Lust CDA3000 - Voltage Load on the Motor Winding; Motor Protection Possibilities

Engineering Guide CDA3000
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3 Selection of inverter module
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3.2.12Voltage load
on the motor
winding
When a standard three-phase AC motor is operated on an inverter the
winding insulation is subjected to higher stress than in a sinusoidal sys-
tem. The reason lies in the periodic switching operations by the inverter
which lead to high rates of rise of voltage (du/dt) and voltage peaks
(Upeak) on the motor winding. This increased voltage load on the motor
winding may shorten the service life of the motors - see the research
report from the ZVEI in the "Bibliography and source reference" section.
Market practice
The rate of rise of voltage of the CDA3000 inverter modules is typically
3-6 kV/µs. For applications with special motors we provide a wide range
of motor chokes (see 6.2).
Our experience shows that no problems arise in connection with IEC
standard motors with vacuum-saturated windings and insulated winding
heads. However, the decisive factor in each individual case is the specifi-
cations of the motor manufacturer!
3.2.13Motor
protection
possibilities
The following chart presents a summary of frequently occurring overload
types and the possibilities for protection offered by various devices (motor
circuit-breakers, thermistor protective relays, inverter functions).
Technology
du/dt
Typical
Problems with IEC
standard motor
1)
Special motors
2)
Inverter technology
with standard tran-
sistors (on the market
for over 15 years)
3-6 kV/µs Not known Isolated cases known
Inverter technology
with IGBTs
10-20 kV/µs Isolated cases known Isolated cases known
Inverter technology
with IGBTs and du/dt
limitation to around 6
kV/µs
3-6 kV/µs Not known Isolated cases known
Inverter technology
with IGBTs and du/dt
motor choke
< 1kV/µs Not known Not known
1) With vacuum-saturated winding insulation (without air bubbles) and insulated winding heads
2) Without vacuum-saturated winding insulation (with air bubbles) and without insulated winding heads
Table 3.22 Practical experience with du/dt voltage load