usa.siemens.com/ids
Integrated Drive Systems
Siemens Integrated Drive Systems optimize system cost when VFD
current derating is required due to operational requirements
How do I derate a VFD when I need to operate in
unusual service conditions such as high ambient or
altitude, high or low frequency or current overload?
Problem description
When sizing a VFD to operate a motor, one of the
important parameters to consider is the operating current
that the motor demands from the VFD. In a SINAMICS
PERFECT HARMONY GH180 VFD, the power cells are
arranged in series per phase, see Figure 1.
Figure 1 Arrangement of Cells in a Nine Cell VFD
*Variable Frequency Drive (VFD) may be used interchangeably with
adjustable frequency drive (AFD).
For usual service environments, the maximum VFD output
current is determined by the maximum cell output current
and transformer kVA rating. With properly sized transformer,
the VFD can achieve output current ratings equal to the cell
maximum continuous rating. In fact, the three different
product lines of GH180 PERFECT HARMONY VFDs are
determined by their cell output current ranges, as seen in
Table 1.
Answers for industry.
Table 1 Power cell rated maximum continuous
output currents
Types of SINAMICS GH180 VFD Product Lines Cell Amps
40 A
70 A
6SR4
100 A
140 A
200 A
Air-cooled VFD
260 A
315 A
375 A
6SR3
Derating current due to high ambient temperature
(air-cooled VFD only)
When the VFD is operating in an ambient temperature greater
than 40 °C but less than 50 °C, the cell output current must be
derated to prevent overheating of the cell IGBTs. The SINAMICS
PERFECT HARMONY GH180 VFD may not be able to operate in
ambient conditions over 50 °C ambient because the control
boards may be damaged or malfunction at high temperature.
Please consult the factory for any operating conditions above
50 °C ambient temperature.
The required motor current is denoted as IMOTOR. If the ambient
temperature is within usual service conditions, then
IREQUIRED 1 = IMOTOR , otherwise:
500 A
660 A
IREQUIRED 1 =
720 A
The usual service environment is the normal environment in
which the VFD is operating. This environment is defined as
having ALL of the following conditions in addition to other
conditions1:
1) The ambient temperature in which the VFD is operating
does not exceed 40 °C (104 °F).
2) The altitude (measured in feet above sea level, or FASL)
at which the VFD is operating does not exceed 3300 ft
(1000m).
3) The VFD operates with continuous output current with
frequency between 10 Hz and 167 Hz.
4) The overload requirement of the application is 110% of the
rated cell current for 1 min / 10 min (1 minute of overload
current for every 10 minutes of nominal operation).
If the VFD is operating in a usual service condition, then the
current values in Table 1 define the maximum continuous
current output for the different cell types in each of the GH180
product configurations. However, if the VFD is operating in an
unusual service environment in which one of the above
conditions is not satisfied, then the maximum output current
of the VFD may need to be derated in order to keep the
temperature of the junction connections inside the IGBTs of
the cells below the manufacturer’s maximum rating. The
derating of the current may result in using a cell with a higher
continuous current cell, which could change the required VFD
product line (see Table 1).
The derating of the cell current can be due to the following
four reasons:
1) High ambient temperature
2) High altitude
3) Frequency considerations
4) High overload requirements.
Note that if a VFD is operating inside a control air-conditioned
house, the ambient temperature is controlled due to the HVAC
system, and temperature derating of the cell current is
normally not necessary.
2
ambient
880 A
1250 A
[(60 – Temperature
20
]
(°C)
0.67
This derating method only applies to the air-cooled VFD. Note
that 720A cells cannot operate above 40 °C ambient temperature.
Therefore, if a VFD is operating at an ambient temperature
greater than 40 °C , and the application requires current greater
than 660A, a water-cooled VFD may need to be used.
For a water-cooled VFD, standard design allows for the cooling
limits are defined by a combination of maximum ambient
air temperature and the coolant inlet temperature. Above
this limit, the cells may not be able to operate because the
control boards may be damaged in this condition, and the
factory should be consulted. As the maximum VFD ambient
temperature increases from 40 °C to 50 °C, the maximum VFD
coolant inlet temperature decreases from 47 °C to 42 °C (See
Figure 2). This relationship between maximum air temperature
and maximum inlet coolant temperature is due to residual
heat removal from the cell cabinet through air-to-liquid heat
exchangers installed on top of the cell cabinet. Please consult
the factory if the operating temperatures are in the conditions
when control boards may fail.
VFD Ambient Temperature (°C)
Water-cooled VFD
IMOTOR
VFD Coolant Inlet Temperature (°C)
Figure 2 Temperature Conditions for Power Cells in a
Water-cooled VFD
Derating current due to high altitude
When the VFD is operating in an altitude higher than 3300 ft
(1000 m), the lower density causes a reduction in heat transfer
capabilities from the surrounding air. This reduction causes the
connections in the cell IGBTs to be more easily overheated, and
therefore the output current must be derated. Furthermore,
voltage strike and creep distances are reduced at higher
altitudes, which may impose an upper limit on the operational
altitude of the power cells.
Continuous Cell Current with Overload Requirements
Some applications may require intermittent overload conditions.
Depending on the magnitude of the overload, cell currents may
need to be derated to stay within the temperature tolerance of
the manufacturer. Usually, the overload condition is defined as
the percentage overload for 1 minute of overload duration per
10 minutes of normal operation (1 min/10 min). Table 2 shows
the cell continuous current derates when different overload
conditions are required by the application.
If the altitude is within usual service conditions less than or
equal to 3300 feet (1000 meters), then IREQUIRED 2 = IREQUIRED 1.
Note that these derates assume ambient temperature < 40 °C
and altitude of below 3300 ft. Therefore, the calculated current
IREQUIRED 3 should be used when selecting the appropriate cell.
If the altitude is above 3300 feet (1000 meters) but below
13,123 feet (4000 meters) for air-cooled 6SR3 VFD or below
10,000 feet (3048 meters) for air-cooled 6SR4 VFD and watercooled VFD, then
Table 2 Cell current derates for different overload
conditions and classes (CL)
Cell Continuous Current
No Overload
Required
110% of
nameplate
1min/10 min
(CL-1)
150% of
nameplate
1min/10min
(CL-2)
40 A
40 A
40 A
70 A
70 A
70 A
100 A
100 A
100 A
140 A
140 A
140 A
If the operating conditions are beyond the altitude current
derating limits, then the factory must be consulted.
200 A
200 A
200 A
260 A
260 A
260 A
Derating current due to operating frequency
To adequately operate the motor, the operational output
frequency (FOUT) of the VFD must match the operating frequency
of the motor. Note that for applications above 330 Hz, the
factory should be consulted.
315 A
315 A
300 A
375 A
375 A
300 A
500 A
500 A
400 A
660 A
660 A
450 A
720 A*
N/A
N/A
880 A
880 A
660 A
1250 A
1250 A
950 A
IREQUIRED 1
(Altitude (ft) -3300)×0.5)
116700
IREQUIRED 2 =
[
]
IREQUIRED 1
(Altitude (m) -1006)×0.5)
15090
IREQUIRED 2 =
[
]
If the required operating frequency is within usual service
conditions between 10 Hz and 167 Hz, then IREQUIRED 3 = IREQUIRED 2,
but if the continuous (i.e. > 1 minute) output frequency FOUT is
between 0.5 Hz and 10 Hz, the new required cell output current
IREQUIRED 3 can be calculated as follows:
IREQUIRED 3 =
IREQUIRED 2
[ 0.5 + (F20 )]
Air-cooled
6SR4 VFD
Air-cooled
6SR3 VFD
Water-cooled
VFD
*720 A cells does not have overload capability
For all other overload conditions not defined here, the overload
current is used as the continuous current (No Overload Required
column in Table 2) to select the appropriate power cell.
OUT
The relationship between the carrier frequency and the VFD
output frequency is typically defined as FC ≥ 3.6 × FOUT, where
FC is the carrier frequency of a single power cell. If output
frequency FOUT is between 167 Hz and 330 Hz, then the carrier
frequency is used to calculate the current derate due to high
switching frequency. Note that for the 720A cell, the maximum
carrier frequency is 400 Hz, which may impact performance
above approximately 111 Hz. The new required VFD cell output
current is calculated as follows:
IREQUIRED 3 =
IREQUIRED 2
600) x 0.2
[1- (F - 600
]
c
Determining Cell Currents After Derates
The method to determine the cell size and current capacity
required for a particular motor current (IMOTOR) after derating
due to the different service conditions are as follows:
1) Calculate the current required by the motor after derating
due to the following conditions if applicable:
a. High Ambient Temperature
b. High Altitude
c. Unusual Operating Frequency
2) Using the calculated derated current, select the appropriate
power cell after considering overload requirements from
Table 2.
3
Siemens Integrated Drive Systems can mitigate this increase
in system cost when the VFD output current must be derated
due to environmental conditions such as ambient temperature,
altitude, frequency, and/or overload requirements. Because
the system is integrated within Siemens products, the motor
can be designed around the VFD.
The motor can be designed to lower the required motor
current and raising the required voltage to satisfy the power
requirement from the customer. This design change may not
have a significant effect on the cost of the motor, since this
change typically does not increase the shaft height, which is
a primary cost lever for the motor.
In our previous case study, if the customer were to purchase an
integrated drive system, the motor can be designed to require
140A and 6.1kV because
258A ×3300V ≈ 6.1V
140A
Case study
In this case study, a customer wanted a SINAMICS GH180 VFD
that can deliver an output current of 258 A to the motor with
3.3 kV. Normally, with this current requirement, we would have
offered the customer an air-cooled 6SR4 nine cell VFD.
However, an unusual service condition was requested where
the VFD must be rated for 170% Overload for 10 seconds in
10 minutes and 150% Overload for 30 seconds in 10 minutes.
Because of the unusual overload conditions, the cell current
must be derated. In this case, the cell is derated to the worst
case scenario, which was the 170% Overload. At 170%
Overload, the VFD must output about 439A. With this derating,
the smallest power cell that can provide the required current is
a 500A cell. The proposal was revised to offer the customer an
air-cooled 6SR3 nine cell VFD with 500A cells.
Integrated Drive System Optimized Solution
General speaking, an air-cooled 6SR3 VFD can be about 20-40%
more costly than an air-cooled 6SR4 VFD, while a water-cooled
VFD can be about 5 times as costly as an air-cooled 6SR4 VFD.
Therefore, from a system cost perspective, it is a significant
system cost reduction if a smaller size VFD can be used for an
application.
For a system that is not integrated and is not optimized for cost,
derating the output current may result in significant system cost
increase due to the change in VFD sizes and product lines.
Siemens Industry, Inc.
3333 Old Milton Parkway
Alpharetta, GA 30005
1-800-241-4453
info.us@siemens.com
usa.siemens.com/ids
140A ×1.7 = 238A
Because a 260A cell is available in the 6SR4 product line, we can
then offer the customer an air-cooled 6SR4 nine cell VFD along
with this motor compared to the original 6SR3 nine cell VFD
with 500A cells.
By providing an integrated drive system, Siemens provides
several benefits for this specific customer:
• The customer receives the same power output at lower cost
compared to if they had only bought the air-cooled 6SR3 nine
cell VFD.
• The customer is receiving a brand new motor to replace their
older existing motor, which will greatly extend the life of their
system.
• The air-cooled 6SR4 nine cell VFD has a 51% smaller footprint
than the air-cooled 6SR3 nine cell VFD. This makes installation
considerably easier, faster, and less expensive.
For Siemens, new business is generated for our factory, and
Siemens motors have gained a foothold with this specific
customer. Siemens Integrated Drive Systems is truly a win-win
situation for both the customer and Siemens.
To learn more about Integrated Drive Systems, visit
www.usa.siemens.com/ids.
Subject to change without prior notice
Order No.: DTAN-00031-1014
Printed in USA
© 2014 Siemens Industry, Inc.
The information provided in this flyer contains merely
general descriptions or characteristics of performance
which in case of actual use do not always apply as described
or which may change as a result of further development
of the products. An obligation to provide the respective
characteristics shall only exist if expressly agreed in the
terms of contract.
All product designations may be trademarks or product
names of Siemens AG or supplier companies whose use by
third parties for their own purposes could violate the rights
of the owners.