Rockwell Automation 1305 DYNAMIC BRAKES - Manual (Page 2)

1305 Dynamic Brake 2
1305-5.11 May, 1999
N
2
RPM
t
2
t
1
N
1
RPM
Motor
Inertia
wk
2M
Load
Inertia
wk 2L
Gear
Ratio
GR
Input
RPM Output
RPM
How Dynamic Braking
Works When a motor turns faster than the synchronous speed set by drive
output frequency, the motor can generate power which is returned to
the drive. Without dynamic braking, power returned to the drive bus
can cause bus voltage to rise above the rated limit of the drive. This
condition can occur if power returned to the drive exceeds 20% of
drive rating. 1305 drives have an overvoltage trip feature to detect this
condition and shut down the drive if necessary.
When dynamic braking is added to 1305 drives, excessive power is
dissipated in the brake resistors. Increased braking (over 20%) can
now take place and an overvoltage trip condition will not occur within
the increased limits of the brake.
The 1305 drive monitors the DC bus. When the drive senses a rise in
bus voltage and braking action is required, the brake will be activated.
Activating the brake adds resistors in parallel to the DC bus, providing
a load to dissipate motor power generated during braking. When the
DC bus voltage is lowered to within acceptable limits and braking is
no longer required, the dynamic brake will be deactivated and the brake
resistors will be disconnect from the DC bus.
The dynamic brake is designed to be activated only when required to
dissipate excessive energy returned to the DC bus. Typically the brake
should be activated (on) only during deceleration and ramp-to-stop.
How to Select a Brake To begin selection the following application information must be
obtained.
The Nameplate Horsepower of the motor HP .
The nameplate base speed of the motor H in RPM .
The speed profile of the motor.
Where:
N
1
= The motor’s minimum speed in PRM.
N
2
= The motor’s maximum speed in PRM.
t
1
= The motor’s cycle time in seconds.
The motor inertia, the nameplate gear reduction ration, and the load
inertia.