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Load Inrush Current Wave and Time
(1) Incandescent Lamp Load
Incandescent lamp
Approx. 1/3 second
Inrush current/rated current:
i/i o ≈ 10 to 15
(4) Motor Load i/i
≈ 5 to 10
o
Steady
Starting state
0.2 to 0.5 second
·
Conditions become more harsh if plugging
or inching is performed since state transitions
are repeated.
·
When using a relay to control a DC motor
and brake, the on time inrush current, steady-
state current and off time brake current differ
depending on whether the load to the motor is
free or locked.
Inrush Current And The Reverse Voltage
When the motor, capacitance, solenoid and lamp load make, the inrush current is generated, which is several
multiple steady state currents.
When the inductive load such as solenoid, the motor, contactor, the reverse voltage which are from hundreds
of to thousands of volts.
Generally in the normal temperature and atmospheric pressure the critical insulation destruction voltage of the
air is 200 to 300V.
Therefore if the reverse voltage exceeds this value, the discharge phenomena between contacts will happen.
Both inrush current and the reverse voltage will greatly damage the contacts and obviously shorten
the relay life.
Therefore the proper use of the contact protection circuit may increase the life of the relay.
Material Transfer Of Contacts
Material transfer of contacts occurs when one contact melts or boils and the contact material transfers to the
other contact.
This often occurs in circuits where electrical arcs are produced at the moment the contacts "make" such as
when the DC current is large for inductive or capacitive loads or when the inrush current is large (several
amperes or several tens of amperes).
When material transfer becomes serious, the deformation of the contact surface can be seen by eyes, as
shown in Figure 9.
Generally, a concave formation appears on the cathode and a convex formation appears on the anode.
34
(2) Mercury Lamp Load
i/i
≈ 3
o
3 to 5
minutes
The discharge tube, transformer, choke coil, capacitor, etc, are combined in
common discharge lamp circuits.
Note that the inrush current may be 20 to 40 times, especially if the power supply
impedance is low in the high power factor type.
(5) Solenoid Load
i/i
≈ 10 to 20
o
Free
Lock
Load
Braking
0.07
to 0.1 second
Note that since inductance is great, the
arc lasts longer when is cut.
The contact may easily burn.
(3) Fluorescent Lamp Load
L
Contacts
C
(for high power factor type)
(6) Electromagnetic Contact Load
i/i
≈ 3 to 10
o
1 to 2 cycles
(1/60 to 1/30 seconds)
1/2 to 2 cycles (1/120 to 1/30 seconds)
80175G_MAN_I3-IE-IN-IX_I7_11-2018_ENG
i/i
≈ 5 to 10
o
10 seconds
or less
(7) Capacitive Load
i/i
≈ 20 to 40
o
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