Soft Starters vs Contactors

A contactor is an electrically controlled electromechanical switch, similar to a relay, whose function is to break or make the electrical connection between a connected load and the main power supply. Contactors are used to switch currents ON and OFF in circuits, where the switched current is at a much higher level than the supply current. For example, a 24 Volts electromagnetic coil in a contactor can be used to control a 230 Volts electric motor switch.

Applications of contactors include controlling AC and DC electric motors, heating and lighting circuits, thermal evaporators, capacitor banks, automated industrial equipment, and other assorted electrical loads, but without providing protection against overload conditions. They are typically used to control electrical equipment requiring to be turned on and off frequently, such as heaters, lights, and motors. For example, contactors are often used to provide centralized control of extensive lighting installations in retail and office buildings.

Note: A contactor can be used as a standalone power control electrical device, or as part of a motor starter.

How Does a Contactor Operate?

To better understand how a contactor operates, it is important to know the functionalities of its three most essential components. These components are: (i) The Electromagnet/ Operating Coil; (ii) Contacts; (iii) The Enclosure or Frame.

• Operating Coil/Electromagnet: This component features a coil of wire wound around an electromagnetic core, it thus acts like an electromagnet. It provides the driving force required by the contactor to close the power contacts. Generally, the operating coil of a contactor has two parts–a fixed part and a movable part–with a spring connection between both parts. The movable part is connected to a metallic rod known as an armature.  

• Contacts: They are the current carrying component of the contactor–the entire load current flows through the contacts. They are classified as auxiliary contacts, power contacts, and contact springs. Power contacts are further divided into two different types; movable contacts and stationary contacts. Note: Latching contactors with two operating coils are used to minimize power consumption in the contactor. In such a case, one operating coil is momentarily energized to close the power circuit contacts, which are subsequently held closed mechanically; the second coil is used to open the power contacts. 

• Enclosure: It houses the electromagnet and contacts, protecting them from oil, dust, explosion hazards, and bad weather. It also prevents users from directly touching the power contacts when energized. The contactor enclosure can be made of ceramic, plastic, Bakelite, or nylon. 

Working Principle of a Contactor

In a nutshell, electric current from an external circuit flows through the contactor coil, exciting the electromagnetic core. The excited electromagnet generates a magnetic field, which causes the core of the contactor to move the armature. In turn, a Normally Closed (NC) contact makes the circuit connection between the movable power contacts and the stationary power contacts. This allows the electric current from the main power supply to flow through the contactor’s power contacts to the connected load.

The spring connection provides a spring return mechanism; whereby, if the spring force is more than the force of the operating coil, both power contacts will be separated from each other. But if the force of the operating coil is more than the spring force, both power contacts will be pulled together. Such that if the external current passing through the contactor is removed, the coil/electromagnet will become de-energized, and the magnetic force will fall to zero. The spring force will thus be higher than the operating coil force, thereby pulling back the armature. This causes the fixed and movable power contacts to disconnect, opening the power circuit. The contactor is designed to facilitate rapid ON-OFF action (close-open action).

What is a Soft Starter?

A soft starter is basically a solid-state electronic device that controls the supply power for starting an AC induction motor. It limits the current input to the motor to control the starting torque. Also, it sets the starting duration, making it possible to start the motor gradually–soft start. A soft starter consists of two major components, namely: 

• Contactor: The main function of the contactor in a soft starter is to control the flow of electric current to the connected AC induction motor. Its function is to repeatedly establish and interrupt (make and break) the motor’s power circuit from the main power supply, depending on the load requirements.  

• Overload Relay: It protects the connected motor from drawing excessive electric current and overheating or burning out. A large initial inrush of current and overheating can cause the motor to burn out, which makes it practically useless. The overload relay in a soft starter prevents this from happening. 

Simply put, a soft starter is a magnetic contactor with an added overload relay. The two components allow it to switch ON or OFF an electric motor or a motor-driven electrical equipment. In addition, the soft starter also provides the necessary overload protection to a given motor circuit.  

Note: If the contactor circuit does not include an overload relay, then it’s not a soft starter, it’s just a contactor. The term “soft starter” normally refers to the complete assembly, including but not limited to: the contactor, overloads, control transformer(if any), fuses(if any), and the enclosure. 

Soft starters are widely used in industrial AC motors that drive high inertia loads requiring a large inrush of current. Such motors are highly susceptible to wearing out and breaking down due to excessive inrush current during their startup phase. Hence, it’s very necessary to equip them with a soft starter to prevent this. Examples of applications where soft starters are used to control industrial AC motors include: centrifugal pumps, crushers and grinders, conveyor belts, fans and similar systems (i.e. blowers and exhausters), mixers and aerators, conveyor belts, and in electric helicopters. 

Construction of a Soft Starter

The soft starter circuit mainly comprises two sections: (i) Power Unit; (ii) Control Unit 

• Power Unit: The power circuit of the soft starter consists of thyristors or Silicon Controlled Rectifiers (SCR) and a heat dissipater, which are controlled by a microprocessor-based logic circuit. This unit is responsible for supplying power to the connected AC motor. It also has power contacts that switch ON and OFF the electric current being supplied from the main power supply to the motor through an overload relay provided on the supply side.  

• Control Unit: This is usually a microprocessor-based logic circuit whose function is to regulate the power circuit contacts to either break or make the connection between the main power supply and the AC motor. It controls how the electromagnetic coil is energized or de-energized to connect or disconnect the power contacts. 

Note: Fast-acting fuses or circuit breakers are sometimes used to protect the thyristors or SCRs in the power unit of the soft starter in the event of power circuit electrical faults such as short-circuit faults.

Working Principle of Soft Starters

Essentially, a soft starter operates by controlling the amount of input voltage running through the motor’s power circuit. It executes this by limiting the current and torque in the connected motor. As a result, the soft starter is able to minimize the voltage applied to the motor. It is also able to allow for a smooth progression of input current by gradually stopping input voltage reduction.

The working principle of solid-state soft starters with thyristors or silicon-controlled rectifiers (SCR) is based on these fast-acting semiconductors. The supply voltage to the connected motor is controlled by adjusting the firing angle of the thyristors or SCRs. To accomplish this, the microprocessor-based control unit of the soft starter sends a command signal to adjust the conduction angle of the thyristors, consequently, the thyristors adjust the voltage applied to the motor stator accordingly. In doing so, the soft starter is able to protect the power circuit of the connected AC motor from high starting currents–large inrush currents.

If the thyristor conduction angle variables are adjusted correctly, then the motor’s operating parameters such as torque and current can be adjusted as per the load demands. This means that the current required to accelerate the connected load will be the minimum necessary, without changing the input frequency to the motor.

Note: Thyristors in soft starters act at two points: zero current control and zero voltage control. The logic circuit of the control unit times the firing command pulses from the last zero value of both the current and voltage waveform. Also, most soft starter-controlled motor systems have a sensor that transforms the input current either in a single phase or for each of its phases.

In addition, solid-state soft starters use a series of Silicon Controlled Rectifiers in a back-to-back configuration to reduce the input voltage to an appropriate amount for the connected motor during startup. The SCRs have an ON state whereby they allow current to flow through them to the motor. They also have an OFF state in which they limit and control the electric current supplied to the motor. So when you power up the motor-driven electrical equipment, these SCRs energize, limit the supply voltage and then deactivate as the equipment reaches full power. As a result, they provide a gradual ramp up to the motor’s rated full speed, reducing overall strain and overheating of the motor.

Moreover, there are mechanical soft starters that rely on clutches and different types of couplings such as magnetic, steel-shot, or fluid couplings to limit the torque in the motor being controlled. This enables the soft starter to reduce the voltage allowed to surge through the motor, enabling the motor to power up more easily and gently.

The functioning of a soft starter can be summarized as follows: 

• Adjusting the voltage being applied to the motor during startup for a pre-defined time 
• Timing the voltage pulse when starting motor loads with high starting torque 
• Reducing rapid voltage to an adjustable level; is particularly useful in reducing hydraulic shocks in motor-driven pumping systems. 
• Protecting connected electric motors against overcurrent, phase failure, and undercurrent conditions.  
• In essence, soft starters are microprocessor-based, designed to decelerate or accelerate and protect three-phase induction AC motors. 

What is the Difference between Soft Starters and Contactors? 

This is a very common question in the electrical field because both contactors and soft starters control electric motors; thus, the two terms are often used interchangeably. Also, the two devices have several identical mechanical elements such as the power contacts, operating coil, and arc chutes… and they use low-level control voltage to electromagnetically pull the power contacts together. In addition, contactors and soft starters are exclusively designed to operate with Normally Open (NO) contacts, so that the electrical power to the load is shut off when the operating coil is de-energized. So, what features make them different? 

The main difference between soft starters and contactors is the use of an overload relay–a sensitive coil that monitors changes in ambient temperature and the amount of heat generated by excessive inrush current to the motor. Contactors don’t include overload relays, but soft starters do include overload relays that protect the connected motors and the motor-driven equipment from overheating. 

Other features that differentiate contactors and soft starters include: 

• A contactor is basically an electrically controlled electromagnetic switch, similar to a standard relay. In comparison, a soft starter is a magnetic contactor with the option of adding different overload relays. In simple terms, a soft starter includes a magnetic contactor as an essential component, while also providing overload and under-voltage protection, as well as power cutoff. Also, most soft starters are solid-state devices with fast-acting semiconductors such as thyristors or Silicon Controlled Rectifiers.  

• Contactors apply a control voltage to the operating coils to make or interrupt the motor’s power circuit. In contrast, soft starters employ overload relays to protect the connected induction motor from load surges by switching it off (through the use of back-to-back SCR configuration) to prevent overheating. 

• Contactors are generally rated by their voltage capacity or by the designed load current per pole (contact). On the other hand, soft starters are typically classified by their current capacity and power consumption rating of the motors with which they are compatible, all while accommodating large inrush current during motor startup without nuisance tripping. That is normally done through a slight delay in the overload relay tripping.