from high in-rush currents, high- sustained currents, and from high voltage spikes.

    Relays can also fail due to poor contact alignment and open coils. Related Posts.

    Bimetallic Strip

    This final continuity check will also verify that the motor line does not have an open phase.

    Take a step-by-step approach to analyze motor starting problems

    If the motor starter line circuits are clear, the problem is on the load side of the starter. Follow the circuit to the load side of the starter. Try to reset each of the motor thermal overloads on the motor starter. If any one of the overloads have tripped you will be able to feel when the overload resets while pressing the reset pushbutton.

    If you completed the continuity check of the load side of the motor contactor described in Step 2 and the circuits are clear, restore the incoming power and start the motor. If the continuity check on the load side of the contactor is shorted phase-to-phase, phase-to-ground or open phase-to-phase, then follow the power circuit to the motor. Disconnect the incoming motor leads at the ac motor electrical conduit box. Check the motor starter leads for a phase-to-phase or a phase-to-ground short.

    If either of these two problems exists, check the motor lead wiring. If the motor starter leads are clear, move on to the motor.

    With the motor still disconnected, check the motor leads for a phase-to-phase or a phase-to-ground short. There is a high probability that if there were no problems found previously in the circuit, the motor has a phase-to-phase short or a short-to-ground fault.

    If you have verified a phase-to-phase or phase-to-ground fault in the motor, replace the motor and start the system. If there appears to be no problem with the ac motor or its power circuit, then test the motor insulation with an insulation tester.

    If the insulation test results are negative, a mechanical problem is probably causing the motor overload. Uncouple the motor from the mechanical load and start the motor. If the motor runs, the problem is in the mechanical drive train.

    If the motor does not run and the motor overloads trip again, the problem is in the motor bearings. Troubleshooting control circuit problems You can also use a process of elimination to isolate and diagnose control circuit problems.

    Determine if all or any of the motor starter overloads are tripped by pressing the motor overload reset pushbutton. If the motor overload s were tripped, try to restart the motor. If the problem was isolated to the motor overload sthe motor should run. If the motor overloads are not tripped, verify that the starter control circuit has the proper secondary voltage using a DMM set to voltage mode and the appropriate range. Using the DMM in continuity mode Ohmsverify the integrity of the control voltage transformer fuse.

    Replace the fuse if blown, and identify the motor starter control circuit problem.

    Quick Answer: How Do You Check Motor Overload 2

    If the starter control transformer fuse is good, proceed to Step 8 of this troubleshooting procedure. Disconnect the motor starter contactor coil and check for a short, ground or open circuit. If a short or ground is found, replace the motor starter contactor coil and start the motor. If the motor start contactor coil is good, with the coil still disconnected, check the control circuit wiring to the contactor coil. Check the relays, if connected with star delta connection and if overheating according to weather or motor over current or due to reverse direction of the motor Check the motor from down side, if submersible motor Check the breaker rating size if less.

    Check the fuses if connected in Star Delta connection. Check the insulating of the cable. Check the power supply. How to Solve the Problem: Now, I am going to explain each one in detail, if you carefully read all the following steps, I am sure you will be able to fix the electric motor tripping problem. This is really an important test, because with the help of this test you can easily find, if the cable is cut, or if the wires or short together.

    If you can read the value of the resistor, then the cable is ok and you can start the motor if there are no more problems in the motor. In this step engineers or technicians can check the motor direction, if it is clockwise or counter clockwise.

    Inrush Current

    If counter clockwise, it means the motor is not working in good condition due to which the electric motor will overheat and with the passage of time may lead to the permanent damage of the electric motor.

    Because of the wrong direction the water or other liquids pressure will be low. So this type of problem can be fixed by changing any of the two wires from the motor terminal connection in the panel box.

    But if the motor rotates in clockwise it means the motor is ok. So, good water pressure ensures that the motor is working in good condition.

    In this picture you can see water pressure is good it means the motor is working in good condition so no need to change the wires. If your motor is connected with a timer for protection purpose you will need to check the timer setting value, if the time value is ok and still the electric motor is tripping then you should check your motor rated current from the motor name plate e.

    But keep in mind that never increase the current setting value above rated current because, if you select a greater value, there will be more chances of motor damaging.

    Also you can check the motor relay connected, if tripping due to hot weather or not working properly then change you can check with hand touch if the relay is hot then change. If the type of electric motor you are using is submersible Submersible motor is the kind of electric motor which is dipped in water or any other liquid and constantly tripping, then you will need to make sure that the water or liquid is clean.

    When a submersible motor is used for the waste water management, there are high chances of tripping due to the wastes present in the water. When a plastic or something else get stuck inside the motor, the motor will trip the breaker, because while the motor is running in normal condition and if something stuck inside it, the motor will need more force due to which it can overload, which leads to the electric motor tripping.

    In a situation like this if there is no electrical circuit breaker the motor will damage in a minute or so. Circuit breaker and contactors are used for safety and protection purposes. Once you buy an electric motor, and you know about the motor specifications, a control and safety circuit can be easily designed.

    You can order a circuit breaker as per the motor rated current. If the motor is still tripping, then you can perform the above or below tests. If everything is ok and the breaker still keeps tripping then you have no other option than changing the circuit breaker.

    If your motors are connected in Star Delta connection or only Star connection or Delta connection and they are connected with electric fuses for more safety or protection purposes and if the fuses are blown then change the fuses to new one. But how you will know about the fuses if they are ok or blown? First take a multimeter and set the digital multimeter knob farkon cin gindi the ohms or continuity.

    Connect the test leads of the digital multimeter with the opposite ends of the fuse. Checking the insulation of the cable is also most important test, because sometimes when the weather changes from cold to hot the cable becomes more hot, due to which it may lead to the electric motor tripping, this may also due to the exceeded current, when the current crosses the rated value, the cable gets hotter.

    So this is the responsibility of the design engineer to select proper cable size with good insulation to ensure the normal operation of the electric motor.


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