Ramsay Test Motors and Controls 4

Question 1

A three-phase induction motor is running at 1,740 RPM on a 60 Hz supply with a synchronous speed of 1,800 RPM. If the load increases and slip rises to 5%, what is the new rotor speed?

Accepted Answer

1,710 RPM

Answer

Slip of 5% means the rotor speed is 95% of synchronous speed: 1,800 × 0.95 = 1,710 RPM. The original 1,740 RPM corresponds to 3.33% slip. As load increases, slip increases, so rotor speed decreases to 1,710 RPM.

Question 2

In a wye-delta (Y-Δ) reduced-voltage starter, the motor is initially connected in wye during starting. Compared to full-voltage delta starting, what is the line current drawn during the wye start phase?

Accepted Answer

1/3 of full-voltage delta starting current

Answer

When connected in wye, each winding sees 1/√3 of line voltage, reducing winding current to 1/√3. Because line current in wye equals winding current (unlike delta), and voltage is also reduced by 1/√3, the net effect on line current is (1/√3)² = 1/3 of the delta full-voltage line current.

Question 3

A DC shunt motor is operating at rated load and suddenly loses its field excitation while still connected to the supply voltage. What is the most likely outcome?

Accepted Answer

The motor will overspeed dangerously

Answer

In a DC shunt motor, speed is inversely proportional to field flux (N ∝ V/Φ). If field excitation is lost (Φ → 0), the motor attempts to reach extremely high (theoretically infinite) speed. In practice this causes dangerous overspeeding and potential mechanical destruction — a condition called 'field loss runaway.'

Question 4

A two-speed motor using the Dahlander (pole-changing) winding method switches from low speed (4-pole) to high speed (2-pole). Which of the following correctly describes the winding reconnection?

Accepted Answer

Series delta to parallel wye

Answer

In a Dahlander pole-changing motor, low speed uses a series delta connection (4-pole), which halves to 2-pole at high speed by reconnecting to parallel wye. This internally creates twice as many effective poles in low-speed mode and doubles synchronous speed when switched to the parallel wye configuration.

Question 5

A technician notices that a three-phase motor draws balanced current on all three phases at no load, but under full load one phase reads 20% higher than the other two. The most likely cause is:

Accepted Answer

A shorted turn in one stator winding

Answer

A shorted turn in one stator winding reduces the impedance of that phase, allowing more current to flow specifically under loaded conditions when back-EMF effects are most significant. Open rotor bars cause torque pulsation and vibration but result in imbalanced rotor effects, not a single-phase stator overcurrent. Supply voltage unbalance would show imbalance at no load too.

Ramsay Test Practice Test

Ramsay Test Practice Test

Ramsay Test Motors and Controls 4