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Summary
Preface
P. 6
Chapter 2
P. 17
Technical data
Chapter 5
P. 76
Reception, storage
and installation
Chapter 4
P. 54
General information
Chapter 3
P. 47
Application notes
Chapter 6
P. 115
Options
Chapter 7
P. 129
Preventive maintenance
Chapter 1
P. 15
Overview
and installation diagram
Answer Drives 1000
118
6
6.2.3 RESISTOR SELECTION
When the speed (frequency) of an asynchronous motor is reduced rapidly,
the motor acts as an asynchronous generator re-applying power to the
drive.
Some of the energy is dissipated in the motor (with torque corresponding
to approximately 10-20% of the rated torque of the motor), the rest is
stored on the capacitors of the drive intermediate circuit with a voltage
increase at their terminals. A dynamic brake is used to prevent it from
reaching too high voltage values and to avoid the intervention of the drive
maximum voltage protection.
An electronic circuit detects the voltage on the capacitor bank of the drive
and, when its voltage exceeds a predefined value, it enables the power
transistor of the braking unit that inserts the braking resistor in parallel to
the capacitor bank.
As soon as the DC voltage returns to its normal values, the power
transistor is disabled.
The ohmic value of the resistor and its power depend on the type of drive,
the inertia of the moving masses, the load torque and the deceleration
time.
The figure below provides definitions for the terms used when a motor
is braked from an initial speed N1, to a final speed N2. In case of failure
of the power braking switch, the resistor is directly connected on the
network via the rectifier jumper. In a similar situation, the resistor heats
up to a high temperature and a failure of the resistor itself may occur.
To avoid this situation, the resistor must be protected by an adequate
temperature relay: its intervention must open the main switch that
protects the input line.
The cables of the thermal device must be screened and of a length not
exceeding the resistor cables length
It is compulsory, for safety problems, to provide a contactor on the
drive power supply input and make sure that it opens in case of over-
temperature of the resistor.
The figure below shows a simple connection example.
FIG. 6.2.2.2
The formulas for the resistors power calculation are given below.
6.2.3 RESISTOR SELECTION
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