Mitsubishi Electric MELSERVO-JN Series Instruction Manual page 306

12. SERVO MOTOR
(b) Tentative selection and verification of surge absorber
1) Maximum permissible circuit voltage of varistor
Tentatively select a varistor whose maximum allowable voltage is larger than Vb [V].
2) Brake current (Ib)
Vb
Ib = R [A]
3) Energy (E) generated in the brake coil
L Ib 2
E = [J]
2
4) Varistor limit voltage (Vi)
From the energy (E) generated in the brake coil and the varistor characteristic diagram, calculate the
varistor limit voltage (Vi) when the brake current (Ib) flows into the tentatively selected varistor during
opening of the circuit.
Vi is favorable when the varistor limit voltage (Vi)[V] is smaller than the desired suppressed voltage
(Vs)[V].
If Vi is not smaller than Vs, reselect a varistor or improve the withstand voltage of devices.
5) Surge current width ( )
Given that the varistor absorbs all energies, the surge current width ( ) is as follows.
E
= Vi Ib [s]
6) Inspection of surge life of varistor
From the varistor characteristic diagram, calculate the guaranteed current value (Ip) in which the
number of the surge application life is N at the surge current width ( ). Calculate the ratio (Ip/Ib) of the
guaranteed current value (Ip) to the brake current (Ib).
If an enough margin is ensured for Ip/Ib, the number of the surge application life N [Time] can be
considered as favorable.
(4) Others
A leakage magnetic flux occurs at the shaft end of the servo motor with an electromagnetic brake. Note that
chips, screws and other magnetic substances are attracted.
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