Front cover

Choosing the Right UPS for

your Lenovo ThinkServer
System

Introduces key terminology Reviews major UPS technologies

Provides guidelines for UPS Describes important

selection recommendations for ThinkServer
compatibility

Justin Ringuette

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Abstract

This guide provides an overview of uninterruptible power supply (UPS) technologies and the
information necessary to select a UPS that is compatible with a Lenovo ThinkServer system.
Although the information that is presented in this guide is universal, the recommendations are
for ThinkServer products only and do not apply to Lenovo System x servers because these
servers use different power supplies that might not have the same requirements as those
described in this paper.

At Lenovo Press, we bring together experts to produce technical publications around topics of
importance to you, which provides information and best practices for the use of Lenovo
products and solutions to solve IT challenges.

For more information about our most recent publications, see this website:
http://lenovopress.com

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Power and Watts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Waveform shape and Total Harmonic Distortion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Power Factor Correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
UPS outputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Summary of recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
About the author. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Notices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Trademarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2 Choosing the Right UPS for your Lenovo ThinkServer System

Introduction
An uninterruptible power supply (UPS), also known as a “battery backup”, is designed to
provide continuous power if there is a power loss. It provides continuous power 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 difference gives UPS units a
shorter run time, which often is measured in minutes, but allows for extremely fast failover
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 comes
online.

Regardless of its design, all UPS units have the following basic parts in common:
򐂰 A battery that stores energy for emergency use
򐂰 A charger that charges the battery during normal operation
򐂰 An inverter that converts the battery’s Direct Current (DC) into the Alternating Current
(AC) that is used by its devices

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

  

  




 

  




Figure 1 Example of a Standby UPS in operation

In addition to this basic function, UPS units 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
that is provided by the inverter. The power is measured in Watts (W) and the shape is
measured by Total Harmonic Distortion (THD).

© Copyright Lenovo 2015. All rights reserved. 3

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, which is measured in Volts (V) and current that is flowing
through it, which is measured in Amps (A). For a UPS to power its devices if there is a power
failure, the UPS must have a wattage rating at least as high as the sum of all of the devices
that are connected to it.

For a Lenovo ThinkServer®, its power rating is the maximum power consumption of its power
supply unit (PSU). For servers with redundant power supplies, the maximum power
consumption of one power supply is sufficient as both PSUs do not consume their maximum
power rating at the same time under normal operation. However, when a server with
redundant power supplies is first connected to AC power, both PSUs briefly consume their
maximum power as they charge up.

To ensure the correct long-term operation of equipment, Lenovo recommends that you
choose a UPS with a power output that is 15 - 20% greater than the sum of the maximum
power consumption of all devices that are connected to it, as shown in the following equation:
UPS Power Output (in Watts) >= UPS Maximum Wanted Output + 15-20%

Waveform shape and Total Harmonic Distortion

Describing the shape of an electrical signal can be complicated, but it is normally done by
referring to a graph of the signal’s voltage as it changes over time. This graph is called the
“voltage waveform”. For AC power, the waveform is a pure sine wave that repeats
approximately 60 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 approximately
156 V and 60 Hz.

     


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Figure 2 Example sine waveform

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

4 Choosing the Right UPS for your Lenovo ThinkServer System

 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 can be
as high as 5%.

Related to THD is dead time, which is the amount of time that a signal spends at 0 V and
therefore provides no power. This state contributes to a signal’s THD but is especially
important to electrical devices because the devices rely on continuous power to function. UPS
units 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

Lenovo recommends the following THD configuration:
򐂰 Total Harmonic Distortion (THD): <= 10%
򐂰 Switch-over Time: <= 2 ms
򐂰 Dead Time: <= 2 ms

Power Factor Correction

Inside a Lenovo ThinkServer PSU is a device that is called an active Power Factor Corrector
(active PFC), which is responsible for ensuring that the rapid on-off switching of electronics
inside the server does not affect the power quality for other devices that are on the same
circuit. A PFC ensures good power quality by trying to correct the server’s power factor, which
is a measure of power quality that is 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 might 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 place increased stress on the power supply that can lead to
equipment failure over time.

UPS outputs
Although the Wattage of a UPS is an important characteristic, the shape of the waveform that
is providing that power also is an important consideration for sensitive electronics, such as
servers. This shape is determined by the UPS inverter and comes in the following main
categories (not all of which are well-suited to powering server PSUs):
򐂰 Square wave
򐂰 Simulated sine wave
򐂰 Pure sine wave

These waves have the same frequency and average voltage, but different waveforms and,
therefore, different THDs and dead times. These waves are described next.

5
Square wave
Square waves are the simplest type of wave, with only two voltage levels and sharp, nearly
vertical transitions between them. These waves are easy to produce from a DC battery, but
have extremely high THD and are not suitable for Lenovo ThinkServer systems. Figure 3
shows a square wave.

    


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Figure 3 Square wave

Simulated sine wave

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

       


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Figure 4 Simulated sine wave with dead time between the peaks

6 Choosing the Right UPS for your Lenovo ThinkServer System

Pure sine wave
A pure sine wave output is a true, curved sinusoid rather than a series of steps, as shown in
Figure 5. It accurately simulates the sine wave voltage that is normally provided by a power
company.

     


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Figure 5 Pure sine wave with a lack of sharp edges or sudden amplitude changes

ThinkServer power supplies require a UPS that supplies power with a THD below 10% and
switch-over and dead times below 2 ms. Most pure sine UPS units meet these requirements;
however, you should confirm with your UPS vendor that their offerings meet the requirement.

Summary of recommendations
Lenovo recommends that any UPS that is used with a Lenovo ThinkServer system meets the
following specifications:
򐂰 UPS Power: UPS Maximum Wanted Output + 15-20%
򐂰 Total Harmonic Distortion (THD): <= 10%
򐂰 Dead Time: <= 2 ms
򐂰 Switch-over Time: <= 2 ms

Failure to meet these specifications can result in unwanted behaviors, including unexpected
system shutdowns and PSU failures.

As a result of these guidelines, Lenovo does not recommend the use of square or
simulated-sine wave UPS units.

Because of their waveforms, square and simulated sine wave UPS units rarely have a
sufficiently low TDH. Pure sine wave UPS units are more likely to meet the specifications that
are listed in this section but still must be checked to ensure compatibility.

Note: For more information about UPS compatibility for Lenovo System x®, see this
website:
http://powerquality.eaton.com/ibmups/default.asp

7
About the author
Justin Ringuette is a Lenovo ThinkServer Product Engineer with a focus on power systems.
He has a Bachelor’s degree in Computer Engineering and Electrical Engineering from North
Carolina State University and one year of experience in Lenovo ThinkCentre® Product
Engineering before joining the ThinkServer team in 2013.

Thanks the following people for their assistance in writing this paper:
򐂰 David Watts, Lenovo Press
򐂰 Stanley Liu, Product Delivery
򐂰 ZD Zeng, Power Development

8 Choosing the Right UPS for your Lenovo ThinkServer System

Notices
Lenovo may not offer the products, services, or features discussed in this document in all countries. Consult
your local Lenovo representative for information on the products and services currently available in your area.
Any reference to a Lenovo product, program, or service is not intended to state or imply that only that Lenovo
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Lenovo (United States), Inc.

1009 Think Place - Building One
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Attention: Lenovo Director of Licensing

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Any performance data contained herein was determined in a controlled environment. Therefore, the result
obtained in other operating environments may vary significantly. Some measurements may have been made
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© Copyright Lenovo 2015. All rights reserved.

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Administration (GSA) ADP Schedule Contract 9
This document REDP-5221-00 was created or updated on June 26, 2015.

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10 Choosing the Right UPS for your Lenovo ThinkServer System