# Back Motor Effect

Definition

The back motor effect (Counter-electromotive force) is the counter-torque that opposes the rotational motion of the coils in a generator when the generator is under load.

The back motor effect depends upon the load attached to the generator. The even more load, the more current will be created which will certainly enhance the counter-torque. But to produce even more current, the regular torque will likewise increase.

A generator is a source of electrical energy production. Generators convert mechanical energy into electric power, whereas motors transform electrical energy into mechanical energy. Thus, it is not unexpected that motors and also generators have the same general construction.

A large turbine is turned by high-pressure steam or a waterfall. The shaft of the turbine is attached to the coil which rotates in a magnetic field. The coil converts the mechanical energy of the turbine into electrical energy. So, the generator supplies the current to an external circuit.

The devices in the circuit that consumes electrical energy are called Load. The greater the load, the larger the current is supplied to the generator.

The counter-torque acts to slow the generator shaft. If the load boosts and also the input torque (from e.g., a generator driving the shaft) stays the same, that net torque will certainly reduce the generator down.

Usually, a governor of some kind acts to enhance the torque so the rate, regularity, and output voltage remain constant. That’s the governor has to act to boost the input torque and power to match the load.

If there is no active element increasing the input torque, the shaft will slow, the voltage will go down, which will decrease the current as well as power out, till the generator eventually enters into equilibrium at a slower speed and reduced voltage and frequency.

Yet generally we choose to manage those to maintain them constantly.

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##### For Open Circuit

The generator does not supply electrical energy and a very small force is required to rotate the coil for an open circuit.

##### For Closed Circuit

A current is drawn through the coil. The magnetic force acts on the current-carrying coil in a closed circuit.

##### Explanation of Back Motor Effect

When the coil rotates, F1 – the force acts on the left side of the coil whereas force F2 acts on its right side. These forces are such that they produce anti-clockwise torque that opposes the rotational motion of the coil.

##### Back effect and Law of conservation of Energy

This is what we called the back motor effect in generators. The larger the current drawn, the greater is the counter-torque produced, so more mechanical energy is required to rotate the coil with the constant angular speed. This is according to the law of conservation of energy.

### MCQs:

• 1. What is the back motor effect also known as?
• A) Electromagnetic induction
• B) Counter-electromotive force
• C) Electric resistance
• D) Inductive reactance
• 2. What does the back motor effect oppose in a generator?
• A) Electrical resistance
• B) Input torque
• D) Voltage drop
• 3. In a generator, what does the back motor effect depend upon?
• A) Input voltage
• B) Generator speed
• D) Output frequency
• 4. What does the load in a circuit refer to?
• A) Voltage drop across components
• B) Electrical resistance of wires
• C) Devices consuming electrical energy
• D) Capacitance of the circuit
• Answer: C) Devices consuming electrical energy
• 5. What happens to the generator shaft due to the counter-torque of the back motor effect?
• A) Speed increases
• B) Speed remains constant
• C) Speed decreases
• D) Speed fluctuates
• 6. When does the generator supply electrical energy?
• A) For an open circuit
• B) For a closed circuit
• C) Both for an open and closed circuit
• D) Neither for an open nor closed circuit
• Answer: B) For a closed circuit
• 7. What is the effect of load increase on the generator’s net torque?
• A) Net torque increases
• B) Net torque decreases
• C) Net torque remains constant
• D) Net torque fluctuates
• Answer: B) Net torque decreases
• 8. What is the force required to rotate the coil in an open circuit?
• A) High force
• B) Low force
• C) No force
• D) Variable force
• 9. What happens to the generator’s voltage and frequency if the load increases without adjusting the input torque?
• A) Voltage and frequency increase
• B) Voltage and frequency decrease
• C) Voltage decreases, frequency increases
• D) Voltage increases, frequency decreases
• Answer: B) Voltage and frequency decrease
• 10. What type of torque does the back motor effect produce in a generator?
• A) Clockwise torque
• B) Anti-clockwise torque
• C) Static torque
• D) Variable torque
• 11. According to the law of conservation of energy, what happens when more current is drawn from the generator?
• A) Mechanical energy decreases
• B) Mechanical energy remains constant
• C) Mechanical energy increases
• D) Mechanical energy fluctuates
• Answer: C) Mechanical energy increases
• 12. What is the back motor effect in generators a result of?
• A) Increase in load current
• B) Decrease in input voltage
• C) Reduction in coil resistance
• D) Enhancement in generator speed
• 13. What term refers to the force acting on the current-carrying coil in a closed circuit?
• A) Counter-force
• B) Electromotive force
• C) Magnetic force
• D) Mechanical force
• 14. Which factor determines the magnitude of the counter-torque in a generator?
• A) Generator speed
• B) Input voltage
• D) Magnetic field strength
• 15. What is the primary source of mechanical energy in a generator?
• A) Electrical resistance
• B) Input torque
• D) Rotation of the turbine shaft
• Answer: D) Rotation of the turbine shaft
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### Summary

The Back Motor Effect, also known as counter-electromotive force, refers to the counter-torque generated in a generator that opposes the rotational motion of coils when the generator is under load. The magnitude of this effect depends on the load attached to the generator, with higher loads resulting in increased current production and consequently, greater counter-torque.

Generators serve as sources of electrical energy production by converting mechanical energy into electric power. They share a similar construction with motors, which convert electrical energy into mechanical energy. A generator typically consists of a turbine, rotated by high-pressure steam or waterfalls, connected to coils that rotate in a magnetic field. The mechanical energy from the turbine is converted into electrical energy by the coils, supplying current to an external circuit.

Load refers to the devices in the circuit that consume electrical energy. The greater the load, the larger the current supplied to the generator. The counter-torque produced by the back motor effect acts to slow down the generator shaft. If the load increases while the input torque remains constant, the net torque decreases, causing the generator to slow down.

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In the absence of mechanisms to increase input torque, such as a governor, the generator shaft slows down, leading to a decrease in voltage and frequency output until the generator reaches equilibrium at a slower speed and lower voltage and frequency. However, mechanisms are typically employed to maintain these parameters constant.

In an open circuit, the generator does not supply electrical energy, requiring only a small force to rotate the coil. In a closed circuit, current is drawn through the coil, and magnetic force acts on the current-carrying coil.

The back motor effect occurs due to the interaction of forces on the coil during rotation, producing an anti-clockwise torque that opposes the coil’s motion. This effect aligns with the law of conservation of energy, as the greater the current drawn, the higher the counter-torque produced, necessitating more mechanical energy to maintain the coil’s constant angular speed.