AirCompDX

Electrical

Starters, contactors, motor wiring, and NEC quick tables.

Electrical tools
Motor Data (NEC)
FLC, wire, breaker, starter
Motor Amperage
FLA from HP/kW
Voltage Drop
Run length vs. %VD
Motor Reference
Brands, frames, wiring
Contactor Data
Coil ohms, ratings, crossref
Motor Troubleshooting
Humming, tripping, no-start
Controllers
Faults, passwords, manuals
NEC Code Lookup
Article summaries + NFPA links
Three-phase formulas
Amps from HP (3Φ)I = (HP × 746) / (√3 × V × η × PF)
Amps from kW (3Φ)I = (kW × 1000) / (√3 × V × PF)
Amps from HP (1Φ)I = (HP × 746) / (V × η × PF)
Input kW (3Φ)kW = (√3 × V × I × PF) / 1000
kVA (3Φ)kVA = (√3 × V × I) / 1000
Power factorPF = kW / kVA
Voltage drop (3Φ)VD = (√3 × K × I × L) / cmil · K≈12.9 Cu, 21.2 Al
Voltage unbalance%UB = (max deviation from avg / avg) × 100
Delta winding currentI_winding = I_line × 0.577
Slip%s = (Ns − Nr) / Ns × 100 · Ns = 120f / poles
kWh cost$ = kW × hours × rate
Common voltage systems
SystemLine-lineLine-neutralNotes
120/240 1Φ240 V120 VSmall recips ≤5 HP
208Y/120 3Φ208 V120 VLight commercial; motors run hot
240Δ 3Φ240 V120/208**High-leg B = 208 V to ground
480Y/277 3Φ480 V277 VMost industrial compressors
600Y/347 3Φ600 V347 VCanada

Utilization voltage tolerance: ±10% of nameplate. A 460 V motor on a 480 V system is normal; a 460 V motor seeing 415 V is not.

Wye-Delta (Star-Delta) starter

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).
Wye–delta starter — power circuitSchematic
L1L2L3MlineOL · heater = FLA × 0.58T1T2T3T4T5T63Φ MOTOR — 6 LEADS — WYE (START)D — DELTA (RUN)T4→T3 · T5→T1T6→T2Interlock: S and D never in together
M stays in the whole run. S closes first to short T4-T5-T6 into a star point (33% voltage per winding). After the transition timer, S drops out and D closes to reconnect T4→T3, T5→T1, T6→T2 for delta run. S and D are electrically and mechanically interlocked.
Winding configurationsSchematic
WYE (START)star ptT1T2T3V_coil = V_line / √3 = 0.577 VDELTA (RUN)T1T2T3I_coil = I_line × 0.577
Wye (star): each winding sees line ÷ √3, so start current and torque fall to ~33%. Delta: each winding sees full line voltage and carries line current × 0.577 — the reason wye-delta overload heaters are sized at 58% of motor FLA.
Across-the-line (full voltage)

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

Starting current vs. timeSchematic
0%200%400%600%800%0s5s10s15s20s%FLA100% FLAAcross-the-lineWye–deltaSoft start (350% limit)VFD
Typical inrush signatures for a 3Φ compressor motor. Across-the-line peaks 600-800% FLA; wye-delta cuts start current to ~33% but shows a transition spike; a soft starter holds a flat current limit; a VFD starts near full-load current with no inrush.
Soft starter

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.

  • Typical setup: current limit 300-400% FLA, ramp 5-15 s, kick-start off unless the pump won't break away.
  • Bypass contactor closes at full voltage — if it fails, SCRs cook. Check bypass aux feedback on nuisance overtemp faults.
  • Shorted SCR reads continuity phase-to-phase with the unit off — the motor will single-phase or creep.
VFD (Variable Frequency Drive)

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.

  • Never put a contactor between drive and motor while running, and never PF-correct on the motor side.
  • Keep motor cable under ~50 ft unshielded / 100 ft shielded before adding a load reactor or dV/dt filter.
  • Bearing fluting on non-inverter motors ⇒ add a shaft grounding ring; look for frosted/washboard races.
  • Common faults: OU (overvoltage on decel — extend decel/add brake resistor), OC (short decel or output short), OH (blocked heatsink filter).
  • Min speed matters — screw airends need ~20-30% minimum for oil pressure and cooling.
VFD power path & field checksSchematic
3Φ LINERECT6-pulse diodeDC BUScaps · 1.35×VacIGBTPWM inverterMinverter dutyhold-off before touchingshielded cable · bond both endsFaults: OU = decel too fast · OC = output short · OH = blocked heatsinkMin speed 20-30% for airend oil flow
Rectifier → DC bus → IGBT inverter. The DC bus stores lethal charge after disconnect — wait the drive's stated discharge time and verify bus volts. Reflected-wave stress on long motor cable is what kills non-inverter-duty windings; add a dV/dt or load reactor beyond the cable limits.
Motor lead connections (dual-voltage)
9-lead wye (T1-T9)
Low volts (230): L1=T1+T7, L2=T2+T8, L3=T3+T9, tie T4-T5-T6 · High volts (460): L1=T1, L2=T2, L3=T3, tie T4-T7, T5-T8, T6-T9
9-lead delta (T1-T9)
Low: L1=T1+T6+T7, L2=T2+T4+T8, L3=T3+T5+T9 · High: L1=T1, L2=T2, L3=T3, tie T4-T7, T5-T8, T6-T9
12-lead (wye-delta capable)
Wye-delta start uses T1-T12. Low-volt delta run: 1-6-7-12, 2-4-8-10, 3-5-9-11.

Always follow the nameplate/connection plate — brands vary. Rotation reverses by swapping any two line leads.

9-lead wye motor — dual voltageSchematic
LOW VOLTS (230 V)T1T2T3T4T5T6T7T8T9L1=T1+T7 · L2=T2+T8 · L3=T3+T9tie T4-T5-T6 together (star)HIGH VOLTS (460 V)T1T2T3T4T5T6T7T8T9L1=T1 · L2=T2 · L3=T3tie T4-T7 · T5-T8 · T6-T9
Low voltage runs the two half-windings in parallel; high voltage puts them in series. Solid ties are the line connections, dashed ties are internal jumper groups. Always confirm against the connection plate — delta-wound 9-lead motors use a different map.
Single-phase motors & capacitors
  • CSIR/CSCR: start cap (high µF, short duty, black case) drops out via centrifugal switch or potential relay; run cap (low µF, metal oval, continuous).
  • Cap sizing: replace within ±10% µF for run caps, and use equal-or-slightly-higher µF and ≥ same voltage for start caps.
  • Motor hums, won't start, spins if nudged ⇒ open start cap, failed start switch/relay, or stuck unloader.
  • Test caps with a meter on µF, discharged, one lead lifted. Bulged top or leaking = replace.
1Φ capacitor-start / capacitor-run (CSCR)Schematic
L1L2/NRUN windingRUN CAPlow µFcentrifugal swor pot. relaySTART CAPhigh µFSTART winding
Run winding is always in circuit; the start winding is shifted ~90° by the capacitors. The start cap is dropped out by a centrifugal switch or potential relay once the motor reaches ~75% speed — if it stays in, it fails within seconds.
Contactor sizing & troubleshooting
  • 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.
Overload protection
  • 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.
NEC sizing quick rules
Branch conductor125% of FLC (430.22)
Inverse-time breaker250% FLC max (430.52)
Dual-element fuse175% FLC max
Non-time-delay fuse300% FLC max
Overload115% FLC (SF<1.15) · 125% (SF≥1.15)
Disconnect115% of FLC minimum (430.110)
Use table FLCNot nameplate amps, for wire/OCPD (430.6)

Full FLC tables, wire size, and starter picks are in the Motor Data calculator.

Motor nameplate decode
  • 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).
Quick checks (multimeter)
  • 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.
Fault → likely cause
SymptomCheck first
Trips instantly on startShorted winding, welded contactor, wrong OL setting, locked airend
Trips after minutesOL undersized, high ambient, high discharge pressure, loose lug heating
Hums, no rotationSingle-phasing (blown fuse / open contact), start cap (1Φ), seized rotor
Unbalanced ampsLoose termination, bad contact tip, unbalanced supply voltage
Breaker trips, OL doesn'tGround fault or short — megger before re-energizing
Control voltage deadBlown CPT fuse, E-stop, HP switch, thermal switch open
Runs backwardsTwo lines swapped after service — verify rotation arrow before loading
Nuisance drive faultsLoose ground, missing shield bond, cable too long, blocked filter
Lockout / safety
  • LOTO the disconnect, then verify absent voltage phase-to-phase and phase-to-ground with a tested meter (live-dead-live).
  • Bleed receiver and sump pressure before opening any panel that shares a frame with pressurized components.
  • VFD DC bus holds charge — wait for the drive's stated discharge time (typically 5-15 min) and confirm bus volts.
  • Arc-flash PPE per the equipment label for any energized troubleshooting; 480 V panels are not glove-optional.