4.1 Synchronous Motors & AC Generators (Alternators)

4.1 Synchronous Motors & AC Generators (Alternators)

Under Republic Act No. 7920 (the Electrical Engineering Law), "electrical machines" is one of the enumerated technical subjects tested in the board examination for Certified Electricians and Master Electricians. Rotating AC machines fall into two broad families — induction machines and synchronous machines — and the exam frequently tests a candidate's ability to distinguish the two, especially their speed behavior and their role in real installations.

How a Synchronous Machine Differs from an Induction Machine

Both machine types create a rotating magnetic field in the stator when balanced three-phase (or properly phase-shifted single-phase) AC is applied to the stator windings. The speed of that rotating field is the synchronous speed, given by:

Ns = 120f / P

where Ns is synchronous speed in revolutions per minute (rpm), f is the supply frequency in hertz (Hz) — 60 Hz under the Philippine Electrical Code — and P is the number of magnetic poles.

In an induction machine, the rotor has no independent source of excitation. The rotating stator field induces voltage and current in the rotor bars (by transformer action), and that induced current is what produces torque. For current to be induced, the rotor must physically rotate slower than the field — this speed difference is called slip. An induction motor's actual speed is therefore always somewhat below Ns and changes with load.

A synchronous machine works differently. Its rotor carries its own independent magnetic field — most commonly produced by DC excitation windings fed through slip rings (or a brushless rotating exciter), though smaller synchronous machines may instead use permanent magnets (a permanent-magnet synchronous motor, or PMSM) or simply rely on the reluctance of a salient-pole rotor with no separate excitation at all (a reluctance motor). Because the rotor's own field locks magnetically onto the rotating stator field like two meshed gears, the rotor turns at exactly synchronous speed — zero slip — for every load within the machine's capability. This is the defining trait tested on the board exam: a synchronous motor's speed is fixed by supply frequency and pole count alone, not by load.

One practical consequence: a synchronous motor is not self-starting. At standstill, the DC-excited rotor field and the rapidly rotating stator field produce zero net average torque — the rotor is alternately pulled forward and backward within each electrical cycle. Field synchronous motors solve this by starting as an induction motor first, using damper (amortisseur) windings embedded in the rotor pole faces to develop starting torque, then applying DC excitation only after the rotor has accelerated close to synchronous speed. Variable-frequency drives (VFDs) offer another modern starting method by ramping frequency up gradually from zero.

Constant Speed and Power-Factor Correction

Because a synchronous motor runs at exactly Ns regardless of mechanical load (up to its pull-out torque limit, beyond which it loses synchronism and stalls), it is favored for applications demanding precise, constant speed — large compressors, blowers, and mill drives.

The synchronous motor's second distinguishing feature is its ability to correct power factor. Varying the DC field current changes how much reactive power the motor exchanges with the supply, without changing its real power output or speed:

• Under-excited — the motor behaves inductively and draws lagging current, like most other motor loads.
• Normally excited — the motor operates near unity power factor.
• Over-excited — the motor behaves capacitively and draws leading current, actively supplying reactive power (VARs) back to the system.

Plotting armature current against field current at constant load produces the familiar V-curve, with the minimum current point at unity power factor. Because an over-excited synchronous motor supplies leading VARs, plants sometimes install a synchronous motor sized for zero mechanical load purely to correct plant power factor — this application is called a synchronous condenser (or synchronous capacitor). A master electrician evaluating a facility's power-factor penalty on its electric bill should recognize that an existing over-excited synchronous motor already on site is doing useful corrective work, distinct from any capacitor banks that may also be installed.

AC Generators (Alternators)

Run the same machine in reverse and it becomes an alternator — the synchronous generator that produces essentially all of the world's utility-scale AC power. A prime mover (steam turbine, hydro turbine, or diesel/gas engine) mechanically turns the DC-excited rotor field. As the rotating field sweeps past the stationary stator (armature) windings, it induces a three-phase electromotive force (EMF) in them by Faraday's law of electromagnetic induction — exactly the reverse process of motor action, but with mechanical power going in and electrical power coming out.

Utility generators nearly universally use the rotating-field, stationary-armature arrangement rather than a rotating armature. This is because the armature carries the machine's full output voltage and current — often tens of kilovolts and thousands of amperes — and it is far simpler and safer to bring that heavy power output out through fixed terminals on a stationary winding than through sliding contacts. Only the comparatively small, low-voltage DC excitation current needs to reach the rotor, which is easily done through slip rings or, in modern machines, a brushless exciter (a small rotating AC generator whose output is rectified on the shaft itself, eliminating brushes entirely).

Because synchronous alternators lock to grid frequency and can be synchronized to run in parallel with other alternators and the utility grid, they are the standard choice for baseload and dispatchable generation of every kind — coal, gas, hydro, and nuclear plants all use synchronous alternators.

Standby and Genset Alternators

Master electricians regularly encounter smaller synchronous alternators in standby generator sets (gensets) for hospitals, buildings, and industrial facilities. A diesel or gas engine drives a synchronous alternator equipped with a brushless exciter and an automatic voltage regulator (AVR), which continuously adjusts field excitation to hold output voltage steady as load changes. Before a genset can be paralleled with the utility service or with other gensets, its voltage, frequency, phase sequence, and phase angle must all be matched — a process called synchronizing — because closing the paralleling breaker on a mismatched machine can cause a damaging inrush of circulating current. Automatic synchronizing panels perform this check electronically before permitting breaker closure.