15. SERVO MOTOR
15 - 3
(b) Tentative selection and verification of surge absorber
1) Maximum allowable circuit voltage of varistor
Tentatively select a varistor whose maximum allowable voltage is larger than Vb [V].
2) Lock current (Ib)
Ib =
Vb
R
[A]
3) Energy (E) generated by lock coil
E =
2
L × lb
2
[J]
4) Varistor limit voltage (Vi)
From the energy (E) generated in the lock coil and the varister characteristic diagram, calculate
the varistor limit voltage (Vi) when the lock current (Ib) flows into the tentatively selected varistor
during opening of the circuit. Please refer to the varistor characteristic diagram to the varistor
manufacturer.
The desired suppressed voltage (Vs) is the sum of the 24 VDC ± 10% used and the other
devices (relays etc.) used by the user.
Please confirm the specification of the equipment to be used.
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.
Regarding the characteristics characteristic diagram, specification, selection of the varistor, it is
necessary to check with the varistor manufacturer.
5)
Surge current width (τ)
Given that the varistor absorbs all energies, the surge current width (τ) will be as follows.
τ =
E
Vi × lb
[S]
6) Examining surge life of varister
From the varistor characteristic diagram, the guaranteed current value (Ip) in which the number
of the surge application life is N at the surge current width (τ). Calculate the guaranteed current
value (Ip) ratio to lock 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 will occur at the shaft end of the servo motor equipped with a lock. Note that
chips, screws, etc. are attracted.
