Load And Rating; Basics; Is The Thyristor Power Supply Unit Overloaded - Bosch Rexroth PST 6 Serie Betriebsanleitung

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5.3

Load and rating

5.3.1

Basics

5.3.2

Is the thyristor power supply unit overloaded?

| Electric Drives
and Controls
In general, semi‐conductors have an almost infinite life if they are used
within their specifications.
As a result of thermal load, however, mechanical forces act on the
semi‐conductor chips, and this may influence their life depending on the
component size.
For this reason, semi‐conductor manufacturers normally provide life
curves showing the number of temperature cycles until the calculated
end of life – in dependence on the junction temperature rise. In this
case, one temperature cycle corresponds to one spot weld.
These conditions were taken into account in the design of our systems.
Our load diagrams are therefore extended by a peak current curve with
calculatory maximum life. The calculated life takes into account the
thermal rise which is influenced by the welding current, secondary cir­
cuit resistance, weld time ts and ambient temperature, among others.
Our curves are specified for 45°C ambient temperature; however, e.g.
25°C are realistic which results in three times the life.
In principle, the life would have to increase in systems with the "colder"
water cooling as opposed to air cooling. Depending on the application,
however, water cooling may lead to a bigger temperature drop after a
spot weld (due to the higher cooling capacity of water during the cooling
phase). This effect results in a bigger temperature rise and may thus
reduce the life. However the possibility is given here to only let the coo­
ling water flow in the course of the welding schedule.
The maximum load of a thyristor power supply unit is generally specified
by the assignment of types to specific maximum welding transformer
sizes.
Nevertheless it must be ensured that the present application does not
lead to overload of the units.
Overload by inadequate project engineering
Possibility of damage to the device, reduction of useful life!
" Check the actual load of the thyristor power supply unit!
Damages due to overload will result in the extinction of any
warranty claims.
This can be checked using load diagrams. They represent the
• primary output currents (I
• duty cycle (ED in %) for an ambient temperature of 45°C (also refer to
page 246)
that can be switched by the thyristor power supply unit.
The resulting secondary currents (welding currents) can be calcula­
ted by multiplication by the respective transformer ratio.
PST 6xx0
NOTICE
in A) as a function of the
PRIM
| 1070080029 / 05