Technical data

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88
Manual – Gear Units and Gearmotors
Circuit breakers and protective equipment
Project Planning for AC Motors
Motor protection Selecting the correct protection device is a significant factor in determining the opera-
tional reliability of the motor. We distinguish between protection devices that are current-
dependent and those that depend on the motor temperature. Current-dependent
protection devices include fuses or motor circuit breakers. Temperature dependent
protection devices are PTC thermistors or bimetallic switches (thermostats) in the
winding. PTC thermistors or bimetallic switches respond when the maximum permitted
winding temperature is reached. Their advantage is that temperatures are measured
right where they occur.
Motor circuit
breakers
Motor circuit breakers offer adequate protection against overload in standard operation
with a low starting frequency, brief start-ups and starting currents that are not excessive.
The motor circuit breaker is set to the rated motor current.
Motor circuit breakers are not adequate as the sole means of protection given switching
operation with a high starting frequency (> 60 1/h) and for high inertia starting. In these
cases, we recommend you use positive temperature coefficient (PTC) thermistors TF in
addition.
PTC thermistor Three positive temperature coefficient (PTC) thermistors TF (PTC, characteristic curve
according to DIN 44080) are connected in series in the motor and connected from the
terminal box to the TF/TH input of the inverter or to a trip switch in the control cabinet.
Motor protection with positive temperature coefficient (PTC) thermistors TF provide
comprehensive protection against thermal overload. Motors protected in this way can be
used for high inertia starting, switching and braking operation as well as with fluctuating
mains power supply. A motor circuit breaker is usually installed in addition to the TF.
SEW-EURODRIVE recommends always using motors equipped with TF for inverter op-
eration.
Bimetallic switch Three bimetallic switches TH, connected in series in the motor, are looped directly into
the motor monitoring circuit from the terminal box.
Fuses Fuses do not protect the motor from overload. Their only purpose is short-circuit
protection.
The following table provides an overview of the various protection devices used for
various causes.
MOVIMOT
®
protection devices
• MOVIMOT
®
integrate protective equipment to prevent thermal damage.
• No other external devices are required for motor protection.
= no protection
= limited protection
= comprehensive
protection
Current dependent
protection device
Temperature dependent
protection device
Fuse
Protective
circuit breaker
PTC
thermistor (TF)
Bimetallic
switch (TH)
Over-currents up to 200 % I
N
High inertia starting, reversal
Switching operation up to Z =
30 1/h
Stalling
Single phasing
Voltage deviation
Frequency deviation
Insufficient motor cooling