| The kinds of loads and their current characteristics, together with the
ON-OFF frequency, affect the contact so much that it is absolutely
necessary to know the electrical characteristics of the loads to be used. |
| |
| (a) Resistive Load |
inrush current/nominal current = 1 |
(b) Lamp Load
In discharge lamp
circuits of high power factor and low power source impedance, inrush
current of 20 to 40 times normal current can occur. The transient effect
resulting from the combination of choke, capacitor, transformer, and
discharge tube should be calculated to determine true load requirement.
(Example
of high power-factor type) |
 |
(c) Motor Load
Because the transient
state is repeated by plugging and inching, the condition becomes more
severe. |
 |
| (d) Solenoid Load |
 |
| (e) Electromagnetic Contactor Load |
 |
| (f) Capacitor Load |
 |
| (g) DC Load |
- When the inductive load (circuit) is broken, high voltage is
usually generated between contacts, causing arc discharge or spark
discharge. If this discharge energy is large, removal or wear of the
metal of the contacts is caused, which may sometimes result in
deterioration of the contact function.
Removal of the metal
means that one
contact is fused and removed to the other contact. With an increase
of the on-off frequency, concave and convex formations (as
illustrated above) appear on the contacts, until finally they are
locked together, making on-off operation impossible. In order to
prevent this, it is recommended to use an arc extinguishing circuit
as described later. It is also effective to reverse positive voltage
and negative voltage applied to the contacts.
- When closing a capacitive load (circuit) or a circuit which
carries current to the load through a long cable, a charge current
usually flows through the contacts, causing arc discharge. This
discharge energy leads to wear or fusion of the contacts. In order
to control such discharge current, a spark extinguishing current
using CR-type or diode varistor is used when the DC load circuit is
open, and a surge suppressor is used when the capacitive load
circuit is closed.
|
(h) Counter voltage of
DC relays
 |
If input is cut off in DC relays, a counter voltage is
developed across the coil as a result of the collapse of the magnetic
field. If the coil is used in a transistor circuit, the reverse voltage
produced from the coil can cause a serious circuit malfunction. This
counter voltage can be reduced considerably by connecting a capacitor or
a diode in parallel with the coil. The level of reduction must be
determined either by calculation if the coil data is available or by
experiment. |
| Coil
Voltage |
Counter
Voltage
(Approx.) |
| DYC-RELAY |
100 V DC |
1,800 V |
| 48 V DC |
1,300 V |
| 24 V DC |
900 V |
| 12 V DC |
600 V |
| 6 V DC |
400 V |
| DY3-RELAY |
100 V DC |
1,300 V |
| 48 V DC |
800 V |
| 24 V DC |
600 V |
| 12 V DC |
400 V |
| 6 V DC |
300 V |
| DY1-RELAY |
24 V DC |
350 V |
| 12 V DC |
250 V |
| 6 V DC |
180 V |
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16475
Canyon Hills Road #3