Overload relay

Monday, 08 January 2007

Overload relays are electrical switches typically employed in industrial settings to protect electrical equipment from damage due to overheating in turn caused by excessive current flow. Overload relays are provided for protecting components connected to an electrical circuit in the event the current flowing through the circuit exceeds a predetermined level. An overload relay monitors the current flowing in the protected circuit and sends a signal to cause a contactor in the protected circuit to open when the current flowing in the protected circuit is higher than a preselected level. Overload relays are more than simple circuit interrupters, they are sensors which, upon determining the existence of an overload or other undesirable circuit condition, break a circuit and in turn provide a control or an indicating function. Overload relays are specialized circuit breakers used with industrial motors to protect the motors from damages caused by overload or electrical faults. In a typical case, the electrical equipment is a three-phase motor which is connected to a power source through another relay commonly referred to as a contactor. The contactor is controlled by another switch which is typically remotely located. Overload relays of various sorts have long been utilized in connection with the operation of electrical equipment, particularly electrical equipment drawing relatively high levels of power. Single-phase and multi-phase (e.g., three-phase) power systems typically include an overload relay for interrupting power in the power conductors when a fault condition occurs, such as a ground fault, phase loss, overcurrent, or undercurrent condition. For instance, a three phase induction motor is often linked to a power source through a relay commonly referred to as a contactor. A typical contactor includes a separate power path for each of the three motor phases. Contactor motion is typically provided magnetically as the result of power flow through a coil where the current though the coil is controlled by a control switch. In this case, the contactor is a heavy duty relay having three contact sets for breaking each of the three-phases of power upon movement of a yoke member within a contactor coil, the yoke member and coil together forming an electrical solenoid. With an electronic relay, it is possible to protect multiphase motors by cutting off their power supply for example when a current overload arises on at least one phase of the motor or an imbalance between the phase currents occurs.

Overload relays are normally used in conjunction with an electromechanical contactor, that may be used to disconnect power from equipment, for example, from a three-phase motor, when an overload condition exists. Electric motors are one type of electrical load which can be started and stopped using a contactor. The contactor includes a contact associated with each phase conductor connected to the motor. A contact of an overload relay is typically connected in series with the coil of the contactor to cause the contactor to open when an overload condition is sensed. The overload relay senses an overload condition by monitoring the current in each of the three-phases received by the motor windings. For a three-phase motor, the contactor would include three contacts which are opened and closed in unison. The overload relay includes current sensing elements that are wired in series with the three phases passing through the contactor. In this way, the overload relay can monitor current flowing in the three phases through the contactor, and based on current magnitude and duration, may interrupt the current flow through the contactor armature circuit to open the contactor contacts when an overload occurs. The mechanical motion required to open and close the contacts is provided by a solenoid including a coil. The coil is controlled by a basic circuit which includes a normally closed stop button, a normally open start button, and an overload switch. When an overload condition is experienced, power is supplied to a solenoid in the electromechanical trip mechanism causing a plunger to retract, which subsequently, through a series of levers or other mechanical components, causes the normally closed contacts to open. Many overload relays have been designed such that, once tripped, the relay remains open to prevent current flow to the contactor until the relay is manually reset by a system operator. A common resetting device is a reset push button selectable by an operator to reset the relay thereby allowing current to flow to and to close the contactor coil which in turn provides current to the linked equipment. An overload relay is usually designed to operate over a wide range of values and the user must set the trip current based upon the specifications of the motor in use. The trip current defines the value at which the relay is triggered into breaking the circuit between the load and the power. The trip point of the overload relay is selected by moving a pointer from one position on a scale to another position on the scale. The pointer is connected to a variable resistor in the overload sensing circuit such that as the pointer is moved from one position to another along the scale the resistance of the variable resistor changes. The two most critical elements in the overload sensing circuit are the current transformers through which a current proportional to that flowing in the protected circuit is induced and the variable resistor which changes circuit characteristics such that the relay will initiate a trip at the selected overload current. In addition to the mechanical components, a fully featured relay assembly also typically includes a printed circuit board (PCB) including control circuitry for tripping and automatically resetting the relay, current sensors and various types of terminals for linking to power lines, the contactor and perhaps indicating lights.

A variety of types of overload relays are available, ranging from simple thermal overload relays to more complex, solid-state relays which may include some intelligence and/or reporting capabilities. A thermal overload relay is a bimetallic device which provides motor protection for running and stalled rotor overloads. A strip bimetal in the overload relay is electrically heated by heater elements which carry the motor currents. Bimetal overload relays include a snap action electrical switch which has a contact that is movable between an unactuated and an actuated position to make or break electrical connection with a stationary contact. This movable contact is mechanically coupled to a main bimetal element that is responsive to changes in temperature to operate the electrical switch. Excess heat is generated in the heater elements by an overloaded motor. The bimetals deflect to thermally open the normally closed contact, thereby opening a coil circuit of a magnetic contactor which disconnects the overloaded motor from the line. Thereafter the relay may be reset by pressing and releasing a reset rod. With advances in electronic circuitry, the bi-metallic element has been replaced with more complex circuitry. Overload relays have been designed to utilize electronic circuitry responsive to signals derived from the secondary windings of current transformers whose primary windings carry the motor phase currents. Such circuitry may sample current flow to the motor on a periodic basis and provide sophisticated overload prediction based not only on a simple thresholding but on more complex trend analyses. The output of this circuitry is typically a low-powered overload signal. The electronic circuitry processes these signals on a current-time integral basis to determine when a current overload condition is sufficiently persistent to require interruption of the motor circuit. In order for this overload signal to control the contactor coil current, a solid state switch may be required, adding to the complexity and cost of the overload relay. The electronic circuitry can be readily designed to recognize not only overload conditions, but also high fault current conditions calling for circuit interruption without delay and hazardous ground fault conditions. Bigmetal and eutectic overload relays include heater elements in each phase which open when an excessive current flowing through the heater elements causes the element to exceed a specific temperature. Solid-state relays, on the other hand, include electronic devices for monitoring phase current and for determining, based on the monitored current, whether a fault condition has occurred. Solid-state relays typically can be configured to provide protection for ground fault, undercurrent and phase loss conditions, in addition to overcurrent conditions. Solid state overload relays are commonly available in relatively compact, affordable packages that can be easily installed and serviced. In addition to circuitry for detecting fault conditions, such relays also commonly include power supply circuitry for storing energy from the load circuit being controlled.

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