Dynamic Braking
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 characteristics: |
| • 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 |
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| serves as a warning signal that we have exceeded the deceleration |
| capabilities of the system. SJ300 inverters rated 15hp (11kW) and below |
| have a |
| 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, devel- |
| oping 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 assem- |
| bly, which includes an integral thermal fuse and thermally activated |
| alarm relay for safety. However, be careful to avoid overheating its resis- |
| tor. The thermal fuse and thermal relay are safeguards for extreme condi- |
| tions, but the inverter can maintain braking usage in a safe zone. |
Braking
Resistor
Motor Control | Accessories |
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The inverter controls braking via a duty cycle method (percent of the time braking is ON versus total time). Parameter B090 sets the dynamic braking usage ratio. In the graph to the right, the example shows three uses of dynamic braking in a
BRD |
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| Duty cycle = | (t1 + t2 + t3 + | ) ⋅ 100 | |||||||||||||||||||||||
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| 100 seconds |
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Please note the following (for
•When B090 is set for 0%, dynamic braking is not performed.
•When the duty cycle value exceeds the limit set by B090, the inverter will trip (ending the dynamic braking).
•The cable from the external resistor to the inverter must not exceed 5 m (16 ft.) length.
•The wires from the DB resistor to the inverter must not be bundled together with control or signal wires.
NOTE: Inverters rated 20hp (15kW) and above