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An inductive load is a type of electrical load that makes use of one or more inductors, which resist changes in electric current. Some examples are transformers, electric motors, electromagnets and electromechanical relays.
Inductors are components that store electrical energy. When a voltage differential is applied across an inductor’s leads, the inductor converts electricity into an electromagnetic field. When the voltage differential is removed from the leads, the inductor will attempt to maintain the amount of electrical current flowing through it. It will discharge when the electromagnetic field collapses, or if an electrical pathway is created between the two inductor leads.
Some inductive loads — such as the electromagnet in an electromechanical relay — might feed a power surge back into the circuit when power is disconnected from the load, which can damage the circuit. To prevent this surge from reaching the circuit, a load can be wired with a rectifier diode in parallel with the inductor. When the power is switched off, the diode dissipates the power surge by providing a one-way electrical path across the inductor. The diode will dissipate electrical power until the electromagnetic field collapses or until the power surge current is insufficient to activate the diode.
An electric motor is an inductive load that is used to convert electricity into physical work. It requires more power to start turning the rotor initially than it requires to keep an already-turning rotor in motion. When voltage is applied to the leads on an electric motor, the motor generates a change in magnetic flux. This change induces an electromotive force that opposes the forward-turning force that would start the motor turning; this phenomenon is called a back electromotive force (EMF). After several seconds, an electric motor will have overcome the some of the impedance caused by a back EMF, and the motor will function as designed.
Back EMF causes some of the power from the power source to be wasted. For this reason, an inductive load such as an alternating current (AC) electrical motor will use only about 70% of the electrical energy to do actual work. This means that such loads will require a power supply that can provide enough electrical power to start the motor. This power supply must also provide enough power for the motor to perform physical work as needed.