Overload protection(2)

Fuses and circuit breakers are protective devices used to protect circuits against short circuits, ground faults, and overloads. In the event of a short circuit, a properly sized fuse or circuit breaker will immediately open the circuit. There is, however, a dilemma that occurs when applying fuses and circuit breakers in motor control circuits. The protective device must be capable of allowing the motor to exceed its full load
rating for a short time. Otherwise, the motor will trip each time it is started. In this situation it is possible for a motor to encounter an overload condition which does not draw enough current to open the fuse or trip the circuit breaker. This overload condition could easily cause enough heat to damage the motor.

Overload Relays
Overload relays are designed to meet the special protective needs of motor control circuits. Overload relays:
• allow harmless temporary overloads, such as motor starting, without disrupting the circuit
• will trip and open a circuit if current is high enough to cause motor damage over a period of time
• can be reset once the overload is removed.

Bimetal Overloads:
 Overload protection is accomplished with the use of a bimetal strip. This component consists of a small heater element wired in series with the motor and a bimetal strip that can be used as a trip lever. A bimetal strip is made of two dissimilar metals bonded together. The two metals have different thermal expansion characteristics, so the bimetal bends at a given rate when heated.


Under normal operating conditions the heat generated by the heater element will be insufficient to cause the bimetal strip to bend enough to trip the overload relay.

As current rises, heat also rises. The hotter the bimetal becomes, the more it bends. In an overload condition the heat generated from the heater will cause the bimetal strip to bend until the mechanism is tripped, stopping the motor.

Some overload relays that are equipped with a bimetal strip are designed to automatically reset the circuit when the bimetal strip has cooled and reshaped itself, restarting the motor. If the cause of the overload still exists, the motor will trip again and reset at given intervals. Care must be exercised in the selection of this type of overload as repeated cycling will eventually damage the motor.

Overload Protection(1)

Overload Protection
  Current and Temperature Current flow in a conductor always generates heat due to resistance. The greater the current flow, the hotter the conductor. Excess heat is damaging to electrical components. For that reason, conductors have a rated continuous current carrying capacity or ampacity. Over-current protection devices
are used to protect conductors from excessive current flow.

Thermal overload relays are designed to protect the conductors (windings) in a motor. These protective devices are designed to keep the flow of current in a circuit at a safe level to prevent the circuit conductors from overheating,excessive current is referred to as over-current.

over-current is any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault

Short Circuits:
 When two bare conductors touch, a short circuit occurs. When a short circuit occurs, resistance drops to almost zero. Short-circuit current can be thousands of times higher than normal operating current.

Overload Conditions:
 An overload occurs when too many devices are operated on a single circuit or a piece of electrical equipment is made to work harder than it is designed for. For example, a motor rated for 7 amperes may draw 15, 30, or more amperes in an overload condition. In the following illustration a package has become
jammed on a conveyor causing the motor to work harder and draw more current. Because the motor is drawing more current it heats up. Damage will occur to the motor in a short time if the problem is not corrected or the circuit is not shut down by the overload relay.

Temporary Overload Due to starting current:
 Electric motors are rated according to the amount of current to Starting Current they will draw at full load. When most motors start, they draw current in excess of the motor’s full-load current rating. Motors are designed to tolerate this overload current for a short period of time. Many motors require 6 times (600%) the full-load current rating to start. Some newer, high-efficiency motors may require higher starting currents. As the motor accelerates to operating speed, the current drops off quickly. The time it takes for a motor to accelerate to operating speed depends on the operating characteristics of the motor and the driven load. A
motor, for example, might require 600% of full-load current and take 8 seconds to reach operating speed.