Electrical Motors and Control Systems: Types, Operation, and Applications
Three-Phase Asynchronous Motors: Rotor Winding
Three-phase asynchronous motors comprise a stator winding and a rotor winding, star-connected at one end, with three free terminal ends connected to three collector rings. These rings have three brushes that slide across them. The terminals of the brushes are connected to three-phase resistances external to the motor, allowing for regulation of the motor start. This type of motor has been widely used in medium-power applications (above 50 HP), achieving a low starting current (less than 2.5 times the nominal current) while obtaining a good starting torque (less than 2.5 times the nominal torque).
The resistors are formed by a set of properly connected plates to achieve the calculated ohmic value and power dissipation.
Braking Systems of Electric Motors
Current Braking: In this type of braking applied to a three-phase motor with a wound rotor, two possibilities are studied for its winding connection. The current (Icc) will brake to a value equivalent to the effective alternating current (Ief) absorbed by the motor at its nominal value.
Pneumatic Brake: In this case, braking is achieved by pneumatic (air) pressure, chosen according to the inertia to be stopped. The brake is driven by a double-acting pneumatic cylinder. One direction for stopping and another for releasing. The control of the maneuver must be synchronized with the maneuver of the engine to avoid gaps and lack of synchronism.
Hydraulic Brake: This form of braking is a variation of pneumatic braking in which air has been replaced by hydraulic fluid. Hydraulic braking is more assertive than air braking, as hydraulic fluid has lower compressibility than pneumatic fluid.
Mechanical Brake: In this case, braking is achieved by manual actuation on a lever that applies a shoe to a disk, causing braking. This is a rarely used case. The signs of starting or stopping the engine that drives the braking are received by the operator through indicator lights.
Three-Phase Motor with Shorted Rotor Connected to a Single Phase
This configuration is used for low-power three-phase motors. The starting torque is reduced to between 40% and 50% of the rated torque. Reversing the connection is achieved by changing the condenser. Power is 10% to 20% of what would correspond to a three-phase network connection. With a voltage of 230V and 50Hz, a condenser capacity of 70 microfarads per kW should be used. The capacitor is dimensioned for a voltage of 1.25 times the network voltage. For a 400V, 50Hz network, 20 microfarads per kW are needed. For a 132V, 50Hz network, 200 microfarads per kW are needed.
In a single-phase motor with a paper capacitor, the capacitor increases the phase difference between the currents in the main and auxiliary windings, thereby improving performance during the starting period.
Stepper Motors
A stepper motor is an electromechanical device that converts a series of electrical impulses into discrete angular movements. This means it can move a specific number of degrees (steps) depending on its control inputs. The step size can vary from 90° to as small as 1.8°. Stepper motors can be governed by impulses from logical systems.
They consist of a rotor with permanent magnets and a stator with exciting coils. The coils are part of the stator, and the rotor is a permanent magnet. All switching (or excitation of the coils) must be externally managed by a controller.
There are two types of permanent magnet stepper motors: bipolar and unipolar.
- Bipolar: These have four output cables (A, B, C, and D). Control is performed using an H-bridge.
- Unipolar: These have 5 or 6-wire outputs (A, B, C, D, and one or two common wires), depending on their internal wiring. This type is characterized by its simple control.
Servo Motor: A servo motor is an actuator that can be positioned at any point within its operating range and remain stable in that position. It consists of a DC motor, a gearbox, and a control circuit. Its operating range is usually less than a full turn.
Brushless Motors
A brushless motor is a synchronous electric motor that uses electronic switching instead of DC brushes to change the polarity in the rotor.
Its operation is based on sequentially feeding each of the stator phases in synchrony with the rotor movement. The permanent magnets follow the movement of the stator’s magnetic field, which in turn depends on the displacement of the rotor. The control elements include a sensor for switching angle, a resolver or a tachometer generator (for speed control), and a variable speed drive.
Transformers
A transformer is an electrical machine that can increase or decrease the voltage in an alternating current electrical circuit while maintaining the frequency. Transformers are based on the phenomenon of electromagnetic induction. In their simplest form, they consist of two coils wound on a closed core of soft iron or silicon iron. The coils or windings are called primary and secondary, depending on whether they are for the input or output of the system, respectively.
Rectifiers
A rectifier is an element or circuit that converts alternating current (AC) to direct current (DC). This is accomplished using rectifier diodes. Rectifiers can be half-wave, using only one of the half-cycles of the current, or full-wave, where both half-cycles are utilized.
Grounding Installation
A grounding installation is the direct electrical connection, without any protection or fuses, of a conductive element to a ground electrode formed by groups of electrodes buried in the ground.
The grounding should be in the form of a closed ring around the perimeter of the building at a minimum depth of 0.8 meters.
The value of the ground resistance should be such that no mass can lead to contact voltages higher than:
- 24V in conductive premises or locations.
- 50V in all other cases.
If the installation conditions could result in higher contact voltages, rapid elimination of the fault must be ensured by suitable current-operated tripping devices.
Due to its importance from a safety standpoint, any grounding installation must be compulsorily checked by the authorities at the time of commissioning.
Conductors Used in Indoor Installations
Conductors will be made of copper and insulated, at least for a rated voltage of 750V for rigid conductors and 440V for flexible conductors. The sections used are:
- 1.5 mm² for lighting.
- 2.5 mm² for sockets.
- 4 mm² for washing machines and water heaters.
- 6 mm² for ovens and stoves.
Protective Conductor: Made of copper with the same insulation as active conductors. It must be installed in the same conduit as the active conductors.
Identification of Conductors:
- Blue – Neutral
- Brown, Black, and Gray – Phase
- Yellow and Green – Protection
Installation System:
- Built-in facilities: Insulated cables under flexible conduit.
- Surface facilities: Insulated cables under flexible or rigid conduit, under closed protective channels, or busbar trunking.
Regulations:
- A single neutral wire cannot be used for multiple circuits.
- All conductors must be sectioned at any point in the installation from which they are derived, using an appropriate device such as a connecting terminal, to allow complete separation of each part of the circuit.
- Sockets in a room must be connected to the same phase.
- All materials placed on the installation’s conductors (machinery, trims, etc.) must be insulated.
- Embedded installations must use special boxes for embedding. If these are metal boxes, they must be insulated or grounded internally.
- Installation of these devices in metal frames is permitted, provided that the equipment is designed to prevent the metal frame from becoming energized and is connected to the grounding system.
- The use of such equipment in racks or walls made of wood or other insulating material will comply with ITC-BT 49.
Intensity and Voltage Relays
Intensity Relays: These devices detect electrical quantities, compare them to default settings, and emit a signal. The intensity is detected by an electromagnetic device. If the value exceeds the limit, it activates a pair of NO + NC contacts. Depending on the application, these contacts are retained in that position or return to their initial state when the intensity decreases.
Types: Overcurrent relay, indirect current measurement relay.
Applications: As a stress reducer to control the stages of motor starting in three-phase motors and the start of wound-rotor motors.
Voltage Relays: These devices detect electrical quantities, compare them to default settings, and emit a signal. They have the same operating system as intensity relays, except that they measure voltage at their terminals. If the relay is for three-phase voltage, all three phases are connected for control, and the relay itself is powered by one of the phases or a single-phase voltage.
Types: Undervoltage relay, phase voltage relay, and voltage selector relay.
Applications: Mobile machinery (agricultural equipment, construction, etc.).
Programmable Logic Controller (PLC)
A PLC (Programmable Logic Controller) is any electronic machine designed to monitor, in real-time, sequential processes in an industrial environment. Its management and programming can be carried out by electrical or electronic technicians without computer knowledge. PLCs perform logical functions such as series, parallel, timing, counting, and more complex operations like calculations and regulations.
Peripherals: Peripherals are not directly involved in the operation of the PLC but facilitate the operator’s work. The most commonly used peripherals are cassette decks, printers, EPROM memory cartridges, displays, operator panels (OP), and EEPROM.
Input Module: Sensors are connected to the input module. The state of the inputs is periodically transferred to the input image memory. The information received is sent to the CPU to be processed according to the program.
Output Module: The output module is responsible for activating and deactivating the actuators. Information sent from the inputs to the CPU, once processed, is sent to the output image memory. This information is then sent to the output interface to activate the connected actuators.
Digital and Analog Signals
Digital Signal: A signal that takes the values 0 or 1. Example: A switch that is at rest (value 0) or activated (value 1).
Analog Signal: A signal that can take different values within a range. Example: A temperature detector that can provide different values depending on the temperature.
- A bit is a digit in the binary numbering system.
- A byte is 8 bits.
- A word is 16 bits or 2 bytes.
- A double word is 32 bits or 4 bytes.
Distribution of Bits in a Byte and a Word
Example: Q0.2 = 1, Q0.4 = 1, Q1.3 = 1
Q0.0 Q0.1 Q0.2 Q0.3 Q0.4 Q0.5 Q0.6 Q0.7 Q1.0 Q1.1 Q1.2 Q1.3 Q1.4 Q1.5 Q1.6 Q1.7
Binary number: 0 0 1 0 1 0 0 0 0 0 0 1 0 0 0 0
Decimal value: 1 2 4 8 16 32 64 128 256 512 1024 … … … … …
Binary value = 00000100010100 = Decimal value = 1024 + 16 + 4 = 1044
Elements Used in Electrical Assemblies
Thermal Relay: An overload detection device based on its thermal effect.
Fuses: Protective devices against overcurrents. Applied to the protection of electric motors, they supplement the protection provided by the thermal relay. An important feature of a fuse is its rated current, defined as the current value above which the fuse will melt in a given time.
Circuit Breaker: A circuit breaker disconnects power from the network manually, when subjected to an overload, or when a short circuit occurs in the power line.
Pushbuttons: Electrical control devices that, when manually operated, close or open one or more contacts and return to their original position when released.
Selectors: Switches with two or more stable positions that are maintained after operation. Similar to a switch, but a pushbutton is monostable, while a selector has two or more stable positions.
Switches: Electrical control devices that, when operated, make or break a connection and maintain their position until a new action is performed.
Limit Switches: Electromechanical switching devices similar to pushbuttons but are operated by elements of machines or components instead of manually by an operator.
A contactor is an electromechanical component designed to establish or interrupt the flow of current, either in the power circuit or the control circuit.
Proximity Detectors
Proximity switches are switches that cause the static connection or disconnection of a load due to the proximity of materials. There are three main types of proximity sensors:
- Inductive: These consist of an open magnetic core that embraces a coil. A high-frequency electric current runs through the coil. When a conductive element is brought near the sensitive area, there is a damping of the frequency. An amplifier detects this change and converts it into a connection or disconnection signal. Applications: Hydraulic presses, welding, presses.
- Capacitive: When an object approaches the sensor, the probe’s capacitance increases, causing the output circuit’s oscillator to trigger. These are used for non-metallic materials.
- Photoelectric: These use photocells as sensing elements. Their main use is as position sensors. Operation is based on the generation of a light beam by an emitter, which is projected onto a photoreceptor or a reflective device.
- Photoelectric Barrier: Has a separate transmitter and receiver.
- Photoelectric Reflex: Combines the emitter and receiver in one unit and requires a reflector device that reflects light in the same direction it comes from. This increases the sensing distance compared to the barrier type.
- Photoelectric Self-Reflexive: The object itself acts as a reflector. It has a lower detection distance than the other two but offers high response speed and color identification capabilities. Applications: Welding, assembly, presses, hydraulics, quality control.
- Magnetic: These use the magnetoresistive effect, where a magnetic material changes its resistance in the presence of an external magnetic field. They provide an excellent way to accurately measure linear and angular displacements. These sensors are mainly applied in detecting the position of pneumatic cylinders. Applications: Moisture detectors, PowerPoint presentations.
Difference between PNP and NPN:
- A sensor with an NPN output activates the output by connecting or disconnecting the load to the positive power supply.
- A sensor with a PNP output activates the output by connecting or disconnecting the load to the negative power supply.
Definitions of Electrical Components
Switch: An electrical control device that, when manually operated, makes or breaks a connection and maintains that state until operated again.
Pushbutton: An electrical control device that, when manually operated, opens or closes one or more contacts and returns to its original position when released.
Single-Phase Outlet: Consists of a phase, neutral, and protective (earth) connection. Used with a single-phase voltage of 230V.
Three-Phase Outlet: Consists of three phases and an earth connection. Used with a three-phase AC voltage of 400V.
Four-Wire Outlet: Consists of three phases and a neutral connection. The voltage between phases is 400V, and the voltage between phase and neutral is 230V.
Protective Conductor: Recognized as the grounding conductor and is characterized by its yellow-green color.
Bonding: The connection of all metal parts to the grounding system.