5.1 State THREE mechanical inspections to be conducted on a three-phase motor after installation, but before commissioning - NSC Electrical Technology Power Systems - Question 5 - 2020 - Paper 1

1. It is cheaper and more robust compared to other types of motors.

2. It has slightly higher efficiency and power factor.

3. These motors are explosion-proof since the risk of sparking is eliminated by the absence of slip rings and brushes.

The control circuit in FIGURE 5.3 consists of a sequence motor starter configured without a timer and includes the components depicted for controlling motors.

The function of the stop button is to disconnect the supply from the control circuit and stop both motors.

The function of MC1 (N/O1) is to allow current to flow in the parallel circuit even after the start button is released. It also maintains the circuit in contact, enabling the motor to continue running.

1. When start button 1 is pressed, current flows through the stop button and O/I1.

2. MC1 (Motor 1) will energize.

3. MC1 N/O1 and MC2 N/O2 will close, allowing Motor 1 to start running.

4. When start button 2 is pressed, MC2 (Motor 2) will energize and close its contact, switching Motor 2 on.

5. The two motors will run respectively.

The synchronous speed (ns) can be calculated using the formula: n_s = rac{120 imes f}{p} Substituting the values: n_s = rac{120 imes 50}{6} = 500 ext{ rpm}

The rotor speed (nr) can be calculated using: $n_r = n_s imes (1 - s)$ Substituting the values: $n_r = 500 imes (1 - 0.05) = 475 ext{ rpm}$

The line current (IL) can be calculated using: I_L = rac{ ext{P}}{rac{ ext{√3} imes V_L imes ext{Cos} θ}{1000}} Substituting the values: I_L = rac{18000}{rac{ ext{√3} imes 380 imes 0.8}{1000}} = 34.14 ext{ A}

The apparent power (S) can be calculated using: S = rac{V_L imes I_L}{1000} Substituting the values: S = rac{ ext{√3} imes 380 imes 34.14}{1000} = 22.5 ext{ kVA}