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What is drive system efficiency and
why is it important?
Problem description
Drive system efficiency is the ratio of output power
produced by the motor shaft to the input power of the
drive system. A drive system includes a motor and a
Variable Frequency Drive (VFD)*, but in addition it can
also include an input filter, an external transformer,
an output filter, and/or an especially long cable. To
understand drive system efficiency, the power losses of
individual components within the drive system must be
independently understood.
Power losses from motors
The power losses from the motor are generated by
electromagnetic properties and the material properties of
the motor. In general, the power losses from the motor can
be classified as the following different types of losses:
1. Stator Losses – Heating (I2R) losses are generated
within stator winding proportional to its electrical
resistance and square of load current.
2. Rotor Losses – Heating (I2R) losses are generated within
rotor squirrel cage proportional to its electrical
resistance and square of load current.
3. Core Losses – Iron magnetization losses are generated
by the motor core as a function of frequency and
induced eletromagnetic field strengh of the stator
winding.
4. Stray Losses – High frequency harmonic losses are
proportional to electromagnetic flux leakage occurring
in motor air gap and winding fields.
5. Windage and Friction Losses – Mechanical losses are
generated by rotor bearing frictions and shaft-mounted
fan airflow power consumption.
*Variable Frequency Drive (VFD) maybe used interchangeably with
Adjustable Frequency Drive (AFD).
Answers for industry.
Power losses from VFDs
The power losses from VFDs can be generated by power
electronic devices (converters) that convert the input voltage
and current to the desired variable output voltage current.
Different topology of these devices in the VFD produces
different properties, including output harmonic distortions.
Converter Losses – Conduction loss in the switching devices,
which is proportional to the current, and switching losses,
which are proportional to the current and switching frequency,
are generated by the circuit components of VFD output power
converter.
Some VFDs also have an integrated transformer in the design.
This transformer contributes to the overall power loss of the
VFD.
Drive system efficiency
The power losses of the individual components directly affect
drive system efficiency. In general, efficiency is defined as the
ratio of output power to input power of a system. In a lossless
system, this ratio would equal 1.00 or 100%. However, in the
real world, losses are ever-present. Input power can be seen as
the output power plus the power losses of a system, which can
also be shown as the product of the efficiencies of each
component in the system, as shown in Figure 1.
Because of the complexity of the drive system, evaluating drive
system efficiency can be complicated. Consideration must be
taken for the effects of the combination of the system
components efficiencies on the entire system to determine the
most optimized system for efficiency.
Transformer losses – Core losses similar to those of the motor
stator, eddy current losses, which are proportional to current
and frequency squared, and heating losses (I2R) are generated
by the winding and iron core of VFD input power transformer.
It is important to note that although some VFDs can operate
without a transformer, other VFDs may need to operate with
an external nonintegrated transformer. However, when
advertising the efficiency of their VFDs, the manufacturer may
omit the transformer power losses because the transformer is
external to the VFD. Therefore, although the VFD that requires
the external transformer is reported as having a higher
efficiency, the overall system efficiency may be lower when the
external transformer efficiency is considered with the VFD.
Losses from input/output filters
Depending on the VFD design, the drive system may require an
input filter to satisfy certain industry standard requirements,
such as IEEE-519. Depending on the topology of the VFD, the
drive system may need an output filter after the VFD to reduce
output harmonics to protect standard motors or to satisfy
customer requirements. A reduction in output harmonics may
also decrease the stray losses of the motor, thereby making the
motor more efficient.
There are losses that are introduced by the addition of an input
and/or output filter. Depending on the types of filters used, the
causes for their losses are different. For example, passive filters
may have heating and eddy current losses similar to
transformer losses, while active filters may have electronic
device losses similar to the converter losses of the VFD. When
these filters are used, their losses must be taken into
consideration when considering the efficiency of the drive
system.
Losses from long cable
In some applications, the VFD is located far away from the
motor because of specific application requirements or the
harsh environment in which the motor is operating may
damage the VFD. Therefore, a long cable may be used to
connect the motor to the VFD. The losses in the long cable are
mainly caused by heating losses (I2R), and these losses are
proportional to the resistance of the cable, which is
proportional to the length of the cable.
2
Figure 1 Drive system efficiency
Problem example
In a real-world based hypothetical scenario, an end-user
purchases a motor and a VFD from two different
manufacturers. The 5000 hp, 3600 rpm, 4000 volt motor
outputs 3730 kilowatts of power (Pout) to drive its load. Under
service factor load, the motor dissipates 190 kilowatts of heat
(Plosses). Based on the equation from Figure 1, the input power
(Pin) must be 3920 kilowatts which yields a rated efficiency of
0.95 or 95%.
The VFD is a medium voltage drive that must output 3920
kilowatts of power (Pout) to drive the 5000 hp motor. At rated
frequency and current the VFD dissipates 101 kilowatts of heat
(Plosses). Based on the equation from Figure 3, the input power
(Pin) must be 4000 kilowatts which yields a rated efficiency of
0.97.5 or 97.5%. So, when the motor and VFD are operating
together one would expect for the system efficiency to be 0.93
or 93% given that the output power (Pout) is 3730 kilowatts
and the input power (Pin) is 4000 kilowatts.
However, the system efficiency under rated operation was
reported to be 0.90 or 90%. Where did the additional losses
come from? Because the motor and VFD were not necessarily
designed for optimal operation with one another, there were
additional core and stray losses induced in the motor from a
large amount of harmonic distortion at the output of the VFD
converter. Also, such reporting can vary with data coming from
different industry equipment. Furthermore, the harmonic
distortions may cause a voltage surge that would damage the
insulations of the motor. In the addition, the input of the
system does not satisfy IEEE-519 requirements. Although, the
motor and VFD were able to operate together, the drive system
efficiency deviations would incur additional energy costs over
the life of the drive system, and the system cannot be install in
all facilities that requires the system to satisfy IEEE-519. Since
most of the losses show up as heat, additional cooling for
some of the system components may be required.
Problem solution
Resolving this issue requires a sound technical understanding
of the entire drive system. This can be an overwhelming task
for even the most knowledgeable engineer. One method to
address this issue would be to install a supplemental harmonic
filter in the VFD cabinet if there is space available to filter the
output harmonic distortions to the motor. If this is not feasible,
a suitable after-market harmonic filtering system can be
retrofit next to the VFD cabinet with special interfacing.
Furthermore, an input filter will need to be installed to satisfy
the IEEE-519 requirements.
However, adding these two components into the drive system
affects the overall system efficiency. The typical efficiency of
input and output filters is 0.995 or 99.5%. Assuming to the
output filter filters sufficient harmonic distortions so that the
motor is at nominal efficiency, the system efficiency can be
calculated using the equation in Figure 2 as follows:
ηSystem 1=ηInput × ηVFD × ηOutput × ηMotor
Filter
Filter
= 0.995 × 0.975 × 0.995 × 0.95 = 0.917
= 91.7%
Integrated Drive System optimized solution
The greatest advantage of Siemens Integrated Drive System is
the knowledge, the expertise, and the ability to evaluate the
efficiency of the entire drive system and optimize the system
efficiency through design. In the problem example, the
Integrated Drive System solution would be to use a SINAMICS
GH180 VFD with an VFD efficiency of 97%. At first glance, it
would appear that SINAMICS GH180 has a lower efficiency
than the VFD in the Problem Example, and therefore the
efficiency of the Integrated Drive System is lower than the
Non-integrated Drive System in Problem Solution. However,
the SINAMICS GH180 is designed such that it does not require
an input filter to satisfy IEEE-519, and the output voltage can
be operated with a standard motor without an output filter.
Therefore, using the equation in Figure 2, the efficiency of the
IDS system is calculated as follows:
ηSystem IDS = ηVFD × ηMotor
= 0.97 × 0.95 = 0.9215
= 92.15%
Efficiency Gain = ηSystem IDS – ηSystem 1
= 0.45%
The Integrated Drive System is about 0.45% more efficient
than the Non-integrated Drive System composed of the input
and output filters. We can calculate the costs saved by this
efficiency savings with assumptions stated in scenarios shown
in Figures 2 and 3.
Drive system operating half of the time under continuous
duty at 91.7% efficiency with an average energy cost of
$0.09 per kWh would cost approximately $1,604,551
per year.
3730 kW
0.917
•
4383 hrs.
•
yr.
$0.09
kWh
=
$1,604,551
yr.
Figure 2 Yearly cost at 91.7% efficiency
Drive system operating half of the time under continuous
duty at 92.5% efficiency with an average energy cost of
$0.09 per kWh would cost approximately $1,596,715
per year.
3730 kW
0.925
•
4383 hrs.
•
yr.
$0.09
kWh
=
$1,596,715
yr.
Figure 3 Yearly cost at 92.15% efficiency
This comparison yields an average savings of $7,836 per year
for the life of the system. Furthermore, greater efficiency can
be obtained from the Integrated Drive System through design
by either oversizing the transformer or designing the motorVFD system such that it uses the least amount of current to
produce the required power to the customer. However, these
methods may increase the initial capital cost of the system
(increase in transformer cabinet size, increase in number of
cells, etc.). Therefore, the tradeoffs between initial capital
cost and the returns from efficiency savings will need to be
evaluated before design changes are implemented.
Besides the efficiency advantage, the Integrated Drive System
also provides the following benefits to the customer versus
Non-integrated Drive System:
1.Smaller foot-print, which can translate to construction cost
savings for the customer.
2.Reduction in complexity, which mitigates risks that are
caused as a result of a complex drive system.
Through these benefits, the customer obtains the most
production out of their investment in the drive system.
Siemens, in turn, gains the business of the customer. Siemens
Integrated Drive System is truly a win-win situation for both
the customer and Siemens.
3
Get the most production out
of your drive system
By choosing a Siemens Integrated
Drive System, you get the knowledge,
the expertise, and the ability to
evaluate the efficiency of the entire
drive system and optimize the efficieny
through design.
Siemens Industry, Inc.
3333 Old Milton Parkway
Alpharetta, GA 30005
1-800-241-4453
[email protected]
usa.siemens.com/ids
Results:
•
•
•
•
•
•
Increased availability
Higher revenue
Reduction in complexity
Lower risk
Increased efficiency and enhanced reliability
Lower total cost of ownership
Subject to change without prior notice
Order No.: DTAN-00020-0814
Printed in USA
© 2014 Siemens Industry, Inc.
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