3 Phase AC MOTOR
Table of Contents
- Introduction — What is a 3-phase AC motor?
- Key concepts and Why it matters in AE/JE exams
- Important one-liners (30+)
- Related posts you may like
- Join our channels & Donate
- Ads & Keywords
Important One-liner Tricks & Short Notes (most asked in AE/JE)
1
Synchronous speed (Ns) in RPM = (120 × f) / P — quick trick: double frequency -> double speed, halve poles -> double speed.
2
Slip (s) = (Ns − N) / Ns — for induction motors slip is small (typically 1–6%) at rated load; memorize typical ranges for exam questions.
3
Induced EMF in an AC motor is proportional to flux × speed; reduce speed → reduce EMF (used in V/f control explanation).
4
Full-load torque ∝ (Power) / (speed) — for same power, lower speed yields higher torque; useful for gearbox selection examples.
5
Locked-rotor current (starting current) can be 6–8× rated current; remember typical value 6–8 times for MCQ options.
6
Star (Y) connection gives higher phase voltage in delta-star relation: Vline = √3 × Vphase; useful for wiring problems.
7
Delta connection provides higher starting torque; remember: delta = more torque at same line voltage (exam tip).
8
Power in 3-phase balanced system: P = √3 × VL × IL × cosφ — memorize for quick power calculations in single step.
9
Phase sequence reversal changes rotation direction; quick fix in lab: swap any two supply leads to reverse motor direction.
10
Efficiency (%) = Output/Input × 100; typical 3-phase induction motor efficiency at full load ~85–95% (match option ranges in MCQs).
11
Starting methods: DOL (Direct On Line) gives highest starting current; star-delta reduces starting voltage and current by ~1/√3 factor.
12
Rotor bars in squirrel-cage motors produce induced current by relative slip; think of rotor as short-circuited secondary winding.
13
Broken rotor bar symptoms: vibration, reduced starting torque, increased current; common MCQ distractor: 'increased power factor' is incorrect.
14
Power factor correction in motor circuits usually handled at supply side using capacitors; don't confuse with commutator machines.
15
Heat rise class and insulation class (e.g., Class F, H): higher class → higher allowable temperature; memorize common class pairs for exam choices.
16
Locked-rotor torque & pull-up torque: pull-up torque is the minimum torque between standstill and breakdown speed — memorize definitions to avoid trap options.
17
Breaking methods for AC motors: plugging, regenerative braking, and dynamic resistive braking — example: DC injection for AC induction motors at stop.
18
Synchronous motors run at exact synchronous speed (no slip) — used for power factor correction and constant speed applications; contrast with induction motor's slip.
19
Starting torque vs. running torque: some motors have high starting torque but lower efficiency; match question wording carefully in exams.
20
kVA rating of motor = (√3 × VL × IL) / 1000; remember units and conversion to kW using power factor and efficiency for answer steps.
21
Polarity and phase-shift in synchronous machines: remember the right-hand rule for direction of induced EMF with flux and conductor motion — easy trick diagram to sketch in exams.
22
Slip-ring rotor allows external resistance insertion to control starting torque and current — common MCQ: where to add resistance? — answer: rotor circuit of slip-ring induction motor.
23
Thermal overload relay trips based on motor current and time — match characteristic curves for coordination with MCCB in practical protection questions.
24
Nameplate data quick-read: rated voltage, current, frequency, speed, power factor and service factor — examiners often ask which value is NOT on nameplate (watch options).
25
Motor insulation failure often due to moisture and overheating; MCQ tip: 'moisture' and 'overload' are commonly paired as root causes in options.
26
IEC vs. NEMA ratings: be careful about units and classes — some competitive exam options mix standards to confuse you (learn the common pairs).
27
Motor cooling (IC code): IC 01, IC 411 etc. — memorize common ones for MCQ recognition; e.g., IC 411 indicates forced ventilation arrangements.
28
Efficiency testing methods: brake test, dynamometer test; the brake test is widely used for practical efficiency measurement — helpful for short-answer clarity.
29
Motor vibration diagnosis: misalignment, unbalance, looseness — typical MCQ asks which symptom indicates which cause; remember unbalance → high amplitude at 1×RPM.
30
Harmonics from nonlinear loads increase motor heating and reduce life; exam tip: choose answers mentioning both heating and torque pulsation when present.
31
Variable Frequency Drives (VFD) control speed by changing supply frequency; remember V/f control keeps flux roughly constant — common descriptive MCQ topic.
32
Motor coupling selection: flexible couplings tolerate misalignment; rigid couplings do not — pick flexible in scenarios with shaft misalignment mentioned.
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