Hitachi SJ2002 Series, Instruction Manual

Page: 207 / 306

SJ200
2
Inverter
Motor Control
Acce
ssor
ie
s
5–5
Dynamic Braking
Introduction
The purpose of dynamic braking is to improve the ability of the
inverter to stop (decelerate) the motor and load. This becomes neces-
sary when an application has some or all of the following characteris-
tics:
• High load inertia compared to the available motor torque
• The application requires frequent or sudden changes in speed
• System losses are not great enough to slow the motor as needed
When the inverter reduces its output frequency to decelerate the load,
the motor can temporarily become a generator. This occurs when the
motor rotation frequency is higher than the inverter output frequency.
This condition can cause the inverter DC bus voltage to rise, resulting
in an over-voltage trip. In many applications, the over-voltage condi-
tion serves as a warning signal that we have exceeded the deceleration
capabilities of the system. SJ200
2
inverters have a built-in braking
unit, which sends the regenerative energy from the motor during
deceleration to the optional braking resistor(s). External braking units
may also be used if higher braking torques and/or duty cycles are
required. The dynamic braking resistor serves as a load, developing
heat to stop the motor just as brakes on an automobile develop heat
during braking.
The braking resistor is the main component of a braking resistor
assembly that includes a fuse and thermally activated alarm relay for
safety. However, be careful to avoid overheating its resistor. The fuse
and thermal relay are safeguards for extreme conditions, but the inverter can maintain
braking usage in a safe zone.
Braking
Resistor