30.21
Back EMF
Generators convert mechanical energy into electrical energy, whereas motors convert electrical energy into mechanical energy. A motor works by sending…
A motor converts electrical energy to mechanical energy. It consists of a current-carrying coil placed inside a uniform magnetic field.
When a current is passed through a coil, the magnetic field exerts a torque on the loop, rotating the coil inside the magnetic field. The magnetic flux through the rotating coil changes, inducing an emf.
According to Lenz's law, the induced emf opposes the current causing the rotation of the loop. Thus, the induced emf is called the back emf.
The total voltage available for supplying the current is the difference between the applied voltage and the induced back emf.
When the motor turns ON, the induced emf is zero, with maximum current through the loop. When the coil starts rotating, the induced emf reduces the current through the coil.
Suppose a current of 5 amperes passes through a coil with 20 ohm resistance at 100 volts. When the coil rotates at maximum speed, the back emf of 50 volts is induced, which reduces the current passing through the coil to 2.5 amperes.
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Q1: What is back EMF and why does it occur in a motor?
Back EMF is an induced electromotive force that opposes the applied voltage in a motor. When a current-carrying coil rotates inside a magnetic field, the changing magnetic flux through the coil induces an EMF according to Faraday's law. By Lenz's law, this induced EMF opposes the current causing the rotation, hence the name back EMF.
Q2: How does back EMF affect the current flowing through a motor?
Back EMF reduces the current available to drive the motor. The total voltage for supplying current equals the applied voltage minus the back EMF. When a motor starts, back EMF is zero, allowing maximum current through the coil. As the coil rotates faster, back EMF increases, reducing the net voltage and current draw proportionally.
Q3: Why does a motor draw maximum current when first turned on?
When a motor is first turned on, the induced back EMF is zero because the coil is not yet rotating. Without back EMF to oppose it, the full applied voltage drives through the coil's resistance, producing maximum current. As the motor accelerates and the coil begins rotating, back EMF builds up and reduces the current draw.
Q4: What happens to back EMF when a mechanical load is applied to a motor?
When a mechanical load slows the motor, the back EMF decreases because the coil rotates more slowly. With lower back EMF, more current flows through the coil, enabling the motor to produce additional torque and perform more work. However, excessive current from sustained low speed can overheat and damage the motor.
Q5: How does back EMF relate to motor efficiency at different speeds?
Back EMF increases with motor speed, approaching the applied voltage at maximum speed. When back EMF nearly equals the driving EMF, the motor uses minimal current and energy, overcoming only friction losses. This high-speed, low-current condition represents efficient operation, while low-speed operation draws excessive current and generates heat.
Q6: Can you calculate the current in a motor given applied voltage, resistance, and back EMF?
Yes, use the formula: Current = (Applied Voltage − Back EMF) / Resistance. For example, at 100 volts applied with 20 ohm resistance and 50 volts back EMF, the current is (100 − 50) / 20 = 2.5 amperes. This demonstrates how back EMF directly reduces the current available to drive the motor's mechanical output.
Q7: Why does a motor act as a generator when its coil rotates?
A motor acts as a generator because rotating the coil through a magnetic field changes the magnetic flux, inducing an EMF consistent with induced electric fields. This self-generated EMF is the back EMF. Whether the shaft rotates from the motor's own torque or external input like a belt drive, the changing flux produces an induced voltage opposing the applied voltage.