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The purpose of this guide is to provide an overview of UPS technologies and the information necessary to select a UPS compatible with a Lenovo ThinkServer system.  While the information presented in this guide is universal, the recommendations are only for ThinkServer products and do not apply to Lenovo System x servers as these servers use different power supplies which may not have the same requirements as those described here.  UPS compatibility information for Lenovo System x can be found at the following address:



  • UPS = Uninterruptable Power Supply
  • PSU = Power Supply Unit
  • AC = Alternating Current
  • DC = Direct Current
  • THD = Total Harmonic Distortion
  • PFC = Power Factor Corrector


Uninterruptable Power Supplies


What are they?

An uninterruptable power supply (UPS), also known as a “battery backup”, is designed to provide continuous power in the event of power loss.  It does this by charging an internal battery during normal operation and then discharging the battery when a power failure occurs to keep connected devices running.  A UPS is similar to a generator in that both provide emergency power except that a UPS discharges stored electrical energy while a generator produces power from other sources, such as fuel.  This gives UPSes a shorter run-time, normally measured in minutes, but allows for extremely fast fail-over without an interruption in service.  For critical applications a UPS and generator are often used together with the UPS providing seamless backup power until the generator has come online.

Regardless of its design, all UPSes have a few basic parts in common:

  • batterythat stores energy for emergency use
  • A chargerthat charges the battery during normal operation
  • An inverterthat converts the battery’s Direct Current (DC) into the Alternating Current (AC) used by its devices.

Figure 1 shows the two operating modes of a UPS and the major components in the UPS.


UPS Standby.png

 Figure 1: An example diagram of a Standby UPS in operation. 


In addition to this basic function, UPSes can have other features such as overvoltage protection or faster switching to battery power.  Most important for compatibility with Lenovo servers and other complex digital electronics are the power and shape of the AC electricity provided by the inverter.  The power is measured in Watts (W) and the shape is measured by Total Harmonic Distortion (THD).


Power and Watts

Watts (W) are the basic unit of power consumption for electronics and they are defined as the product of voltage across a device (measured in Volts [V]) and current flowing through it (measured in Amps [A]).  In order for a UPS to be able to power its devices in the event of a power failure, the UPS must have a wattage rating at least as high as the sum of all the devices connected to it.  For a Lenovo ThinkServer, its power rating is the maximum power consumption of its power supply unit (PSU).  For redundant-enabled servers, the maximum power consumption of one power supply is sufficient as both PSUs will not consume their maximum power rating at the same time under normal operation.  However when a redundant-enabled server is first connected to AC power both PSUs will briefly consume their maximum power as they charge up.


In order to ensure the correct operation of equipment long-term, it is recommended to choose a UPS with a power output that is 15-20% greater than the sum of the maximum power consumption of all devices which will be connected to it.


Lenovo power recommendation:

The power capacity that Lenovo recommends that you select for your UPS is 15-20% more than the rated maximum that the UPS unit offers.UPS Power Output (in Watts) >= UPS Maximum Desired Output + 15-20%


Waveform Shape and Total Harmonic Distortion

Describing the shape of an electrical signal can be complicated but it’s normally done by referring to a graph of the signal’s voltage as it changes over time.  This graph is called the “voltage waveform” and for AC power, the waveform is a pure sine wave that repeats about sixty times every second (the exact frequency varies between countries).  This repetition is called the signal’s frequency and is measured in Hertz (Hz).


Figure 2 shows an example waveform with a peak voltage and frequency of about 156V and 60Hz.



 Figure 2: An example sinewave


The waveform of the AC signal provided by a utility company closely approximates a pure sine wave but some signals – including those from certain designs of UPS – can look very different and only match an ideal sine wave in frequency and average voltage.  This is enough for simple electronics like light bulbs but it can be a problem for more sensitive devices, including servers.


Total Harmonic Distortion (THD) provides a way to measure how different a signal’s shape is from the sine wave ideal.  It expresses this difference as a percentage with a higher percentage representing that the signal is more distorted.  An ideal sine wave has a THD of 0% and normal utility power may be as high as 5%.


Related to THD is dead time which is the amount of time that a signal spends at 0V and therefore provides no power.  This contributes to a signal’s THD but is especially important to electrical devices since they rely on continuous power to function.  UPSs also have a switch-over time, which is the dead time that occurs between a loss of power and the UPS’ inverter turning on.


Lenovo THD recommendation:

  • Total Harmonic Distortion (THD) <= 10%
  • Switch-over Time <= 2ms
  • Dead Time <= 2ms

Power Factor Correction

Inside a Lenovo ThinkServer PSU is a device called an active Power Factor Corrector (active PFC) which is responsible for ensuring that the rapid on-off switching of electronics inside the server doesn’t affect the power quality for other devices on the same circuit.  A PFC ensures good power quality by trying to correct the server’s power factor – a measure of power quality used for loads instead of sources.  Active PFCs are a common component in server power supplies and computer power supplies in general but they function poorly when supplied by power that has a high THD.


PSUs with active PFCs installed may shut off or draw excessive current when supplied from these sources.  This excessive current draw is within the safe operating range of Lenovo ThinkServer systems but will place increased stress on the power supply, potentially leading to equipment failure over time.


UPS Outputs

While the Wattage of a UPS is an important characteristic, the shape of the waveform providing that power is also an important consideration for sensitive electronics like servers.  This shape is determined by the UPS inverter and comes in three main categories, not all of which are well-suited to powering server PSUs.  These categories are square wave, simulated sine wave, and pure sine wave.  All of them have the same frequency and average voltage but different waveforms and therefore different THDs and dead times.


Square Wave

Square waves are the simplest, with only two voltage levels and sharp, nearly vertical transitions between them.  These are easy to produce from a DC battery but have extremely high THD, and so are not suitable for Lenovo ThinkServer systems.  Figure 3 shows a square wave.



Figure 3: A square wave


Simulated Sine Wave

Simulated sine waves also change suddenly from one voltage to another but unlike true square waves they use three or more voltage levels to produce a smoother shape.  These are also called ‘stepped square’, ‘stepped sine’, or ‘modified sine’ waves.  They have less THD than square waves but are still rarely less than 10%.  Simulated sine waves may also have long periods of dead time.


Simulated sine wave.jpg

Figure 4: The voltage output of a Simulated Sine wave UPS, note the dead time between the peaks


Pure Sine Wave

A pure sine wave output is a true, curved sinusoid rather than a series of steps and accurately simulates the sine wave voltage normally provided by a power company. 


Most pure sine UPSes have a THD below 10% and switch-over and dead times below 2ms, making them suitable for ThinkServer power supplies.  However not all pure sine wave UPSes are guaranteed to meet these requirements so it is recommended to confirm them with the UPS vendor before using one with a Lenovo ThinkServer system.



Figure 5: A pure sine wave.  Note the lack of sharp edges or sudden amplitude changes.


Summary of recommendations

Lenovo recommends that any UPS used with a Lenovo ThinkServer system meets the following specifications:

  • UPS Power = UPS Maximum Desired Output + 15-20%
  • Total Harmonic Distortion (THD) <= 10%
  • Dead Time <= 2ms
  • Switch-over Time <= 2ms

Failure to meet these specifications may result in undesired behaviors including unexpected system shutdowns and PSU failures. 


As a consequence of these guidelines, Lenovo does not recommend using square or simulated-sine wave UPS units. 


Due to their waveforms, square and simulated sine wave UPSs rarely have a sufficiently low TDH.  Pure sine wave UPSes are more likely to meet the above specifications but should still be checked to ensure compatibility.

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