Water Distribution Grade 1 Certification Practice Exam

Which types of motors would be expected to have the lowest demand for a starting current?

1. Squirrel cage induction motor

2. Wound-rotor induction motor

3. Permanent split capacitor motor

4. Universal motor

The correct answer is: Wound-rotor induction motor

The choice of the wound-rotor induction motor being expected to have the lowest demand for a starting current is based on its unique construction and operational characteristics. A wound-rotor induction motor features rotor windings that can be externally connected to variable resistances. This allows the operator to adjust the rotor impedance at startup, which effectively limits the starting current. By adding resistance to the rotor circuit, the inrush current can be reduced significantly, making it much more manageable and preventing damage to the motor windings due to high initial currents. In contrast, the other options generally exhibit higher starting currents. For example, the squirrel cage induction motor often experiences a high starting current due to its simple construction without additional means to control current at startup. The permanent split capacitor motor also tends to have a moderate starting current as it relies on a capacitor for phase shift to start, presenting a direct connection to the supply voltage. Universal motors, capable of operating on both AC and DC, also demand a substantial starting current as they often lack the same level of control over inrush current as the wound-rotor design. This understanding highlights the functional advantages of the wound-rotor induction motor in applications where managing starting current is critical for both efficiency and protection of motor