13-2
Application Notes
07-01-08-02-E-V0500 631 Digital Servo Drive
Dynamic Braking
The energy of a moving system flows back into the drive while decelerating. The DC-Bus
capacitors are able to take a small value. The remainder is converted to heat by a high power
resistor switched across the DC link.
Switching on and off of this brake resistor depends on the DC-Bus voltage.
The load of the resistor is simulated and supervised electronically (EASYRIDER
<
).
Peak power (Pmax) and continuous power (Pd) ratings have to be sufficient to meet the
requirements of the application.
Example Brake Resistor Calculation
n1
RPM
T
tb1
t [sec]
Movement
I [A]
Ib
t [sec]
Braking Current
Example Values
n1 = 3000 RPM
tb1 = 0.1 seconds
T = 2.0 seconds
J = 0.0005 kgm²
Total Inertia
Data
RL = 0.3 Ohm
Speed at Brake-Start
Braking Time
Cycle-Time
Cable-Resistance
Braking-Current
Ib = 3.2A
Motor-Resistance
Rph = 3.6 Ohm
STEP 1 : Calculating actual brake power
(an approximation - capacitor load, friction and drive losses are neglected)
Example
(values see above)
Calculation
Pkin = 0.0055 * 0.0005 * 3000²/0.1
Pkin = 247W
Power of Motion:
Pkin
Pkin
Pkin
Pkin = 0.0055 * J * n1² / tb1 [W]
Pvmot = 3.2² * (3.6 + 0.3)
Pvmot = 40W
Motor Losses:
Pvmot
Pvmot
Pvmot
Pvmot = Ib² * (Ri + RL) [W]
Pd = 0.9 * (247 - 40) * 0.1 / 2
Pd = 9.3W
Continuous Power:
Pd = 0.9 * (Pkin-Pvmot) * tb1 / T [W]
Pd = 0.9 * (Pkin-Pvmot) * tb1 / T [W]
Pd = 0.9 * (Pkin-Pvmot) * tb1 / T [W]
Pd = 0.9 * (Pkin-Pvmot) * tb1 / T [W]
Pmax = (1.8 * 247) - 40
Pmax = 405W
Peak Power:
Pmax = (1.8 * Pkin) - Pvmot [W]
Pmax = (1.8 * Pkin) - Pvmot [W]
Pmax = (1.8 * Pkin) - Pvmot [W]
Pmax = (1.8 * Pkin) - Pvmot [W]
units used:
J
total inertia [kgm²]
n1
speed at Brake-Start [RPM]
tb1 braking time [Sec]
T
cycle time [Sec]
Ib
brake-current [A]
Rph resistance of motor (between terminals) [
Ω
]
RL
line resistance of motor cable [
Ω
]
