6.1 Three-Phase Motors, Nameplates, and FLA

Industrial ammonia plants run almost everything that moves on three-phase induction motors: rotary screw compressors, liquid-overfeed pumps, condenser fans, evaporative-condenser spray pumps, and auxiliary oil pumps. In North American rooms the utilization voltage at the motor control center is typically 480 V. The motor nameplate usually says 460 V. That 20 V gap is intentional. NEMA designs 460 V motors to run on a 480 V system after feeder drop, so a 460 V nameplate on a 480 V plant is the expected pairing, not a mismatch.

A three-phase motor has three stator windings spaced 120 electrical degrees apart. Balanced three-phase voltage produces a rotating magnetic field at synchronous speed. The rotor runs slightly slower than that field; the difference is slip, and slip is what produces torque. CIRO does not ask you to derive an equivalent circuit. It does expect you to read a nameplate, interpret FLA, convert between horsepower and kilowatts, and notice when a running motor does not match the numbers on an operating screen.

Line-to-line versus line-to-neutral

On a 480 V three-phase wye distribution system:

• Line-to-line voltage (V_L-L) is 480 V. Measure it between any two phases (A-B, B-C, or C-A).
• Line-to-neutral voltage (V_L-N) is V_L-L / √3 ≈ 277 V. Measure it from a phase conductor to the grounded neutral.

Most plant motors are three-wire loads. They never use the neutral; the windings see line-to-line voltage. Lighting and some 277 V control power do use the neutral. A classic troubleshooting error is calling a healthy system low voltage because a meter from phase to ground reads 277 V. That reading is normal on a 480/277 V wye. If you need motor voltage, measure phase-to-phase at the motor terminals under load.

Delta-connected plant distribution has no useful neutral for 277 V loads. Do not assume every 480 V bus is wye until you look at the transformer nameplate and the grounding scheme.

Wye versus delta motor windings

Wye and delta describe how the three stator windings are tied together:

• In delta, each winding is connected across two lines, so each winding sees full line-to-line voltage. Line current is √3 times the current in one winding.
• In wye, the windings meet at a common star point. Each winding sees line-to-line voltage divided by √3. Line current equals winding current.

Many dual-voltage motors (230/460 V) use series or parallel groups of coils rather than a simple six-lead wye-delta. Wye-delta starting, covered in the next section, is a starter strategy: the same motor is started in wye so each winding sees reduced voltage, then switched to delta for full-voltage running. Do not mix up a dual-voltage connection diagram in the conduit box with a wye-delta starter in the MCC.

Nine-lead dual-voltage motors are common on pumps and fans. Six-lead motors are what wye-delta starters need. If a large screw is specified for wye-delta starting, the motor must bring out the leads for that starter. You cannot wye-delta start a motor that only has three leads in the box.

What the nameplate is for

A typical 460 V three-phase screw-compressor motor carries the markings below. Operators who skip the nameplate and guess FLA from a sister machine set overloads wrong and misread screens.

FLA (full-load amperes) is the current the motor draws when it delivers rated horsepower at rated voltage and rated frequency. It is not locked-rotor current, not the overload catalog number, and not the feeder-breaker rating. If measured current is above FLA at rated voltage while the driven machine is at full load, the motor is producing more than nameplate horsepower, voltage is low, or a power-quality problem (unbalance or single-phasing) is present.

Service factor of 1.15 means the motor can deliver 15% more than rated horsepower under the manufacturer's service-factor conditions, usually rated voltage and frequency in a 40°C ambient. Running in the service factor every day cooks insulation. Treat SF as emergency margin for a hot day or a dirty condenser, not as design capacity you plan around.

Insulation class sets the thermal ceiling of the enamel and slot insulation. Class B is 130°C, Class F is 155°C, Class H is 180°C. A common rule of thumb is that every extra 10°C of winding temperature halves insulation life. Voltage unbalance, packed TEFC fins, high machinery-room ambient, and single-phasing all show up as temperature, not as a new nameplate number.

Efficiency and power factor are not decorations. Efficiency tells you how much electrical power you must buy to get the shaft kW. Power factor tells you how much current (and kVA) that kW requires. A screen that shows kW without PF will not match a clamp-on ammeter unless you do the three-phase math.

Worked conversion: 300 HP, 480 V, PF 0.86, 93% efficiency

CIRO operating screens for a large screw often present this family of numbers: a 300 HP compressor motor, 480 VAC supply, power factor about 0.86, and motor efficiency about 93%. Those four figures let you convert horsepower, kilowatts, and current. They are plant data, not a copied exam stem.

Step 1 — shaft output in kilowatts

kilowatts out = HP × 0.746

kilowatts out = 300 × 0.746 = 223.8 kW at rated horsepower.

Step 2 — electrical input the meter sees

kilowatts in = 223.8 / 0.93 ≈ 240.6 kW

Input is always larger than shaft output. If a fully loaded 300 HP screw shows roughly 241 kW on the power meter, the screen is consistent with 93% efficiency. If the same motor shows 280 kW input at 100% slide valve with normal oil and discharge temperatures, the motor is doing more work than nameplate (high head pressure, extra mass flow) or something is wasting power (mechanical drag, very low voltage, poor power quality).

Step 3 — three-phase line current at 480 V

I = (kW_in × 1000) / (√3 × V_L-L × PF)

I = 240,600 / (1.732 × 480 × 0.86)

I = 240,600 / 713.5 ≈ 337 A

Nameplate FLA is stated at nameplate voltage, usually 460 V, so repeat the same formula at 460 V:

I_nameplate ≈ 223,800 / (1.732 × 460 × 0.93 × 0.86) ≈ 351 A

A 460 V FLA near the mid-300 A range is therefore consistent with this motor. On a 480 V bus, current for the same kW is slightly lower because voltage is in the denominator. Always compare current at the voltage you actually have. Do not call the motor overloaded solely because a 480 V ammeter reading is a few percent below a 460 V FLA number.

Step 4 — kVA versus kW

kVA = kW / PF = 240.6 / 0.86 ≈ 280 kVA

Feeders, starters, and transformers are sized on current and therefore on kVA, not on kW alone. Poor power factor means more amperes for the same useful kilowatts. That is why a screen that lists PF 0.86 next to a 300 HP screw is operationally useful: it tells you the current (and the thermal load on cables and overloads) is higher than a naive kW-only estimate.

Inrush versus FLA

When a full-voltage (across-the-line) starter closes, the motor is not yet turning. Locked-rotor current on a NEMA Design B motor is typically about six times FLA (the NEMA band is roughly 6 to 7 times). That inrush lasts only while the motor accelerates. Overload relays are inverse-time devices: they must ride through inrush and trip on sustained current near or above FLA. Short-circuit protection (fuses or instantaneous breaker trip) must be high enough not to open on inrush, yet still protect the feeder. The next section covers wye-delta and other reduced-voltage methods that cut starting current.

Operator checks that belong on a round

• Read FLA from this motor's nameplate, not from a similar compressor.
• Measure voltage at the motor terminals under load, not only at the MCC stab.
• Record amps on all three phases. Unequal amps belong in the unbalance and single-phasing sections.
• If amps exceed FLA, look at the load first: slide-valve position, discharge pressure, condenser performance, pump dead-head, or a stuck unloader.
• Compare nameplate r/min to a tachometer or VFD speed feedback. A large extra slip at load is overload or low voltage.
• Keep TEFC fins and filters clean. A packed motor is an overload waiting to happen even when the electrical numbers look modest.
• Use efficiency and PF from the nameplate or the screen when you convert HP to kW and kW to amperes. Guessing 100% efficiency understates input power by the lost 7% in the example above.