Electrical Flashcards

A three-phase motor is running but drawing significantly unbalanced currents across its three legs. Phase A reads 18A, Phase B reads 12A, and Phase C reads 19A. The supply voltages are confirmed balanced. What is the MOST likely cause?

Answer: A shorted turn in one of the stator windings

A shorted turn in a stator winding reduces the impedance of that winding, drawing more current in the affected phases while causing unbalanced magnetic flux. A single-phasing condition (open fuse or open overload) would typically cause much more severe imbalance and the motor would likely overheat rapidly. A loose connection would more commonly cause high resistance and lower current on that leg, not this pattern of moderate imbalance across all three phases.

When using a clamp-on ammeter to measure current in a DC circuit, the reading is unstable and fluctuating even though the load appears steady. What is the MOST likely explanation?

Answer: The clamp-on meter is an AC-only model being used on a DC circuit

Standard clamp-on ammeters use a current transformer that only works with AC—they require a changing magnetic field to induce a secondary current. When used on DC, they produce erratic or zero readings. True DC clamp meters use Hall-effect sensors and are specifically rated for DC measurement. A conductor's insulation thickness does not affect the magnetic field sensed by the clamp jaw.

A 480V/120V control transformer is sized at 500VA. The primary fuse is 2A. A technician replaces it with a 5A fuse 'to stop nuisance blowing.' Shortly after, the transformer fails. Which of the following BEST explains the failure mechanism?

Answer: The oversized fuse allowed sustained inrush or fault current that overheated the transformer windings

The primary fuse on a control transformer is sized to protect the transformer's windings from overcurrent. A 500VA transformer on 480V primary draws about 1.04A at full load—a 2A fuse gives standard 200% inrush headroom. Replacing it with a 5A fuse means a fault or severe overload on the secondary can sustain current well above the transformer's thermal limit before the fuse clears, cooking the windings. The fuse rating does not alter voltage or cause core saturation under normal operation.

An electrician measures 277V between a phase conductor and ground in a 480V wye system. After isolating the panel, megohm testing reveals Phase B has 0.5 MΩ to ground while Phases A and C show >500 MΩ. What does the 0.5 MΩ reading most likely indicate?

Answer: Degraded or moisture-contaminated insulation on Phase B conductors or equipment

A megohm reading of 0.5 MΩ on an isolated phase is well below the IEEE 43 minimum acceptable value of 1 MΩ (and well below the preferred 100 MΩ+ for new wiring), indicating insulation breakdown—commonly from moisture ingress, heat damage, or mechanical abrasion. Capacitive leakage in conduit typically appears as a consistent, very high resistance across all phases, not an isolated low reading. An open circuit would show extremely high resistance, not low resistance to ground.

A variable frequency drive (VFD) is installed on a pump motor. The drive trips on 'DC bus overvoltage' fault during motor deceleration. The deceleration ramp is set to 5 seconds. What is the MOST appropriate corrective action?

Answer: Increase the deceleration time ramp to allow the DC bus voltage to dissipate more gradually

During rapid deceleration, the motor acts as a generator, feeding energy back into the VFD's DC bus. If the deceleration ramp is too fast, the bus voltage rises above the drive's overvoltage trip threshold. Increasing the deceleration time allows the bus capacitors to dissipate energy through the drive's braking resistor (if equipped) or reduces the regenerative energy rate enough to stay below the trip point. A larger input fuse addresses supply-side current, not regenerated energy. Carrier frequency affects motor heating and acoustic noise, not DC bus voltage during braking.

In a ladder logic rung, a normally-closed (NC) contact wired in series with a motor starter coil represents a thermal overload relay contact. The motor will NOT start even though the overload has been manually reset. The overload LED indicator shows 'healthy.' What should the technician check FIRST?

Answer: Whether the overload relay's reset mode is set to 'manual' vs. 'automatic' and if the reset was fully seated

Many overload relays have both manual and automatic reset modes, and a manual reset requires the button to be pressed fully and held until it latches. A partial press may allow the LED to show 'healthy' (meaning no active fault is detected by the relay's monitoring circuit) while the NC contact remains open if the mechanical trip mechanism was not fully re-latched. This is a common field mistake. While the other options are valid troubleshooting steps, the reset confirmation on the device that was just operated is the logical first check per a systematic fault-isolation approach.