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Integrated Drive Systems (IDS)
High Voltage Stresses
Siemens IDS ensure voltage stress compatibility between motor and drive
Eliminate voltage stress risks with motors and drives
optimized in Integrated Drive System
Because modern semiconductor devices can switch on and
off quickly (within microseconds), a voltage spike seen at
the motor terminals can have an extremely fast rise time to
an abnormally high peak voltage (referred to as a “wave
front” or “dv/dt”). This is much higher in magnitude, dv/dt
and frequency, versus what a motor would normally see on
a sine wave. As a result, this abnormally high peak may
overly stress a motor’s insulation and shorten its lifespan.
An example of a voltage spike from a variable frequency
drive (VFD) that could be seen at the motor terminals is
shown in Figure 1. This waveform does not have a smooth
transition from 0 voltage (U) to the peak voltage. Instead, it
has a rapid rise (dv/dt) and a high peak over shoot at the
beginning of the pulse. This high dv/dt directly affects the
stresses between the copper strands in the individual coils,
commonly referred to as turns as shown in Figure 2.
U
U
U1pk/pk
Figure 1 Five Step Phase to Phase Voltage at the Terminals
of a Machine Fed by a 3-Level Converter
(Reproduced from IEC 60034-18-41)
Figure 2 Stator Winding
Answers for industry.
These individual turns in the coil of the winding may develop
high voltage potentials which exceed the insulation system’s
capability depending on the magnitude and rise time (dv/dt).
The magnitude of these voltage spikes can determine the levels
of voltage stress seen between adjacent coils in the motor’s end
windings and within the stator slots. The voltage stress may
also create higher levels of stress between the windings and
the ground, which is normally associated with the core or
frame.
Problem example
For example, let’s say an order is placed for a 4,000 volt inverter
duty motor, but the VFD was purchased from third party. The
voltage characteristics of the drive provided are 6,700 volts
phase to phase peak voltage. The National Electrical
Manufacturers Association (NEMA) MG 1 Part 31 states:
“When operated under usual service conditions (31.1.2), where
the inverter input nominal voltage does not exceed rated motor
voltage, stator winding insulation systems for definite purpose
inverter fed motors shall be designed to operate under the
following limits at the motor terminals.” “Motors with base
rating voltage Vrated >600 volts: Vpeak ≤ 2.04 * Vrated”
For a 4,000 volt motor, the requirement for Vpeak ≤ 2.04 *
Vrated = 2.04* 4,000= 8,160 volts.”
IEC standards 61800-8 and 60034-18 state that one needs to
calculate the expected voltage based on the system and drive
topology, and choose an impulse voltage class. All required
formulas are provided in the standard. This is a lot of
information for the end user to coordinate.
If one chooses the NEMA level with only 6,700 volts, then the
motor would be significantly oversized. This means the end
user would have a considerable amount of wasted expense,
and the system would not be optimized.
If the end user elected to use IEC standard 61800-8, they would
need to have extensive knowledge of the product. In one case,
the customer supplied 6,700 volts as the voltage to the motor.
The levels seen were unexpectedly higher due to the end user
accounting only for the output voltage from the drive and not
being aware of the additional voltage due to reflection in the
system. This increased the voltage to just under than 8,000
volts. The system had to be repaired or the customer risked a
life expectancy of less than two years for the system. This
resulted in significant unplanned expense for the end user.
Siemens Industry, Inc.
3333 Old Milton Parkway
Alpharetta, GA 30005
1-800-241-4453
info.us@siemens.com
usa.siemens.com/ids
Solution
The voltage levels seen at the motor terminals are dependent
on the drive system and its topology, and are covered
extensively in international standard IEC 61800-8. The voltage
spike levels that a motor can withstand are covered extensively
in IEC 60034-18-41 and 42, IEC 60034-25 and NEMA MG 1
part 31.
Deciphering these standards and understanding how they can
be made to work together may be a daunting task for the end
user. The end user may not have the time or inclination to
become an expert in all the industry standards as well as
coordinate the VFD and motor manufacturer’s technical details
if each component is not purchased from the same supplier.
As the supplier of the Integrated Drive System (IDS) that
includes both the motor and VFD, Siemens optimizes the drive
system for the application, providing reliable and efficient
components without the unplanned wasted expense.
IDS ensures that the VFD and motor are designed so that the
peak voltages and levels of dv/dt coming out of the drive do not
exceed the design capability of the motor’s insulation. Motor
life expectancy is directly affected by the magnitude of the
voltage and how often the voltage oscillates from a level below
its corona inception or extinction voltages to its peak level
above its corona inception. This is commonly referred to as the
voltage endurance life.
Siemens reviews both products and determines if the problem
is better resolved by putting additional insulation on the
individual turns of a coil to handle the fast rise time, or by
adding additional ground wall insulation to reduce the voltage
stress seen between phases or phase to ground on the motor.
Another option is to investigate if it is more cost effective to
resolve the issue by changing drive topology. Only a thorough
analysis by a knowledgeable and experienced expert can truly
determine the best solution for a specific application.
IDS delivers the most cost effective and reliable solution to
drive train problems. The end user does not need to become
an expert in all of the above mentioned technologies. Siemens
has successfully implemented IDS solutions in industrial
applications around the world, with hundreds of applications
up and running for decades.
Subject to change without prior notice
Order No.: DTAN-00026-1014
Printed in USA
© 2014 Siemens Industry, Inc.
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