Electrical Power Equipment Maintenance and Testing Flashcards

Flashcards covering motor and generator types, NEMA design classifications, insulation standards, and maintenance testing procedures based on Chapter 10 of the lecture notes.

What phenomenon occurs between the stator and rotor under operating conditions when revolving magnetic fields induce voltage?

Mutual induction

According to NEMA standards, what is the starting locked rotor current range for a Design A squirrel-cage motor?

$6\times$ to $10\times$ rated current

What is the slip at rated load for a NEMA Design B motor?

Less than $5\%$

What is the designated high starting torque percentage for a Design C squirrel-cage motor?

$$200\%$$

What is the slip requirement for a NEMA Design D motor at rated load?

$5\%$ or more

Why are single-phase motors generally not self-starting?

They possess only one primary stator winding that produces a pulsating magnetic field rather than a revolving one at standstill.

What are the four common types that identify a single-phase induction motor based on its starting torque method?

Split-phase, capacitor start and run, repulsion, and shaded pole

What is the maximum frequency for a universal motor to operate at approximately the same speed/output on AC as it does on DC?

$$60\,Hz$$

What is the primary purpose of the light series winding in a stabilized shunt-wound DC motor?

To prevent a rise in speed or to obtain a slight reduction in speed from no-load to full-load conditions.

Why should a series-wound DC motor typically not be operated at full voltage while uncoupled from its load?

The torque and speed are load dependent, risking catastrophic speed increases.

How is a drip-proof enclosure defined regarding the angle of descending particles?

Successful operation is not interfered with when drops strike the enclosure at any angle from $0^{\circ}$ to $15^{\circ}$ downward from the vertical.

What is the maximum strike angle for downward vertical moisture in a splash-proof enclosure?

$$100^{\circ}$$

What defines a totally enclosed nonventilated enclosure?

It is not equipped for cooling by means external to the enclosing parts.

In a NEMA pipe-ventilated enclosure, what is it called when air is driven by an external blower?

Force-ventilated

What type of motor operates at a speed practically independent of the load at one or more definite speeds?

Multispeed motor

What are the standard terminal markings for AC stator windings according to Table 10.1?

$$T1, T2, T3, \dots$$

According to Principle 1 of terminal marking, what are the auxiliary winding designations for a single-phase motor?

$$T5, T6, T7, T8$$

What is the standard direction of rotation for synchronous and universal motors when facing the end opposite the drive?

Counterclockwise

What is the formula for calculating synchronous speed ($N_s$) in AC motors?

$$N_s = \frac{120 \times \text{frequency}}{\text{no. of poles}}$$

How is motor slip expressed as a percentage of synchronous speed?

$$\text{Slip (\%)} = \frac{\text{Synchronous rpm} - \text{actual rpm}}{\text{Synchronous rpm}} \times 100$$

What unit is equivalent to $1\,hp$ in mechanical terms?

$$33,000\,ft.lb/min$$

What is the constant used in the formula to calculate Torque when both horsepower and actual speed are known?

$$5252$$

What is the definition of lock-rotor torque?

The minimum torque developed by the motor for all angular positions of the rotor when the stator is energized at rated voltage and frequency.

What torque term refers to the maximum torque developed for $1\,min$ without stalling?

Pull-out torque

Which NEMA design motor is often selected for applications requiring high starting torque, such as reciprocating air compressors?

Design C motor

How are motor insulation systems classified?

On the basis of their ability to withstand the total temperature during the machine's life without deterioration.

What is the rated total temperature for Class B insulation?

$$130^{\circ}C$$

What is the rated total temperature for Class F insulation?

$$155^{\circ}C$$

What is the standard altitude limit for defined temperature rises in motors and generators?

$$3300\,ft$$

What voltage tolerance does NEMA provide for the operation of induction general-purpose motors?

$\pm 10\%$ from the nameplate rating

According to NEMA, what is the maximum combined voltage and frequency deviation allowed?

$$\pm 10\%$$

At what percentage of voltage unbalance is it recommended to derate a motor?

$$4\%$$

What are the six electrical areas identified for comprehensive motor analysis?

Power quality, Power circuit, Insulation, Stator, Rotor, and Air gap

A service factor of $1.15$ typically translates to how much of a temperature increase with a $15\%$ overload?

$$10^{\circ}C$$

What are the typical duration ratings for short-time duty motors?

$$15, 30,\text{ or } 60\,min$$

What is the relationship between ball bearing fatigued life and load?

It varies inversely with the cube of the load.

What dielectric strength challenge is created by PWM switching technology in variable frequency drives?

Reflected voltage waves resulting from supply cables acting as incorrectly tuned transmission lines.

What is the primary indicator of cleanliness in a winding insulation test?

Capacitance (higher capacitance indicates more contamination buildup).

What is the minimum insulation resistance ($IR_{1\,min}$) for most DC armatures and AC windings built after 1970?

$$100\,M\Omega$$

How is the Polarization Index (PI) calculated?

$$\text{PI} = \frac{\text{IR at } 10\,min}{\text{IR at } 1\,min}$$

What PI range is generally considered 'Excellent' for motor insulation?

Between $2$ and $4$

What is the likely cause of a PI ratio greater than $5$ according to EASA?

Dry or brittle insulation that has potentially lost mechanical properties.

What test is performed by rotating the rotor in specific increments to record inductance measurements?

Rotor Influence Check (RIC)

How many inductance measurements per pole group are recommended for a proper RIC resolution?

$$18$$

What defines static air gap eccentricity?

When the centerline of the shaft is at a constant offset from the centerline of the stator.

What defines dynamic air gap eccentricity?

When the centerline of the shaft is at a variable offset from the centerline of the stator.

What is the formula for the eccentricity frequency ($F_{ECC}$)?

$$F_{ECC} = \text{number of rotor bars} \times \frac{\text{rpm}}{60}$$

What percentage of motor failures were attributed to the rotor in an EPRI/General Electric study?

$$10\%$$

In a PWM drive, how fast can voltage rise times be?

As low as $0.1\,\mu s$

What is the term for the ratio of the rms of the harmonic content to the rms of the fundamental quantity?

Total Harmonic Distortion (THD)