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, each of the three coils carries a current that is phase-shifted by 120° relative to the others. As a result, when these currents flow through the stator coils, they generate magnetic fields that rotate around the stator. This rotation occurs due to the sequential energization of the coils, which causes a continuous build-up of the magnetic field, effectively creating a rotating magnetic field at a frequency determined by the supply frequency.

Squirrel cage motors are considered less of a fire hazard due to their construction, which does not require brushes or slip rings that can create sparks. The rotor is contained within an enclosure, and under normal operating conditions, it does not generate excessive heat, which reduces the risk of ignition in potentially explosive environments.

The synchronous speed ( s) of a motor can be calculated using the formula: n_s = rac{60 imes f}{p} Substituting the values: n_s = rac{60 imes 50}{2} = 1500 ext{ RPM}

To calculate the rotor speed, we use the slip formula: $n_r = n_s imes (1 - ext{slip})$ Substituting the known values: $n_r = 1500 imes (1 - 0.05) = 1500 imes 0.95 = 1425 ext{ RPM}$

Efficiency ( η) can be calculated using the formula: ext{Efficiency} = rac{P_{ ext{out}}}{P_{ ext{in}}} imes 100 First, we need to find the output power: $P_{ ext{out}} = P_{ ext{in}} - ext{losses} = 11750 - 1750 = 10000 ext{ W}$ Then, ext{Efficiency} = rac{10000}{11750} imes 100 ext{ which equals } 85.1 ext{ %}

The apparent power (S) can be calculated using the formula: S = rac{V imes I}{ ext{sqrt}(3)} Substituting the values: S = rac{400 imes 20}{ ext{sqrt}(3)} ext{ which equals } 13856.41 ext{ VA or } 13.86 ext{ kVA}

Power factor (PF) can be calculated as: ext{Power factor} = rac{P_{ ext{real}}}{S} Substituting the values: ext{Power factor} = rac{11750}{13856.41} ext{ which equals } 0.85

The output power of the motor can be calculated using: $P_{ ext{out}} = S imes ext{power factor}$ Where, S = rac{V imes I}{ ext{sqrt}(3)} = 13856.41 ext{ VA} Then, $P_{ ext{out}} = 13856.41 imes 0.85 = 11725.94 ext{ W or } 11.725 ext{ kW}$

The motor starter control circuit is identified as the Manual Sequence Starter, which is designed to initiate motor operation through user interaction.

MC1 functions as a contactor that energizes or de-energizes motor 1 by controlling the flow of current in the main (power) circuit. It acts as a switch that connects or disconnects the power supply to motor 1 based on user input.

Having two overload relays allows for independent protection of each motor. This ensures that if one motor experiences an overload and trips, the other motor can continue to operate without interruption.

If the overload relays are connected in a way that ties the two motors together, an overload in one motor can lead to the tripping of the entire system, causing both motors to stop. This could disrupt productivity if one motor is functioning properly.

To ensure that MC2 cannot be energized before MC1, a normally open (N/O) contact of MC1 can be integrated into the circuit. This modification would mean that MC2 can only be activated once MC1 has been engaged, preventing any situation where MC2 is powered while MC1 is off.