Braking
Resistor

5–5

SJ200 Inverter

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 necessary 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 condition serves as a warning signal that we have exceeded the deceleration capabilities of the system. SJ200 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.

 

Accessories

Control Motor