The three checks a synchroscope or sync-check relay makes before it's safe to parallel an incoming generator with a live bus.
| Generator voltage | 415 V |
| Bus voltage | 412 V |
| Generator frequency | 50.05 Hz |
| Bus frequency | 50.00 Hz |
| Phase angle difference | 3° |
| Acceptance windows | Voltage ≤5%, frequency ≤0.2 Hz, phase ≤10° |
The beat period is the time between successive instants where the two sources are exactly in phase — how long an operator watching a synchroscope has between each pass through the 12 o'clock position.
| Check | Value | Window | Result |
|---|---|---|---|
| Voltage difference | 0.73% | ≤5% | OK |
| Frequency difference | 0.05 Hz | ≤0.2 Hz | OK |
| Phase angle | 3° | ≤10° | OK |
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Voltage difference | ≤ 5% | 0.73% | ✓ PASS |
| Frequency difference | ≤ 0.2 Hz | 0.05 Hz | ✓ PASS |
| Phase angle difference | ≤ 10° | 3° | ✓ PASS |
Key insight: The beat period (20 s here) is what actually makes manual synchronizing practical: with the generator running only slightly faster than the bus, the phase relationship drifts slowly and predictably, giving an operator a wide, repeating window to close the breaker right as the phase angle crosses zero — rather than a split-second opportunity.
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Open Generator Sync Check calculator →Running marginally faster (a small positive frequency difference) makes the phase angle drift slowly forward and predictably cross zero, giving the operator or sync-check relay a clean, repeating closing window. If the generator ran slightly slower, the phase would drift backward relative to the bus, which is functionally the same principle but conventionally generators are brought up to speed from below and allowed to drift forward.
Closing significantly out of phase, frequency or voltage causes a large transient current and torque shock to the generator and its prime mover — potentially damaging the windings, shaft or coupling, and in severe cases causing a violent mechanical event. This is exactly why sync-check relays (ANSI device 25) exist to block breaker closure automatically outside the acceptance windows, as a backup to manual synchronizing.