Shihlin SDH Series, Manual

Page: 102 / 220

93
Chapter 6.
The capacity of brake resistor is calculated as follows:
Power of brake resistor (Pbr) ==>
N is the ratio of load inertia to motor shaft. T is duty cycle (Defined by users).
If the ratio of load inertia to motor shaft is N, deceleration from 3000rpm to stop; the
regenerative energy is (N +1) × Es. The brake resistor consumption is (N +1)× Es - Ec joules. Assuming
the duty cycle is T second, then the recommend power of brake resistor is 2 × ((N +1) × Es-Ec) / T.
The calculation procedure is as follows.
J is motor inertia (Unit:kg
‧
m
2
); Wr is the maximum speed of operation cycle. (Unit:rpm)
Step Item Calculation or procedure
1 Choose the duty cycle T With user’s application to decide the repeat operation cycle.
2 Set motor speed Wr Panel operation to read/write this value.
3 Set load to motor inertia ratio N Panel operation to read/write this value. (PA02=0002 valid.)
4 Compute the Es Es = J × Wr
2
/ 182
5 Compute the Ec Refer to the previous table
6 Compute the Pbr 2 × （ (N+1) × Es － Ec ） / T
► Example 1
The drive’s capacity is 400W, duty cycle T is 0.5 second, revolution speed is 3000 rpm, load to motor inertia ratio is 7,
then the necessary power of brake resistor = 2 x ((7 + 1) x 85.1 -11.02 ) / 0.5 = 8.72W.
Since these are less than the capacity (20W) of 400W servo drive’s built-in brake resistor, users could directly use
the built-in brake resistor to consume the regenerative energy.
Note: Due to 3000rpm is the rate speed of 400W servo drive; we could find the Es on the previous table is 1.65 J.
► Example 2
The drive’s capacity is 2KW, duty cycle T is 1 second, revolution speed is 1000 rpm, and load to motor inertia ratio
is 20. Since the revolution speed 1000rpm is less than the rated speed (2000rpm), we need to compute Es = 38.8 x 0.0001
x 1000
2
/ 182 = 21.3J, then the necessary power of brake resistor = 2 x ((20 + 1) x21.3 -41.62) / 1 = 811.36W. Since these
are more than the capacity (50W) of 2KW servodrive’s built-in brake resistor, a 1000W brake resistor is recommended.
Generally, if the load to motor inertia ratio is small (N<=5), the built-in brake resistor is sufficient. If the capacity
of brake resistor is too small, the heat accumulated is growing easily and the temperature of brake resistor rises soon.
When the temperature is higher than a certain value, the brake resistor will be burn out.
(b) With external load
When the external load torque is greater than motor torque, it makes the servo motor output torque
direction is opposite to the rotary direction of servo motor. In this case, the external energy is delivered
to the servo drive through the servo motor. The following figure is an example that the motor runs in
CCW rotation at constant speed when a sudden external load torque change.