5–5

L100 Inverter

Dynamic Braking

Introduction

The purpose of dynamic braking is to improve the ability of the inverter to stop (deceler- ate) 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 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. The L100 inverter can connect to an external braking unit, which sends the regenerative energy from the motor during deceleration to the optional braking resis- tor(s). The dynamic braking resistor serves as a load, developing heat to stop the motor just as brakes on an automobile develop heat during braking.

A switching circuit and power resistor are the main components of the dynamic braking unit 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.

Dynamic Braking Usage

Dynamic braking usage must follow guidelines to avoid overheating. The timing diagram to the right shows the output frequency versus time. Dynamic braking is in effect during the deceleration ramp, and has the following constraints:

Output

Dynamic braking

freq.

• Dynamic braking maximum duty cycle

 

 

 

 

 

 

 

 

Tb

 

 

 

 

t

= 10%, where Tb/Tc ≤ 0.1 sec.

 

 

 

Tc

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

•Dynamic braking maximum continuous ON time Tb ≤ 10 sec.

Selecting Braking Resistors for External Braking Units

200V Class Inverters – The following tables specify the braking options for 200V class L100 inverters and the braking torque for each option. You can connect a single braking unit

Inverter

Braking unit

to the inverter, or two braking units for additional stopping torque.

+ Braking

–

 

 

 

unit