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• A standard three-phase induction or synchronous motor that is connected to a balanced three-phase power system does not need any start or run capacitors to function.
• The capacitors you frequently see “near” three-phase motors are there for two quite different reasons:
Why a three-phase motor usually starts unaided
• The three stator currents are 120 ° apart, naturally producing a rotating magnetic field (RMF); this self-starts the rotor without auxiliary phase-shift components.
• Hence no start/run capacitor is integral to the motor, unlike single-phase split-phase or PSC motors.
Where capacitors are found with three-phase motors
2.1 Power-factor correction (PFC)
• Induction machines draw magnetising current (reactive kVAR) that lags the voltage.
• By connecting a capacitor bank in parallel—at the motor terminals, at a motor control centre (MCC) or at the service entrance—the leading reactive current of the capacitors cancels part of the lagging component.
• Benefits:
– Raises PF from, say, 0.75 lag to ≥ 0.95 lag.
– Lowers line current and I²R losses, increases available capacity of transformers and feeders, and avoids utility penalties.
• Selection rule of thumb:
kVARC = P × [tan (acos PF_initial) – tan (acos PF_target)].
Manufacturers supply tables; never oversize beyond no-load magnetising kVAR to avoid over-voltage or self-excitation.
2.2 Phase conversion / Steinmetz connection
• Where only single-phase service exists, a static phase converter (Steinmetz) adds a “run” capacitor (≈ 25 – 35 µF per HP, 370–440 V ac) between the third motor lead and one live line to synthesize an approximate third phase.
• Often a larger “start” capacitor (2–3× run value) is switched in temporarily for torque boost.
• Expect derating to 50–70 % of nameplate HP and some current imbalance.
• Better solutions: rotary phase converters or VFDs with single-phase input rectifiers.
2.3 Other capacitor uses around three-phase drives
• DC-link capacitors inside VFDs (electrolytic or film) smooth the rectified bus.
• Snubber / surge-suppression networks across contactors or coils.
• EMI filters meeting IEC 61000‐3-2/3.
Typical capacitor specifications
• Metallised polypropylene (self-healing) for continuous-duty PFC and run service, IEC 60252-1 class.
• Start capacitors: non-polar electrolytic, intermittent duty, temperature class B or better.
• Voltage rating ≥ 1.1 × line–neutral RMS.
• Discharge resistors sized to bleed < 50 V in ≤ 60 s per IEC 60831-1.
Harmonics and resonance risks
• Large PFC banks can resonate with feeder inductance near the 5th or 7th harmonic; detuning reactors (tuned to 189 Hz for 50 Hz systems, 227 Hz for 60 Hz) mitigate this.
• VFD-fed motors should have PFC connected upstream of the drive, never on the VFD output.
Latest industry trends
• Smart capacitor banks with thyristor or IGBT switching for stepless kVAR control.
• Integrated PFC inside IEC (IE3+) premium-efficiency motors (“motor-mounted caps”).
• Active front-end (AFE) VFDs that supply leading kVAR electronically, displacing traditional capacitors.
• Use of dry-film, self-healing capacitors with over-pressure disconnects to address fire safety (UL 810, IEC 60831-1/2).
Example: 18.5 kW, 400 V, 0.78 PF(lag) motor
• Real power P ≈ 18.5 kW
• Initial reactive power Q₁ = P·tan (acos 0.78) ≈ 18.5 × 0.66 = 12.2 kVAR
• Target PF = 0.95 ⇒ Q₂ = P·tan (acos 0.95) ≈ 18.5 × 0.33 = 6.1 kVAR
• Required capacitor bank: 12.2 − 6.1 ≈ 6 kVAR (about 22 µF line-to-line at 50 Hz).
• Energy-efficiency regulations (EU Ecodesign, US DOE motor regulations) encourage PF improvement but may limit harmonic injection; capacitor banks must comply with IEC 61000-3-12.
• Safe discharge per IEC 60831; NEC 2017 Art. 460 in the US mandates disconnect and bleed resistors.
• Correct identification of live capacitors crucial for maintenance safety—lock-out/tag-out (LOTO) procedures.
• Static phase-converter methods are stop-gap; for continuous industrial duty use a VFD or utility three-phase service.
• Over-correction into leading PF can create over-voltage on lightly loaded generators.
• Capacitor life halves for every 10 °C rise above rated ambient—ensure adequate ventilation.
• Evaluate active (IGBT) VAR compensation vs. classical banks for dynamic loads such as cranes, spot-welders.
• Explore silicon-carbide (SiC) devices in AFE drives for harmonic-free, unity-PF operation.
• Study IEEE 519-2022 limits on harmonics and their impact on capacitor design.
• Examine self-healing metallised film degradation mechanisms (partial discharge, corona) for lifetime modelling.
Three-phase motors themselves do not rely on capacitors for starting or running; the three-phase supply already provides the needed rotating magnetic field. Capacitors appear in the system mainly to (1) supply reactive power locally and raise the power factor, or (2) synthesize a third phase when only single-phase power is available. Choosing, installing, and protecting these capacitors requires careful calculation, adherence to standards, and periodic maintenance to maximise efficiency and safety.
