Alpha XM 6015 Series Operations Instructions download pdf (Page 12)

1. INTRODUCTION

1.2 Theory of Operation,

continued

1.2.2 Inverter (STANDBY) Operation

When the incoming AC line voltage drops significantly, or a complete power outage

occurs, the control logic’s line monitor activates STANDBY operation. The battery pow-

ered inverter comes on-line (in-phase with the failing AC line) as the transfer / isolation

relay switches to prevent AC power from back-feeding to the utility. During the brief

transfer from LINE to STANDBY operation, the energy contained in the ferro-resonant

transformer continues to supply power to the load. The following changes occur: The

transfer / isolation relay K101 opens to disconnect the AC line from the primary winding of

ferroresonant transformer T1. The control logic drives the inverter transistors on and off at

line frequency. This switching action converts the DC battery current into AC in the

inverter winding of the ferroresonant transformer which provides regulated power to the

load. The control logic, which includes a circuit to protect the inverter transistors from

over-current damage, monitors the condition of the batteries during inverter 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 10.5 VDC / battery (31.5 VDC / set).

When AC line voltage returns, the power module transfers back to LINE operation

within 10 to 50 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. It then de-

energizes the transfer / isolation relay, re-connects the AC line to the primary of the

ferroresonant transformer and disconnects the batteries from 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 is then activated to recharge the batteries in preparation for the

next power outage.

1.2.3 Charger Operation

The XP Series Uninterruptible Power Supply uses a dual-mode, temperature-compen-

sated battery charger. During AC line operation, a charger / inverter winding on

ferroresonant transformer T1 feeds the charger circuit which provides "float" and "equalize"

charge voltages to the batteries. The circuit consists of a switching regulator, inductor L1

and other associated components. The charger (inverter) winding of transformer T1

produces an AC voltage that is regulated by SCRs Q301, Q302 and filtered by inductor L1.

This produces a regulated DC battery charging voltage. The charge current passes

through R104 to provide current-limit sensing for the charging circuit. Fuse F301, located

on the removable, Main Circuit Module assembly, protects the circuit in the event of

charger malfunction or reversal of the battery leads (Refer to the component layout drawing

at the back of the manual).

The standard control logic provides a constant (programmable) float charge to the

batteries. A switch (CHARGE MODE) SW201 located on the front panel of the power

module allows a technician to manually activate the charger’s equalize mode which has a

1.2 hour duration. With the optional APM or USM logic upgrade installed, the equalize

charging mode becomes an automatic user-programmable function.

When the XM Series module resumes LINE operation, the charger quickly recharges

the batteries. The charge current is determined by the acceptance level of the batteries,

but limited to 10 Amps maximum. As the batteries approach full charge, the charger’s

current tapers off to normal float levels.

The three color-coded LEDs on the XP front panel "CHARGER STATUS" block display

charging modes. When lighted, the LEDs indicate FLOAT (green), EQUALIZE (yellow) and

RECHARGE (red). Recharge represents a charge rate greater than 5 Amps.

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Alpha XM 6015 Series Operations Instructions Page 12

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