Rockwell Automation AK DBU - Dynamic Braking Unit 600/690 VAC - Manual (Page 40)

C-2 Design Information
The Peak Regenerative Power can be calculated as:
• Horsepower (English units)
• Watts (The International System of Units, SI)
• Per Unit System (pu) which is relative to a value
The final number must be in watts of power to estimate the resistance
value of the Dynamic Brake Resistor. The following calculations are
demonstrated in SI units.
Gather the following information
• Power rating from motor nameplate in watts, kilowatts, or
horsepower
• Speed rating from motor nameplate in rpm or rps (radians per sec.)
• Required decel time (per
Figure C.1 , t
3
– t
2
). This time is a process
requirement and must be within the capabilities of the drive
programming.
• Motor inertia and load inertia in kg-m 2 or WK 2 in lb.-ft. 2
• Gear ratio (GR) if a gear is present between the motor and load
• Motor shaft speed, torque, and power profile of the drive application
Figure C.1 shows typical application profiles for speed, torque and
power. The examples are for cyclical application that is periodic over t4
seconds. The following variables are defined for
Figure C.1 :
ω(t) = Motor shaft speed in radians per second (rps)
N = Motor shaft speed in Revolutions Per Minute (RPM)
T(t) = Motor shaft torque in Newton-meters
1.0 lb.-ft. = 1.356 N-m
P(t) = Motor shaft power in watts 1.0 HP = 746 watts
ω
b
= Rated angular rotational speed
ω
o
= Angular rotational speed less than ω
b
(can equal 0)
-P
b
= Motor shaft peak regenerative power in watts
ω
2 πN
60
---------- =
Rad
s
---------
Rad
s
---------