Thermal overload relays are installed in the main circuit when electromechanical protection is adequate. This protects the motor in case of phase failure or overload. The thermal overload relay does not directly shut down the load. Instead, it triggers one or several auxiliary contacts which then disconnect the motor.
What is the use of thermal overload relay?
Thermal overload relays are economic electromechanical protection devices for the main circuit. They offer reliable protection for motors in the event of overload or phase failure. The thermal overload relay can make up a compact starting solution together with contactors.
What is the purpose of thermal overload protection at the motor?
The Purpose of a Thermal Protector
The thermal protection is present to turn the motor off when excessive heat is generated within the motor circuitry. This safety feature halts the temperature climb before it can burn up the motor.
What is thermal motor protection relay?
Thermal motor protection relays contain three bimetal strips together with a trip mechanism in a housing made of insulating material. The bimetal strips are heated by the motor current, causing them to bend and activating the trip mechanism after a certain travel which depends on the current-setting of the relay.
What are two types of thermal overload relay?
Thermal overload relays are classified into two types namely solder pot as well as a bimetal strip.
Do all motors need overload protection?
Separate overload protection is not required for a continuous duty motor of 1 hp or less that is neither permanently installed nor started automatically, and is within sight of the controller location.
Why the overload relay is connected in each motor?
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.
How do you check thermal overload on a motor?
Overload Protection Test:
- Measure the normal motor running current (i motor).
- Turn off the motor and let it cool for about 10 minutes.
- Calculate the following ratio: i (motor) / i (overload min FLA).
- Set the overload to its minimum FLA and turn on the motor.
- Wait for the overload to trip.
How do you select a thermal overload motor relay?
Thermal Overload Relay setting = 100% x Full Load Current (Line).
- Thermal Overload Relay setting = 100% x Full Load Current (Line).
- Thermal Overload Relay setting = 100%x6 = 6 Amp.
What is OL in motor control?
The letters often represent the type of device, such as M for motor starter, CR for control relay, OL for overload relay or TR for timing relay.
Which type of protection is used for motor?
Circuit breakers and fuses are use as protection units for the different motors. Overcurrent protection can protect personnel from electric shock, the control equipment of the motor, the conductors of the motor branch circuits and the motor itself from high currents.
What size motors need overload protection?
Motors rated more than 1 hp without integral thermal protection and motors rated 1 hp or less that are automatically started [430.32(C)] must have an overload device sized per the motor nameplate current rating [430.6(A)].
Why contactor is used in motor?
Why are Contactors Used? Contactors are used for high power applications. They allow a lower voltage and current to switch a much higher power circuit, so they are generally larger and more heavy-duty than control relays, enabling them to switch higher power loads on and off for many thousands of cycles (Figure 1).
What causes thermal overload trip?
Causes may include a large change in load (e.g., a scrap grinder is fed too much at a time), misalignment, a broken drive gear, or improper motor drive settings. Power problems (e.g., low voltage or low power factor) also may cause an overload condition. It’s easy enough to determine the correct size overloads.
What is the maximum overload setting of A motor?
The maximum allowable size for the overloads is 9.8A. The overloads can be sized at 140% of the FLA if the overloads trip at rated load or will not allow the motor to start, 7A x 1.4 = 9.8A.
What’s a thermal overload?
A thermal overload relay, commonly called a ‘thermal overload’, is a device which is usually connected directly to a motor contactor. It offers overload protection only, so must be coupled with an upstream magnetic circuit breaker for short circuit protection.
Why is DOL starter used up 5 HP?
The direct on line (DOL) starter method of an induction motor is simple and economical. In this method, the starter is connected directly to supply voltage. By this method, small motors up to 5 kW rating are started to avoid the supply voltage fluctuation.
Which circuit breaker is used for motor protection?
Comparison of MPCB (Motor Protection Circuit Breaker) and MCCB (Moulded Case Circuit Breaker)
| Motor Protection Circuit Breaker | Moulded Case Circuit Breakers |
|---|---|
| MPCBs are specially designed for motor protection. | MCCBs are used for general-purpose circuit protection. |
What is the difference between MCB and OLR?
MCB provides both short circuit fault protection and overload fault protection whereas the overload relay is better for the protection of overload protection but it is not suitable for short circuit fault. It can give protection against the short fault but is not much sensitive than the MCB.
How does a thermal relay work?
A thermal relay works depending upon the above mentioned property of metals. The basic working principle of thermal relay is that, when a bimetallic strip is heated up by a heating coil carrying over current of the system, it bends and makes normally open contacts.
What causes high current in a motor?
Probable causes of high current with load include mechanical overload, excessively high magnetic flux densities and, less frequently, an open rotor. An error in winding data that results in lower-than-design-level magnetic flux also can cause high current with load.
How many types of motor protection relays are there?
Short Circuit Protection (50) Locked Rotor Protection or Blocked Rotor Protection (14) Over Load Protection (49) Phase Unbalance Protection (46)
How is motor overload setting calculated?
Divide by the rated full load current from the motor nameplate. This will be the load factor for the motor. If the motor current is 22A and the rated full load current is 20A, then the load factor is 22/20 = 1.1. This means the motor is overloaded by 10%.
What happens during overvoltage?
Overvoltage occurs when the supply voltage rises above the rated voltage of the equipment or motor. The overvoltage can cause excessive current to flow as well as creating excessive voltage stresses.
How do you protect a circuit from overvoltage?
In order to protect the circuit from overvoltage conditions, Zener diodes are often the first option. A Zener diode follows the same diode theory, which blocks the current flow in the reverse direction.
Where is DOL starter used?
Direct On Line Motor Starter (DOL) is suitable for:
- A direct on line starter can be used if the high inrush current of the motor does not cause excessive voltage drop in the supply circuit.
- DOL starting is sometimes used to start small water pumps, compressors, fans and conveyor belts.
What is 13 and 14 on a contactor?
Many contactor manufacturers use the designations A1 and A2 for the terminals that connect power to the magnetic coil. Likewise, many manufacturers use the designations 13 and 14 for the terminals of normally open auxiliary contacts. Auxiliary contacts are operated by the magnetic coil just like the main contacts.
What causes phase unbalance?
A phase unbalance may be caused by unstable utility supply, unbalanced transformer bank, unevenly distributed single-phase loads on the same power system, or unidentified single-phase to ground faults.
Why does a 3 phase motor need a DOL starter?
A DOL starter (also known as a direct on line starter or across the line starter) is a method of starting a 3 phase induction motor. In a DOL Starter, an induction motor is connected directly across its 3-phase supply, and the DOL starter applies the full line voltage to the motor terminals.
What is the range of DOL starter?
D.O.L Starters: Range: 0.5 to 7.5Hp:
Contactor protects the motor against under voltage where as the relay protects the same against overloads.
Why contactor is used in DOL starter?
DOL part – Contactor
The magnetic contactors that are present act as electromagnetically operated switches that provide a safe and convenient means for connecting branch circuits. They also use electromagnetic energy to close switches. The electromagnet consists of a coil of wire that is placed on the iron core.
Can we use MCCB for motor?
MCCBs can protect motors against overload and short circuits. MCCBs can also provide under-voltages, earth faults and phase failure faults but these features are made available with additional accessories. MPCBs are specially designed for motor protection. MCCBs are used for general purpose circuit protection.
What is tripping current in MCB?
The trip current rating is the minimum current level at which the MCB will trip instantaneously. The trip current must persist for at least 0.1s, which is the requirement for the rating. The trip curve can also be referred to as the I-t tripping characteristic.