Alpha Technologies VMX Series - Manual (Page 17)

17 017-940-B0-001, Rev. A
2.2 Standby Operation
When incoming AC line voltage drops or rises signifi cantly, or a complete power outage
occurs, the VMX line monitor activates standby operation. During the transfer from AC line to
standby operation, the battery powered inverter comes on-line as the isolation relay opens to
prevent AC power from back-feeding to the utility. The energy contained in the ferroresonant
transformer continues to supply power to the load. The following changes also occur within
the VMX Power Supply:
• The control logic drives the inverter fi eld-effect transistors (FETs) on and off. This
switching action converts the DC battery current into AC current in the inverter winding of
the ferroresonant transformer, which provides regulated power to the load.
• The control logic, which includes a microprocessor and other circuits to protect the
inverter FETs from overcurrent damage, monitors the condition of the batteries and the
inverter during standby operation. Since a prolonged AC line outage would severely
discharge the batteries, resulting in permanent damage, the control logic disables the
inverter when the batteries drop to approximately 10.5Vdc per battery.
When acceptable AC line voltage returns, the power supply transfers back to AC line
operation within 10 to 20 seconds. This delay allows the AC line voltage and frequency to
stabilize before the control logic phase-locks the inverter’s output to the utility input. The
control logic then de-energizes the isolation relay, reconnects the AC line to the primary of
the ferroresonant transformer and disables (turns off) the inverter. This results in a smooth,
in-phase transfer back to utility power without interruption of service to the load. The battery
charging circuit then activates to recharge the batteries in preparation for the next power
outage. In the case of a prolonged AC outage where the inverter shuts off due to low battery
voltage, the output will return as soon as the AC line voltage returns.
2.0 Theory of Operation
2.1 AC (Line) Operation
During AC line operation, utility power is routed into the primary winding of the ferroresonant
transformer through the contacts of the transfer isolation relay. At the same time, power is
directed to the rectifi er circuitry in the inverter which provides power for the control circuitry.
The bidirectional inverter also serves as a battery charger during AC line operation. The
ferroresonant transformer and an AC capacitor form the resonant tank circuit, which provides
excellent noise and spike attenuation, output short circuit current limiting, and output voltage
regulation. The ferroresonant transformer produces a quasi square wave output which
resembles a rounded square wave.
To minimize the possibility of the transformer entering unstable operation, the VMX Power Supply
requires a minimum load of at least one ampere connected to the output. Unstable transformers will
self-correct as soon as a load of one ampere or greater is connected to the power supply.
CAUTION!
The duration of battery-backed standby operation depends upon the type and number of batteries and the
load on the power supply.
NOTE: