6.1 Name TWO rotating parts of a three-phase induction motor - NSC Electrical Technology Power Systems - Question 6 - 2024 - Paper 1

When a three-phase supply is connected to the stator windings, the alternating current flowing through each of the coils creates a magnetic field. These coils are spaced 120° apart, causing each coil to be magnetised at different intervals. The combination of these magnetic fields rotates at a frequency corresponding to the supply frequency. For example, with a frequency of 50 Hz, the rotating magnetic field is produced, which is essential for the operation of induction motors.

A squirrel cage rotor does not use brushes or slip rings, which are components that can create sparks during operation. This lack of sparking minimizes the fire hazard, making the squirrel cage rotor safer in explosive environments.

The synchronous speed, $n_s$, can be calculated using the formula:

n_s = rac{60 imes f}{p} Substituting the known values: n_s = rac{60 imes 50}{2} = 1500 ext{ RPM}

To find the rotor speed, $n_r$, we can use the slip formula:

$n_r = n_s imes (1 - ext{slip})$ Considering a slip of 5%: $n_r = 1500 imes (1 - 0.05) = 1500 imes 0.95 = 1425 ext{ RPM}$

The efficiency, $ext{Efficiency}$, can be calculated with the formula:

ext{Efficiency} = rac{P_{out}}{P_{in}} imes 100 Where:

• $P_{out} = P_{in} - ext{losses}$
• $P_{in} = 11750 ext{ W}$
• $ext{losses} = 1750 ext{ W}$ Thus: $P_{out} = 11750 - 1750 = 10000 ext{ W}$ Therefore, $$ ext{Efficiency} = rac{10000}{11750} imes 100 ext{ approx } 85.11 ext{ %} $

The apparent power, $S$, can be calculated using:

$S = ext{V} imes ext{I} imes ext{sqrt}(3)$ Substituting the known values: $S = 400 imes 20 imes ext{sqrt}(3)$ Thus, $S ext{ approx. } 13856.41 VA$

The power factor, $ext{PF}$, can be computed using:

ext{PF} = rac{P_{out}}{S} Substituting: $$ ext{PF} = rac{11750}{13856.41} ext{ approx. } 0.85 $

The output power can be calculated using:

$P_{out} = ext{sqrt}(3) imes ext{V} imes ext{I} imes ext{PF}$ Substituting the known values: $P_{out} = ext{sqrt}(3) imes 400 imes 20 imes 0.85 ext{ approx } 10001.25 ext{ W}$

The motor starter control circuit identified in Figure 6.5 is the manual sequence starter circuit.

MC1 is a contactor that energizes and de-energizes motor 1 in the main power circuit whenever the start button is pressed.

Having two overload relays ensures that each motor is protected by its own overload relay independently, enhancing safety and reliability in case one motor experiences overload.

If one motor is overloaded and stops, the circuit can cause the other motor to also shut down, which might interrupt the entire conveyor system and reduce operational efficiency.

The control circuit can be modified by connecting a normally open (N/O) contact of MC1 in place of the temporary connection. This alteration ensures that MC2 can only be activated after MC1 is energized, preventing operation until it is safe.