Page 334 - CITS - Electronic Mechanic - TT - 2024

ELECTRONICS MECHANIC - CITS

The higher the number of poles, the slower synchronous speed.Relationship between poles, frequency and
speed of synchronous is as follows:
Ns(rpm) = 120 x f /p
Ns = synchronous speed

f = supply frequency
p = number of poles in the stator
Example
a four ( 4 ) pole motor is built to operate at 60 Hz and synchronous speed is:
Ns = 120 x f /p

Ns = 120 x 60 / 4
Ns = 1800 rpm
Slip in electric motor
The difference in rotor speed and synchronous speed is known as slip

we can calculate the slip of a motor using the formula:
S = Ns – Nr / Ns x 100%
S = slip
Ns =stator magnetic field speed
Nr = rotor speed (available on the motor name plate)
How to calculate HORSE POWER for motor?

To calculate the total power output of a motor, we can use the formula:
HP = I x V x Eff / 746
I = Ampere
V = voltage

Eff = efficiency (available on the motor name plate)
Example
1 unit single phase motor,240 volt,20 ampere and 85% efficiency
HP = I x V x Eff / 746
HP = 20 x 240 x 85% / 746

HP = 5.5
iv Three-phase Induction Motor
An electrical motor is an electromechanical device that converts electrical energy into mechanical energy. In the
case of three-phase AC (Alternating Current) operation, the most widely used motor is a 3 phase induction motor,
as this type of motor does not require an additional starting device. These types of motors are known as self-
starting induction motors.
To get a good understanding of the working principle of a three-phase induction motor, it’s essential to understand
the construction of a 3 phase induction motor. A 3 phase induction motor consists of two major parts:
• A stator

• A rotor

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CITS : E & H - Electronics Mechanic - Lesson 205 - 222