Alternator and load sharing

After loads are combined, Unit 27 pushes you to find what the alternator must deliver: line current, power, PF, and sometimes internal quantities depending on the problem set. Treat the alternator as the source that must equal the total upstream demand you calculated.

• Prime mover speed primarily governs frequency (and influences power sharing for paralleled machines)
• Field excitation primarily governs voltage (and reactive power sharing)

Rule of thumb used across the trade: watts from the throttle/governor side; VARs from the field rheostat/AVR side — always within manufacturer procedures.

Before connecting an alternator to a live bus:

1. Phase sequence matches
2. Frequencies match
3. Voltages match
4. Phase angles align (synchronism)

Sync lamps or a synchroscope show when closing is safe. Closing out of sync can damage shafts and windings.

Once paralleled:

• Raising mechanical input on one machine increases its share of real power (kW)
• Raising excitation increases its share of reactive power (kVAR)

If two machines fight (one leading, one lagging hard), circulating VARs heat the units even when the total load is modest.

Capacitor banks change the VAR balance the alternator must supply. On small islands, that interaction is why PF correction and generator controls must be coordinated (previous lesson).

After combining facility loads into P_total and VARs_total, the alternator must supply those totals (plus losses). ILine = S / (√3 ELine) with S=√(P²+VARs²).

Match voltage, frequency, phase angle, and phase sequence before closing. All four must be right.

If amps are high but kW is low on one machine, chase VARs first.

Situation. Two generators share a temporary power board. One runs hot with high amps while the other looks lightly loaded, yet total facility kW seems normal.

What happened. Poor VAR sharing: one AVR set high, the other low, causing circulating reactive current.

Applied lesson. Check kW share and kVAR share separately. Balance governors for kW and AVRs for kVAR per the rental house procedure.

### Teaching pause — say this out loud

Before you leave this lesson, explain the main idea to an imaginary first-month helper in under one minute. If you need the book open to do it, reread How it works once more. Field diagnosis only helps after the concept is yours.

Also sketch the key diagram from memory (triangle, wye/delta, filter shape, or charge curve — whichever this lesson used). Labels beat artistic skill.

### Why this lesson matters on Monday morning

Alternator and load sharing is not trivia. You will meet it when a meter reading looks “impossible,” when a replacement part is almost right, or when a helper asks why the book uses √3 or lead/lag. Master the model here so the next call is pattern recognition, not panic.

Common Monday uses: verify a nameplate against clamps, explain a PF or capacitor change to a customer, or catch a miswired series/parallel or wye/delta assumption before energizing.