Motors and Controls Flashcards

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?

Answer: 1,710 RPM

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.

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?

Answer: 1/3 of full-voltage delta starting current

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.

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?

Answer: The motor will overspeed dangerously

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.'

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?

Answer: Series delta to parallel wye

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.

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:

Answer: A shorted turn in one stator winding

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.

In a control circuit using a latching relay (memory circuit), a momentary STOP pushbutton is wired in series with the coil, and a normally open auxiliary contact of the contactor is wired in parallel with the momentary START button. If the auxiliary contact welds closed due to a fault, what is the safety consequence?

Answer: The motor will start automatically on power restoration after any outage

The auxiliary contact in parallel with the START button serves as the 'seal-in' or holding contact. If it welds closed, the circuit between the coil and supply is permanently completed through that branch — bypassing the START button entirely. When power is restored after any interruption, the coil energizes immediately without requiring a START button press, defeating the anti-restart safety feature.