Electrical

Reduced-voltage starting by first connecting the motor windings in wye (33% of full voltage across each winding, ~33% starting current and torque), then transitioning to delta for run.

• Start (S contactor): wye-point of motor leads T4-T5-T6 tied together. Line contactor (M) energizes T1-T2-T3.
• Run (D contactor): after timer (0.5-2.5 s), S drops out, D pulls in — T4→T3, T5→T1, T6→T2 (per nameplate).
• Mechanical interlock: S and D must NEVER be in at the same time — phase-to-phase short.
• Overload sizing: heater = motor FLA × 0.58 (line current in the delta winding).

  L1 L2 L3
   |  |  |
  [M] line contactor  ← always in when running
   |  |  |
   T1 T2 T3 ──────── motor windings
   T4 T5 T6
   |  |  |
   ├──┼──┤  [S] wye contactor  ← in only during start
   └──┴──┘
   
   then timer transitions:
   [S] out → [D] delta contactor in
   T4↔T3, T5↔T1, T6↔T2

Simplest start. Single contactor + overload. Inrush typically 600-800% FLA. Acceptable up to ~10-15 HP on most utilities; check utility rules above that.

SCR-based reduced voltage. Replaces wye-delta in modern installs. Adjustable ramp time, current limit, and kick-start. No moving parts in the start circuit.

Best for variable-demand screw compressors. Modulates motor speed to match air demand. Always use inverter-duty motor or check insulation rating. Shielded VFD cable, proper grounding, and dV/dt or sine filter for long runs.

• NEMA size by max FLA at applied voltage; IEC by AC-3 rating.
• Chattering ⇒ low coil voltage (<85% rated). Check transformer & wire length.
• Welded tips ⇒ jogging / undersized / arcing on opening. Replace, don't file.
• Burned line lugs ⇒ loose terminations. Torque to spec, IR scan annually.

• Standard: 115% of motor FLA (NEC 430.32). 125% max for marked SF≥1.15.
• Class 10 = trip ≤10 s at 600% (general). Class 20 = 20 s (high-inertia). Class 30 rarely needed for compressors.
• Ambient compensation matters in hot rooms — use ambient-compensated OL.

• HP / kW — output mechanical power.
• FLA — full-load amps at rated voltage.
• SF — service factor (1.0 standard, 1.15 = 15% short-term overload OK).
• Code letter — locked-rotor kVA/HP (A lowest, V highest). Code G typical for compressor TEFC motors.
• Insulation class — B (130°C), F (155°C), H (180°C). Run cooler = longer life (halves every 10°C above rated).
• NEMA design — B (general), C (high starting torque, good for recip compressors with unloaders).

• Winding continuity: 3Φ — all three should read within 5-10% of each other.
• Winding to ground: >1 MΩ minimum (megger at 500/1000 VDC).
• Voltage balance (3Φ): max deviation <1% of average. 2% causes ~8% temp rise.
• Current balance: max deviation <10% of average under load.