JohdotaThe self-holding circuit, also called a latching or seal-in circuit, is the basic circuit of motor control. A short press switches the contactor on, and it stays on until the stop button is pressed.
Power cut
When the control voltage is lost, the contactor de-energises and the self-holding is released. When the power comes back, the motor does not start by itself. It has to be started again with the start button.
Stop has priority
When S0 is placed before the branch, stop always has priority: if both buttons are pressed at the same time, K1 does not energise. This is how motor controls are normally built. If S0 is wired in series with the holding contact only, start has priority.
Common mistakes
- The holding contact is also wired in parallel with S0. Then the stop button does not stop the motor.
- An NO contact is used as the stop button. NC contacts are 11–12 and 21–22, NO contacts 13–14 and 23–24.
- A contact of another contactor is used as the holding contact. The holding must come from a contact of the same contactor.
In your region
Showing notes for
United States, Canada
This is called three-wire control: three wires connect the start button, the stop button and the holding contact of the starter. It gives low-voltage protection: after a power cut or an overload trip, the motor does not restart until someone presses start.
In two-wire control a maintained contact switches the coil directly. The motor stops when the power is lost, but it restarts on its own when the power returns if the contact is still closed.
Source: Siemens: Basics of Control Components, part 4 (STEP 2000 course)
UK & Ireland, Europe, Australia & New Zealand, Other IEC countries
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