ESAA Intermediate Level Exam flashcards

33 cards. Read the term and the definition here. The deck at the top of this page is optional.

Three-phase real power

P = √3 × V_line-line × I_line × power factor. This formula applies to balanced three-phase loads and gives the actual power consumed in watts.

Per-unit impedance

Z_pu = Z_actual / Z_base, where Z_base = V_base² / S_base. Per-unit values simplify transformer and fault calculations across voltage levels.

Wye vs delta currents

In a wye connection, line current equals phase current. In a delta connection, line current is √3 times phase current. Voltages follow the opposite rule.

Power factor correction

Capacitor kVAR = P × (tan θ_initial − tan θ_target). Capacitors supply reactive power locally and reduce line current without changing real power.

Synchronous speed formula

N_s = 120 × f / P, where f is frequency in Hz and P is the number of poles. A four-pole motor at 60 Hz runs at 1800 rpm synchronous.

Motor slip

Slip s = (N_s − N_r)/N_s, usually 1–5% at full load. Slip increases with load and determines the induced rotor current.

NEMA Design B

The general-purpose industrial induction motor: locked-rotor current about 6× FLA, breakdown torque about 200% rated, and slip under 5%.

Motor torque equation

Torque in lb-ft = 5252 × HP / rpm. This relationship links shaft horsepower, speed, and torque for any rotating machine.

NEMA overload class

The class number equals trip time in seconds at 6× FLA from a cold start. Class 10 is standard; Class 20 suits high-inertia loads.

V/f control principle

A VFD holds voltage proportional to frequency to keep motor flux constant. Below base speed torque is constant; above base speed power is constant.

PWM carrier frequency

Higher carrier frequency reduces audible motor noise but increases switching losses, common-mode current, and motor heating. Manufacturer limits apply.

Line reactor purpose

A 3–5% impedance reactor on the VFD input reduces harmonic current and absorbs transients. It also protects the drive's DC bus from surges.

Safe torque off (STO)

STO removes gate pulses to the drive output without a contactor. It is a safety function, not an isolation device, and does not de-energize the drive.

4–20 mA live zero

4 mA represents 0% and 20 mA represents 100% of span. The live zero at 4 mA allows detection of a broken wire or failed transmitter.

Pt100 RTD resistance

A Pt100 RTD reads 100 Ω at 0 °C and changes about 0.385 Ω/°C. Three- and four-wire connections cancel lead resistance errors.

Orifice differential flow

Flow is proportional to the square root of differential pressure. To get 25% flow, only 6.25% of full-scale ΔP is required.

HART communication

HART superimposes a ±0.5 mA frequency-shift digital signal on the 4–20 mA analog loop. The loop needs at least 230 Ω for communication.

PLC scan cycle

The PLC reads inputs, solves the program logic, updates outputs, then performs housekeeping. Scan time depends on program size and instruction mix.

TON vs RTO timers

A TON timer resets its accumulated value when the rung goes false. An RTO retains the accumulated value until a separate reset instruction.

Analog input scaling

Engineering units = ((raw − raw_min)/(raw_max − raw_min)) × (EU_max − EU_min) + EU_min. This maps raw counts into process units.

PID derivative on PV

Applying derivative to the process variable instead of the error avoids derivative kick when the setpoint changes suddenly.

Arc flash boundary

The distance at which incident energy equals 1.2 cal/cm², the threshold for a second-degree burn. PPE selection depends on incident energy.

Let-go current

About 10 mA for an average adult is the let-go threshold. Above this value, a worker cannot release the conductor without help.

NFPA 70E PPE categories

Category 1 = 4 cal/cm², Category 2 = 8, Category 3 = 25, Category 4 = 40. Use an incident-energy analysis when one is available.

Test before touch

Verify the meter on a known live source, test the target circuit, then re-verify on the known source. This confirms the meter works.

NEC 430.32 overload rule

Motor overload protection is set at 125% of nameplate FLA for motors with a 1.15 service factor, and 115% otherwise.

NEC 250.122 EGC sizing

The equipment grounding conductor is sized from the overcurrent device rating, not the conductor ampacity. Table 250.122 gives the values.

Hazardous location classes

Class I covers gases and vapors, Class II dusts, Class III fibers. Division 1 means normal presence; Division 2 means abnormal presence.

NEMA enclosure types

Type 1 is indoor general purpose; Type 4/4X is watertight with 4X corrosion resistant; Type 7/9 covers hazardous gas and dust.

Polarization index

The ratio of 10-minute to 1-minute insulation resistance. A value above 2.0 indicates good insulation; below 1.0 is suspect.

Vibration fault signatures

1× running speed indicates unbalance; 2× indicates misalignment; high-frequency broadband energy indicates bearing defects. Phase analysis confirms the diagnosis.

MTBF and availability

MTBF = total operating time / number of failures. Availability = MTBF / (MTBF + MTTR), expressed as a fraction or percentage.

Thermography delta-T

A 1–3 °C rise suggests a possible deficiency, 4–15 °C a probable one, above 15 °C a major one, and above 40 °C a critical fault.