# How Does Back EMF Affect Motor Performance?

• michaelw
In summary, the motor operates on 117V and draws 12.2A when starting up and 2.30A at normal operating speed. The resistance of the armature coil is 9.59 ohms and the back emf developed at normal speed is 97.9V. The current drawn by the motor at one-third normal speed cannot be calculated as the back emf is 0.
michaelw
A motor is designed to operate on 117 V and draws a current of 12.2 A when it first starts up. At its normal operating speed, the motor draws a current of 2.30 A. Obtain (a) the resistance of the armature coil, (b) the back emf developed at normal speed, and (c) the current drawn by the motor at one-third normal speed.

Please let me know if I am on the right track :)
a) V=IR 117V = 12.2A * R
R = 9.59ohms
b) I = V-emf/R
2.3A = 120V-emf/9.59ohms
emf = 97.9V

But i have no idea how to do 3... would i just divide emf / 3 and find current? (97.9/3)

There is an inconsistency between a) and b). In a) you say V = IR, in b) you say V-emf = IR. Revisit your solution.

emf = 0 in a)

## 1. What is back EMF in a motor?

Back EMF (electromotive force) is a phenomenon that occurs in a motor when the motor's armature windings create an electromagnetic field that opposes the flow of current. This counter-electromotive force is known as back EMF.

## 2. How does back EMF affect a motor?

Back EMF can affect a motor by reducing the amount of current flowing through the motor's windings, which in turn reduces the motor's torque. This can result in slower speeds and lower power output for the motor.

## 3. How is back EMF calculated?

Back EMF can be calculated using the formula: Back EMF = motor speed (in RPM) x motor constant (in volts per RPM). The motor constant is a value specific to each motor and can be found in the motor's datasheet.

## 4. How can back EMF be controlled in a motor?

Back EMF can be controlled in a motor by using a technique called pulse width modulation (PWM). This involves rapidly switching the motor's power supply on and off, which can effectively control the voltage applied to the motor and reduce the effects of back EMF.

## 5. Can back EMF damage a motor?

Yes, back EMF can damage a motor if not properly controlled. The high voltage spikes that occur as a result of back EMF can cause damage to the motor's electronics and decrease the overall lifespan of the motor. It is important to properly manage back EMF in order to prevent damage to the motor.

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