In short, the power is transferred to the rotor winding by stator through induction. There are two main types of induction motors — single-phase induction motors and three-phase induction motors. Its popularity is due to the rugged construction, excellent operating characteristics, speed regulation, and absence of commutator. Like any regular induction motor, this motor also comprises a stator and rotor.
The rotor of the three-phase induction motor is classified as phase wound rotor or slip ring rotor and squirrel cage rotor. Among the two, the squirrel cage rotor is one of the most common ones. Induction motors equipped with a squirrel cage rotor are known as squirrel cage induction motors. Factors such as simplicity, rugged construction, and constant speed in different load sizes have contributed to their popularity.
Like other induction motors, the squirrel cage motor consists of:. When the motor draws excess current, it is referred to as an overload. This may cause overheating of the motor and damage the windings of the motor. Because of this, it is important to protect the motor, motor branch circuit, and motor branch circuit components from overload conditions.
Overload relays protect the motor, motor branch circuit, and motor branch circuit components from excessive heat from the overload condition. Overload relays are part of the motor starter assembly of contactor plus overload relay. They protect the motor by monitoring the current flowing in the circuit. If the current rises above a certain limit over a certain period of time, then the overload relay will trip, operating an auxiliary contact which interrupts the motor control circuit, de-energizing the contactor.
This leads to the removal of the power to the motor. Without power, the motor and motor circuit components do not overheat and become damaged. The overload relay can be reset manually, and some overload relays will reset automatically after a certain period of time. After which, the motor can be restarted.
The overload relay is wired in series with the motor, so the current that flows to the motor when the motor is operating also flows through the overload relay. It will trip at a certain level when there is excess current flowing through it. This causes the circuit between the motor and the power source to open. The overload relay can be manually or automatically reset after a predetermined time duration. The motor can be restarted after the cause of the overload has been identified and rectified.
Many overload relays include bimetallic elements or bimetallic strips, also referred to as heater elements. The bi-metallic strips are made of two types of metals — one with a low coefficient of expansion, and another with a high coefficient of expansion.
These bimetallic strips are heated by a winding around the bimetal strip, which carries the current. Both of the metal strips will expand due to the heat. However, the metal with a high coefficient of expansion will expand more in comparison to the metal with a low coefficient of expansion. This dissimilar expansion of the bimetallic strips causes the bimetal to bend towards the metal with a low coefficient of expansion. As the strip bends, it actuates an auxiliary contact mechanism and causes the overload relay normally closed contact to open.
As a result, the contactor coil circuit is interrupted. The above equation defines that heat produced by the winding will be directly proportional to the time period of the flow of overcurrent through the winding. In other words, the lower the current, the longer it will take the overload relay to trip and the higher the current, the faster the overload relay will trip, in fact it will trip much faster because the operation of the relay is a function of the current squared.
Bimetallic overload relays are often specified when automatic reset of the circuit is required, and occurs because the bimetal has cooled and returned to its original state form. Once this happens the motor can be restarted.
If the cause of overload is not rectified, the relay will trip again, and reset at predetermined intervals. It is important to be careful during the selection of an overload relay, because repeated tripping and reset can reduce the mechanical life of the relay and may cause damage to the motor.
In many applications, the motor is installed at a location with a constant ambient temperature, and the overload relay and motor starter may be installed in a different location, which experiences different ambient temperatures. In such applications, the trip point of the overload relay can vary depending on multiple factors. This will be the load factor for the motor. If the load factor is 1. Check the motor nameplate for a service factor.
Many motors are rated with a service factor of 1. If the motor in the example from step 2 had a service factor of 1. Check the motor nameplate for the ambient temperature rating and the insulation class ratings. Electric motors are commonly rated for operation in an ambient temperature of 40 deg.
C deg. F or 50 deg. Their insulation rating is usually class B or F. Thermistors measure temperature by using resistance. The RTD is a type of instrument used for measuring the temperature, whereas, the thermistor is the thermal resistor whose resistance changes with temperature.
The RTD is made of the metals having a positive temperature coefficient whereas the thermistor is made of the semiconductor materials.
To compensate for lead wire resistance, 3 wire RTDs have a third wire that provides a measurement of the resistance of the lead wire and subtracts this resistance from the read value.
Because 3 wire RTDs are so effective and affordable, they have become the industry standard. Sometimes referred to as resistance thermometers, RTDs are commonly used in laboratory and industrial applications because they provide accurate, reliable measurements across a wide temperature range.
Begin typing your search term above and press enter to search. Press ESC to cancel. Skip to content Home What should motor overloads be set at? Ben Davis May 31, What should motor overloads be set at? What is overload on a motor? How do you calculate star Delta overload? How does a motor overload work? What causes a motor to overload? How do you select a motor overload? How do you calculate overload? What is Class 10 overload protection?
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