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.
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.
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