The SPM-D21 Series consists of microprocessor-based synchronizers designed for use on three-phase AC generators equipped with Woodward or other compatible speed controls and automatic voltage regulators. The SPM-D21 Series provides automatic frequency, phase and voltage matching using either analog or discrete output signals. It combines synchronizing for a GCB (generator circuit breaker) and a MCB (mains circuit breaker), load and power factor control, and generator and mains protection.