As a follow up to our Lighting inverter series, this article will focus on the components of a lighting inverter and how they may differ from a UPS. UPS systems are typically used for critical power applications where both power conditioning and uninterrupted power is required. They typically have shorter run time and rely on batteries to keep the system running in the event of an outage. While there are UPS systems that are UL924 approved, a UPS is typically used in non-lighting applications such as data centers, hospitals and other critical environments, keeping your servers, computers and other equipment operational in the event of a power outage. A lighting inverter is similar to a line-interactive UPS, where the unit passes utility power to support systems until there is an outage, then draws from batteries to provide power during the outage window.
Lighting inverters are typically used for life safety applications and usually have a longer run time requirement than a UPS system. In fact, the UL minimum standard is 90 minutes. They are used to keep the lights on in the event of an outage so occupants can safely egress. Lighting inverters typically do not conditioned power running through them. Rather than a traditional rectifier, there are components that act like a rectifier designed to provide a charging current to the batteries.
Let’s take a closer look at the components in a lighting inverter.
- UPS Module. In normal mode, when utility power is present, the lighting loads are fed from utility, similar to a standby UPS. However, in some situations, the unit may help filter or supplement the incoming power. The UPS module contains an inverter which is an electronic device that changes direct current (DC) to alternating current (AC). There are usually a few other components within the UPS module such an AC distribution module with an input circuit breaker, boost tap transformer, control and monitoring subsystems. These additional components may provide some power conditioning, but are usually not designed for total power conditioning like a true, online UPS system might.
- Battery Module. The battery module contains the battery system required to produce the reserve energy to supply the inverter during abnormal power conditions. In our industry we focus on run time and how long you want your unit to provide power while on battery. For a lighting inverter, there are a variety of battery types that may be used. We’ll discuss batteries in more detail below.
- Battery Charger. The charger converts A/C voltage to D/C current. With utility power present, the battery charger circuit supplies voltage and current to the batteries. For most systems, once the batteries have received a full recharge, a constant trickle charge maintains the batteries at maximum level.
- The inverter converts D/C voltage supplied by the battery to A/C voltage of a precisely stabilized amplitude and frequency that is suitable for powering most lighting loads. Typically, the inverter output voltage is generated by sinusoidal pulse width modulation (PWM). The use of a high carrier frequency for PWM and dedicated A/C filter circuit consisting of a transformer and capacitors, ensure a very low distortion of the output voltage.
- Output Power Transformer. A dry type power transformer provides the inverter A/C output. Transformers are a key component in electrical distribution equipment. Factors to consider would be capacity, voltage rating, insulation system, core & coils and winding insulation system. Not all systems will have an isolation transformer, but if they do, most of the transformers within a UPS or lighting inverter unit are built with copper wiring.
- Display and Controls. The system provides operation monitoring and control, audible alarms and diagnostics. The front mounted control panel includes a display and keypad for user interface. The display is menu driven and allows the operator to easily “watch” the system functions as they occur and check on virtually any aspect of the system’s operation.
- Battery Assembly. While battery configurations can vary significantly in a lighting inverter application, many are a front access, sealed, lead-acid valve regulated battery cell. Batteries are interconnected via buss bars and cables. Similar to a UPS system, temperature has a direct effect on the life of a battery. There are two main battery types that may be found in your lighting inverter:
- VRLA battery. Maintenance free battery that requires no addition of water over the life of the battery. The battery cells are housed in protective molded cases. Life expectancy can vary, but is typically 4 – 7 years.
- Wet cell or nickel cadmium battery. Cell containers are made of translucent, heavy duty polypropylene, making it easier to check the electrolyte level. Life expectancy is designed for 15-20 years at 77-degree F.
There are other accessories to consider, such as a remote meter panel, which will allow greater flexibility to monitor all the system parameters from a remote location. The user can control and program the inverter from a remote location. This might allow a remote user or monitoring system to detect issues without needing to be onsite. Lastly, an external maintenance bypass switch, which permits maintenance personnel to easily bypass the protected equipment directly to the A/C utility power. The make before break switch isolates the system to perform routine maintenance or servicing. These come in handy when you need service on your unit, and can save you money not only by reducing the risk of losing your loads, but also in maintenance costs. Service can be completed during normal business hours rather than evenings or weekends which typically costs more.
These are the main components in a lighting inverter and common accessories that may be included.
For more information on emergency lighting inverters, or to schedule a maintenance inspection on your own, contact Quality Power Solutions today!