Do you know which decisions are critical to UPS performance and reliability?

Applying an uninterruptible power supply (UPS) without considering system issues like output waveform, input voltage regulation, generator sizing, transition with residual voltage, backup generators serving a lightly loaded UPS, and maintenance, can result in poor performance and reduced power quality at your facility.

Many lower-end UPS units are nothing more than standby power systems (SPS), which pass utility power through to the load under normal conditions. When a power outage occurs, the SPS internally switches to its backup power. Because the power from an SPS is interruptible, it isn't a true UPS. However, an SPS is useful for noncritical loads or those that can tolerate short interruptions. Before specifying an SPS for PLC or computer loads, make sure the unit you're considering has a switching time shorter than the ride-through capability of the loads it will serve.

Output waveform.

Because the UPS inverts its internal DC bus voltage to AC, it won't produce a perfect sine wave. Most equipment can handle this distorted voltage without problems, but you will have efficiency losses and additional heating of the load. Look for a UPS with at least a quasi sine wave output. You should test the output under load with a handheld scope or other wave analysis instrument. Regardless of equipment specification, measure the actual output voltage distortion under normal load conditions to determine if your equipment can sustain the UPS output harmonics.

Input voltage.

Input voltage is a system compatibility issue most people overlook. Don't make this mistake. If the normal input voltage is lower than what the UPS expects, it will recognize this normal voltage as an undervoltage and switch to batteries — which is its backup power source — and deplete them when it should be running on utility power. When the batteries run dry, the unit will switch back to utility power, but here you run the risk of a switching transient. This transient may cause an overvoltage, dropping a critical load in the process. Even if that doesn't happen, those depleted batteries won't support the load when an interruption occurs.

How do you avoid this situation? You could apply a power distribution unit with an integral transformer to boost the voltage, or choose a multi-range UPS. Commercially available multi-range UPSs often have an auto-transformer on the input to properly center the acceptable input voltage range on the nominal voltage.

Generator sizing.

Facilities often support critical loads with a UPS that supplies power during outages or sags. Designers often use a generator and an automatic transfer switch (ATS) to complement the UPS for longer interruptions. Upon sensing a utility outage, the ATS sends a start signal to the generator and then switches from utility to generator once the generator has reached proper voltage and frequency.

However, in addition to providing reliable power, the UPS input produces harmonics that affect the generator. Likewise, the generator allows deeper voltage drops and frequency variations when large motors start, which may affect the UPS. To ensure system compatibility, specify a generator de-rated to serve harmonic-producing loads like a UPS. The amount of de-rating depends on the specifics of the UPS you are considering and the load it will supply.

Motor starts may affect the input voltage to the UPS. When a motor starts under generator power, it may affect the input voltage to the UPS by dragging down the voltage and momentarily slowing all but the largest generators. This causes a frequency variation. The rate of change of frequency is the slew rate, measured in Hz/sec. The UPS monitors the slew rate of the input voltage. The UPS continues to operate using this power source, converting the AC power to DC and back to a clean or conditioned AC waveform. However, if the UPS senses a slew rate problem, or any other problem with the input power, it won't switch to static bypass in the event of a failure in the DC path. Instead, it will dump the load.

Changing the allowable slew rate setting to accept the greater frequency shifts will reduce the likelihood of a problem. However, adjusting the slew rate tolerance might adversely affect the UPS loads. One way to minimize slew rate problems during motor starting is to purchase an oversized generator. Another possible solution is to use soft-starts or motor drives to prevent across-the-line starting, and use control logic to stagger any motor starts.

Transition with residual voltage.

Additional problems can arise when a UPS synchronizes to out-of-phase motor residual utility voltages. A DC bus overvoltage may occur, resulting in a frequency error. It isn't the residual voltage alone that causes the problems, but the presence of the residual voltage when your system applies power from another source.

You can address this concern in one of several ways. For example, you can use load-shedding relays to shut off motor loads prior to a transfer of power in either direction. Installing an optional synchronization check relay or in-phase transfer feature in the transfer switch can also solve the frequency error problem. This relay ensures that when two sources are available at the transfer switch, the actual transfer doesn't occur until the two sources are in phase.

Generator serving a lightly loaded UPS.

A generator may not properly regulate its output voltage when serving a lightly loaded UPS with a high power factor (unity or possibly slightly leading). When there is an input filter on the UPS and little other loading on the generator, the generator may oscillate excessively and cause the UPS to cycle because the unit finds the generator power oscillations to be unacceptable. This happens because the excitation current in a generator is very low during periods of light, capacitive load. That low current leads to a lower synchronizing torque and allows greater generator variation, thus oscillation.

Many newer UPSs use input power electronics to minimize this problem. They don't require input filters, so you can apply them without concern for causing excessive capacitive load. If you use an input filter, switch it off during low load conditions. For example, automatically disconnect the input filters — the source of capacitance — at loads below 30% of full UPS load.


If a load is important enough to require a UPS, it's important enough to allow for maintenance without interrupting it. Large UPSs typically have external and internal maintenance bypass switches. Many smaller UPSs are of modular construction, so critical loads can use redundant modules or utility power while you maintain or replace modules.

Economical UPS application isn't a matter of buying the most battery time for the least amount of money — it's a matter of getting the right performance for the time and money spent. When specifying a UPS, begin by identifying the critical loads. Then, look at how much UPS those loads will require. Once you've determined the size of your UPS, you can narrow down your search to those products that satisfy the requirements for compatibility with your system. Once you've installed your UPS to protect your loads, be sure to do the proper maintenance to protect your investment.

Blooming is a senior project engineer with Eaton/Cutler-Hammer, Minneapolis.