Electrical Elements

Various standard electrical components are compatible with CNC machines, enhancing their versatility. These components are not exclusive to CNC machines and can be used in other electrical circuits as well. Below is a list of commonly used electrical components that primarily function as supporting devices for CNC machines:

· Relay
· Contactor
· Overload Relay
· Miniature Circuit Breaker (MCB)
· Motor Protection Circuit Breaker (MPCB)
· Electrical Fuse

Relay:

A relay is an electromagnetic switch used for switching and isolating electric circuits. It can handle high currents in one circuit while controlling a low-current circuit. CNC machines typically use a 24-volt DC control circuit for relays. There are two types of relays: electromechanical and solid-state. Electromechanical relays use magnetic force to switch electrical contacts on and off, while solid-state relays utilize special electronic circuits. The images below depict the two types of relays.      

The relays commonly employed with CNC machines are of the electromechanical type, consisting of two sections: an "Energizing" part and a "Switching" part. The energizing part contains an electromagnetic coil that is energized by an external voltage, attracting a small iron plate against spring tension. The switching elements are connected to the plate, allowing them to turn the switching circuit on or off. When the relay is energized, the electromagnetic coil attracts the iron plate, and when de-energized, the spring tension pushes it back to its initial position.

The voltage rating of the electromagnetic coil can vary depending on the requirements of other appliances. For DC coil voltage relays, there are fixed terminals for positive and negative coil connections, while the terminals are interchangeable for AC voltage relays. Relays come in different types based on their switching contacts or elements, such as single contact, double contact, or triple contact, enabling collective switching. A single switching element typically has three terminals: NC (Normally Closed), NO (Normally Open), and Common (see picture below). In the de-energized state, the NC and Common terminals are interconnected, and in the energized state, the Common terminal is disconnected from the NC terminal and connected to the NO terminal. This allows for turning an electric circuit on or off using the relay's NC, NO, and Common terminals.

Solid-state relays use light-emitting diodes instead of electromagnetic coils. When a solid-state relay is energized, the emitted light from the diode activates a "Solid-state switching element" within the relay, which can turn an external electrical circuit on or off. Solid-state relays offer the advantage of minimal electrical noise due to the absence of physical contact, and they are enclosed in a solid compartment with a shorter response time compared to electromagnetic relays. However, solid-state relays have some drawbacks, such as being limited to single-pole switching and being more expensive than electromagnetic relays.

How does a relay function with a CNC machine?

A relay serves as a switching element between two electrically isolated circuits and finds application in various types within CNC machines. Relays are used to control induction motors and operate hydraulic and pneumatic solenoid valves. When a 24-volt DC signal is received from a programmable logic controller (PLC) output module, it is sent to the energizing part of the relay to activate it. The relay's switching element is then used to turn on or off higher current devices such as contactors and solenoid coils. The image below illustrates how a 220 AC coil voltage connector controls the on/off operation of a 3-phase induction motor through a 24-volt relay.

Contactor:

The basic working principle of an electrical contactor is similar to a relay. It consists of two sections: the energizing segment and the contact part. An electromagnetic coil is connected to the energizing element, while the contact part contains multiple contact terminals that control higher electrical power, usually 3-phase AC. Additionally, two or three auxiliary contacts may be attached to the contactor externally for logical operations. The electromagnetic coil inside the contactor has different voltage ratings, such as 24-volt DC, 110-volt AC, and 220-volt AC. A contactor is commonly used to control a 3-phase electrical induction motor in a CNC machine. The following pictures illustrate how a contactor is connected to an induction motor. The right-hand side picture shows one contact point, while the others display three or four contact points.

In accordance with the previous picture, the electromagnetic coil is energized by receiving the required voltage through the A1 and A2 terminals. When voltage is applied to the coil, the electromagnet pulls the armature firmly against spring tension. The armature moves downward and retracts to its initial resting position when de-energized. The armature is equipped with 3 or 4 movable contact strips that insulate each other and the armature body. When the electromagnet pulls the armature downward, the movable contact strips connect the corresponding fixed contact power terminals. Similar to the previous picture, one contact point is shown, but in reality, there are three or four contacts that connect correspondingly. Referring to the left side picture, the contact strips connect the T1, T2, and T3 terminals to the L1, L2, and L3 terminals, respectively. The L1, L2, and L3 terminals are connected to the 3-phase AC and the induction motor through the T1, T2, and T3 terminals. As a result, the induction motor starts to rotate when the T1, T2, and T3 terminals are connected to the 3-phase AC.

Sometimes an additional contact block is attached to the contactor externally, which may include normally open (NO) and normally closed (NC) types. The auxiliary contact block is typically connected to the main contactor in a way that the contact points inside the auxiliary contact block also change with the movement of the main contactor's armature. These supplementary contacts usually have low current ratings and are used in various relay logic circuits of a machine, as input for PLC logic, or in motor control circuits. A coil suppressor module is always connected in parallel with the electromagnetic coil to neutralize the volt-ampere reactive (VAR) generated by the inductive load of the coil.

Overload relay:

An overload relay is used to measure and protect an induction motor from excessive current. When the current exceeds a predefined value, the relay cuts off the current flow to prevent damage to the motor. The 3-phase power supply to the motor usually passes through the overload relay. Therefore, the overload relay acts as a cutoff switch between the contactor and the induction motor. The following picture shows the schematic diagram of an overload relay, contactor, and induction motor.      

Inside an overload relay, three low-resistance coils are wound around three separate bimetallic strips, known as overload heaters. The 3-phase AC power supplied to the motor passes through these low-resistance coils. As the motor rotates, the current passing through the coils generates heat due to their low resistance, causing the bimetallic strips to warm up. The amount of heat generated depends on the current carried by the motor. If the motor current exceeds a specific level, the temperature of the bimetallic strips reaches a critical point. The bimetallic strips, made of two different metals, bend on one side due to the heating, exerting pressure on a tripping mechanism. Consequently, the tripping mechanism disconnects the linkages between the 3-phase input power line and its output. This action interrupts the 3-phase power supply to the motor through the overload relay. The relay remains in the tripped state until the trip mechanism is reset. To reset the tripped overload relay, a dedicated reset button is provided (refer to the picture below). It is necessary to reset the trip mechanism to restore or reactivate the overload relay. The following image shows an overload relay and its components. 

An overload relay includes separate normally closed (NC) and normally open (NO) contacts (as shown in the picture) that control the motor's control circuit and sometimes provide signals indicating the relay's status (tripped or not). The signals from the NC or NO contacts can be used as input signals to a PLC to detect whether the motor is overloaded. The overload relay is equipped with a "trip current adjust" knob, which allows setting the maximum current that can pass through the relay. A "trip indicator" arrangement is also present, indicating the tripped state of the overload relay externally. For testing purposes only, the overload relay can be manually tripped using the "trip test" button.

Miniature Circuit Breaker or MCB 

A Miniature Circuit Breaker or MCB is a protective device used to safeguard against electrical overload and short circuit situations. It can be used as an alternative to an electrical fuse and a bimetallic overload relay in an electrical circuit. When an electrical overload or short circuit occurs, the MCB automatically trips and disconnects the incoming and outgoing supply lines. This prevents severe damage to devices connected to the outgoing circuit. The MCB must be manually reset after tripping to restore the connection between the supply lines. MCBs are categorized into different types based on the number of poles or contact points, such as single-pole, double-pole, and triple-pole MCBs. Single-pole and double-pole MCBs are used with 24V DC and 220V AC supplies, while three-phase induction motors are connected through a triple-pole MCB. The maximum current allowed to pass through an MCB is predetermined and cannot be changed. The picture below illustrates the internal view of a single-pole circuit breaker.

A triple-pole circuit breaker consists of three separate single-pole breakers assembled together, with individual switch levers fastened collectively. If one MCB trips, the other individual MCBs also trip. Triple-pole MCBs are commonly used for powering three-phase induction motors. Sometimes, an auxiliary trip contact is associated with the MCB to send a signal to the PLC, indicating the tripped MCB. The following picture illustrates how a triple-pole miniature circuit breaker is connected to an induction motor through an overload relay.  

Motor Protection Circuit Breaker or MPCB:

The basic working principle of a Motor Protection Circuit Breaker or MPCB is similar to that of a miniature circuit breaker or MCB. However, an MPCB is specifically designed to protect three-phase induction motors. In the event of an electrical overload or short circuit, the MPCB automatically trips and disconnects the incoming and outgoing electric circuits, thereby safeguarding the connected motor. MPCBs are commonly used in applications where induction motors require frequent switching on and off. They are often employed with CNC machines to power motors used for coolant, lubrication, chip conveyor systems, and other purposes. Some MPCBs are equipped with an auxiliary normally closed (NC) tripping contact, which allows the trip indicator signal to be fetched from that point, enabling the PLC to identify the tripped MPCB. The rating of an MPCB is selected based on the maximum current rating of the induction motor it is intended to protect. There is also a current setting arrangement that allows slight adjustment of the tripping current. MPCBs provide protection to induction motors in case of abnormal conditions in the outgoing electrical circuit, such as overload, short circuits, or single-phasing. The following picture shows an MPCB commonly found in CNC machines.    

Electrical fuse:

An electrical fuse is a protective device used to prevent overcurrent or overload situations in an electrical circuit. Different types of electric fuses are available and used for various purposes, including CNC machines. Some fuses are installed in the incoming supply of CNC controllers and servo amplifiers, while others are found inside the controller and drive amplifier circuits. In most cases, heavy-duty cartridge fuses are commonly used for the incoming supply to the controller and servo amplifiers, while surface mount fuses (SMD fuses) are located on various circuit boards of CNC controllers and servo amplifiers. A typical fuse known as the "Daito fuse" is sometimes used inside the servo amplifier. It is arranged within a small transparent container and is usually a plug-in type. Special Daito fuses are also available, including those with an alarm generation system. The following pictures show some special fuses related to CNC machines and other electrical circuits.