Contactor control with hold-to-run operation
In simple hold-to-run contactor control, the coil is commanded only while the intended button is held and the other conditions are satisfied. Releasing the button removes the command. The basic example has no seal-in contact bypassing it.
This describes an operating function. It does not mean the machine stops mechanically at once or that the button provides a validated safety function.
Construction and operation
NO button S2 completes the control path to coil K1. A common stop path can additionally interrupt it. K1 main contacts switch the suitable load; the small button does not carry that load current.
While S2 is closed, K1 can operate with a suitable supply. On release, the path opens and the ordinary monostable contactor drops out. Its main NO contacts open, but the driven mechanism may coast.
The coil needs the correct voltage and current type. Contacts do not convert voltage. Main contacts and switching frequency must suit the actual duty.
Switching duty and behaviour
A signal load differs from repeatedly starting a motor that has not reached running speed. Frequent jogging can impose significant electrical and thermal duty. Check the actual utilization category and permissible switching frequency.
Hold-to-run describes the command arrangement first. A safety-related enabling or hold-to-run function requires an appropriate machine assessment and design. A conventional button and contactor do not automatically provide it.
A stuck button can maintain the command. If supply returns while the path remains closed, the circuit may operate again. Assess that behaviour in the actual application.
Circuit diagrams and explanations
Diagram and current path
The new control example shows control supply Uc, control fuse F0, NC Stop S0, intended protective contact F1 and NO command S2 before K1. This is a defined functional example, not complete machine protection.
The second image highlights the path while S2 is pressed. Colour denotes the analysed state, not conductor colour. Releasing S2 interrupts the path; no K1 holding branch bypasses it.
F0 is control-circuit protection, not motor overload protection. A motor application additionally needs the appropriate power circuit and coordinated protective devices.
Hold-to-run: control circuit
Hold-to-run: active command path
Wiring principles and selection
Wiring and seal-in comparison
The basic circuit must have no unintended parallel path bypassing S2. Adding a holding contact changes the function: operation may continue after a brief command. The seal-in guide explains that step.
Combined jog/run operation requires a separately defined circuit and switching sequence. Interrupting one holding contact without analysing transitions can produce unintended retention. This page deliberately explains the pure hold-to-run function.
Wire to the verified plan with suitable conductors, terminals and control contacts. Removing a command is not electrical isolation for work.
Functional checks and verification
| State | Expected basic behaviour |
|---|---|
| Supply available, S2 released | K1 off. |
| S2 held, other conditions satisfied | K1 can operate. |
| S2 released | K1 drops out. |
| Stop or intended protection opens | K1 loses supply. |
| Supply returns with S2 still held | Reoperation is possible; assess separately. |
Complete required electrical verification first and secure the load area. Observe response and coasting without entering danger. Apparently quick stopping is not a measured safety validation.
Advantages and limitations
Benefits depend on suitable design. Contact duty, protection and behaviour after interruption must match the application.
Advantages and limits
The basic function is easy to follow and needs no holding contact. With an intact intended button, release removes the command.
Its limits include continuous operation of the control, mechanical coasting and repeated-switching duty. Protective and safety functions need their own suitable design.
Questions and related guides
Common questions
Does release mean safe standstill?
No. Command removal and mechanical standstill are different states.
Why might K1 remain operated?
Possible causes include an unwanted holding path, another maintained command or equipment failure. Investigate against the actual plan rather than improvising bridges.
Related guides
Contactor seal-in circuit · Contactor control from multiple stations
Sources and documentation
Electrical safety: scope, competence and precautions
Put electrical safety first.
- These explanations and simplified diagrams support learning. They are not complete installation designs, site-specific risk assessments or a substitute for competent professional assessment.
- Electrical work can be life-threatening. It requires the competence and authorisation appropriate to the actual task, installation and jurisdiction.
- Errors and omissions can occur. Check the actual equipment documentation, applicable requirements and the complete installation before relying on an example; a working device is not proof of electrical safety.
- Personal protective equipment must suit the assessed hazards and task. It does not replace isolation, appropriate tools, competence or a safe system of work.
- Do not work on exposed live parts on the basis of these guides. Identify every relevant source, isolate it, secure against reconnection and prove dead using suitable equipment and procedures before access.
- Responsibilities for design, installation and verification must be established for the actual work. A website statement does not determine liability or remove duties under applicable law.
- This summary is not exhaustive. Where the circuit, condition or protective arrangement is uncertain, obtain suitable professional assistance before work proceeds.
- Complete the required inspection, testing and documentation before use, and maintain the installation and equipment appropriately.
Electrical terms explained
Electric current
The rate of flow of electric charge, measured in amperes (A). Conventional current direction follows positive charge flow, which is opposite to electron drift in a metallic conductor.
Voltage
The electric potential difference between two points, measured in volts (V). Always identify the reference points, such as line-to-neutral or line-to-earth; a voltage is not a property of one isolated terminal alone.
Resistance
The opposition associated with resistive current flow, measured in ohms (Ω). For a component behaving as an ohmic resistance under the stated conditions, U = IR.
Electrical power
The rate of electrical energy transfer, measured in watts (W). P = UI applies to steady DC; in a sinusoidal single-phase AC circuit, active power is UI cos φ using RMS values. The general average depends on voltage and current waveforms.
Ohm's law
For an ohmic resistance, current is proportional to the applied voltage: I = U/R. Resistance can vary with temperature, and non-linear devices do not necessarily obey one constant-resistance relationship.
Kirchhoff's laws
The current law expresses charge conservation at a circuit node. The voltage law states that the algebraic sum of voltages around a closed loop is zero within the applicable circuit model; induced effects must be represented appropriately.
Direct current (DC)
Current that maintains one direction. Its magnitude may be constant or vary; pulsating DC is still distinguished from current that reverses direction.
Alternating current (AC)
Current whose direction reverses with time, commonly periodically. Public supplies normally use approximately sinusoidal AC, while electronic equipment can produce other waveforms.
Frequency
The number of cycles per second of a periodic quantity, measured in hertz (Hz). Many European public supplies operate at a nominal 50 Hz.
Phase difference
The relative position of periodic quantities of the same frequency, often expressed in degrees or radians. For sinusoidal quantities, a time displacement can be related to that phase angle.
Circuit diagram
A drawing representing circuit components and their electrical connections using symbols. It is distinct from a physical layout or a complete installation design.
Capacitor
A component that stores energy in an electric field. Its capacitance relates stored charge to voltage under the stated conditions; a practical capacitor also has voltage, current and other operating limits.
Strappers in lighting circuits
The pair of conductors linking the changeover contacts in conventional two-way lighting switching, including intermediate switching where used. They carry the alternative switching paths and are not transformer windings; identify their actual functions from the circuit.
Coil or winding
A conductor arranged in turns, commonly to produce magnetic effects. Its behaviour includes inductance as well as the resistance and other properties of the actual winding.
Inductance
The relationship between flux linkage and current in the applicable model, measured in henries (H). Changing current produces an induced voltage; for a constant inductance its magnitude relates to L di/dt with the appropriate sign convention.
Capacitance
The relationship between stored charge and voltage, measured in farads (F). For a constant capacitance, changing voltage produces current according to i = C dv/dt.
Transistor
A semiconductor device used for switching, amplification or other control functions. Its operating principle and control quantities depend on whether it is, for example, a bipolar transistor or a field-effect device.
Semiconductor
A material whose electrical behaviour can be controlled through composition, electric fields, light or other conditions. Semiconductor devices use this behaviour for functions such as rectification, switching and amplification.
Diode
A device with a strongly directional current-voltage characteristic. Real diodes have forward losses, reverse leakage and limits; they do not act as perfect one-way valves at every voltage.
Relay
A device that changes an output in response to a control input. An electromechanical relay uses a coil and contacts, while a solid-state relay uses electronic switching elements.
Switch
A device that makes, breaks or changes electrical connections. Its rating, contact arrangement and permitted duty determine the applications for which it is suitable.
Electrical network
An interconnection of electrical components and branches. Series, parallel and more complex arrangements can be analysed with an appropriate circuit model.
Transformer
A device transferring energy between magnetically coupled windings through changing flux. Turns ratio influences voltage and current relationships; losses, limits and any conductive connection depend on its construction.
Electromagnet
A magnetic arrangement energised by electric current, commonly using a coil and magnetic core. Contactors, relays and actuators use electromagnetic force for mechanical operation.
Impedance
The relationship between voltage and current phasors in a sinusoidal steady-state model, measured in ohms. It combines resistance and reactance and can vary with frequency.
Short circuit
An unintended low-impedance connection between points at different potentials. Current is limited by the complete source and fault-path impedance, not literally unrestricted.
Electric circuit
An arrangement of sources, conductors and components. Current flows where a complete conductive path exists, although a circuit can also be described in an open or disconnected state.
Series connection
Components in one unbranched path carry the same current. Their voltages add algebraically; the individual voltage shares depend on their electrical behaviour.
Parallel connection
Branches connected between the same two nodes share the same voltage. The branch currents depend on their loads and add algebraically at the connection points.
Fuse
An overcurrent protective device with an element designed to melt and interrupt the circuit under specified conditions. Its rating, characteristic and breaking capacity must suit the application.
Overcurrent protection
Protection against overload and short-circuit effects, commonly using fuses or circuit-breakers. A sensing relay may require a separate interrupting device; an ordinary RCCB does not provide this protection by itself.
Earthing
A deliberate connection of a system point or conductive part to earth. Protective earthing forms part of an electric-shock protection arrangement; it must not be confused with neutral, functional earth or a guarantee of zero touch voltage.
Frequency converter or variable-speed drive
Electronic equipment that produces a controlled output frequency, commonly with controlled voltage, to operate a suitable motor. Its pulsed output, protection and compatibility requirements differ from a direct mains supply.
Electromagnetic compatibility (EMC)
The ability of equipment to operate satisfactorily in its electromagnetic environment without causing unacceptable disturbance to other equipment. Both emissions and immunity matter.
Power loss
Energy dissipated per unit time, commonly as heat in conductors, cores, insulation or semiconductor devices. Ideal inductors and capacitors exchange stored energy without average loss; practical components have additional loss mechanisms.
Short-circuit current
The current resulting from a short circuit. Prospective fault current is assessed to select protective devices with adequate interrupting capability and to check thermal and mechanical effects.
Power factor
The ratio of active power to apparent power. For sinusoidal voltage and current it equals cos φ; with distortion it also reflects waveform effects. It is not the same as energy-conversion efficiency.
Insulation resistance
Resistance measured between specified conductive parts separated by insulation under stated test conditions. The test voltage, connected equipment and interpretation must suit the circuit; it is not a complete assessment of insulation performance on its own.