Induction motors are extensively used in industrial applications that demand a certain level of control over it. Motor control technologies are widely used to incorporate motors in different scenarios. From rotating the ordinary ceiling fan to playing an important role in circulating fluid in the process industry through pumps; motors have high demand in today’s changing world.
Let’s start with the most basic question.
What Is A Motor?
A motor is an electromechanical device used to produce rotatory motion by the action of the electromagnetic field. This is used to convert electrical energy into mechanical energy.
An induction motor has a stator winding and a rotor winding. When a 3-phase AC is supplied to the stator winding, it produces a rotating magnetic field. As this changing magnetic field cuts the rotor winding, it produces an opposing current and starts rotating the rotor to oppose the change in the magnetic field. This causes the rotor to rotate and produce rotational motion.
Motor Control Circuits And Devices
All large and small scale industries have a motor control center (MCC) panel to control a motor. Various circuits and devices are enclosed in it, some of which have been discussed here.
A control circuit is a circuit used to control the motor operation and provide necessary protections. A control circuit uses 220V or 120V to operate. This voltage can be taken from any phase of the three-phase used.
A contactor is an electromechanical device used to switch an electrical load. It is widely used for controlling the switching of an electric motor. Contactors are used for high power or high current (greater than 10A) applications.
A contractor has the following 03 contacts.
- Power contacts: It is a 3 –phase normally open (NO) contact, used to switch on and off an electrical load such as a motor.
- Auxiliary Contacts: These are NO (normally open) or NC (normally closed) contacts. They are used in the control circuit. They are used to indicate the start/ stop/ fault conditions of an electrical load.
- Supply Coil: It is a 120VAC or 220VAC coil, which controls the power contacts and auxiliary contacts when supplied current.
The contractor has a contactor coil which acts as an electromagnet. When an alternate current (AC) or a switching direct current (DC) passes through the coil, it produces an electromagnetic field. This electromagnetic field moves the moving coil towards the fixed coil and the power contacts are changed from NO (Normally Open) contacts to NC (Normally closed) contacts.
Motor Starting Methods
On operating an induction motor, as it starts reaching full load speed, it conducts 5-6 times more starting current than normal operating current to produce high torque. This could burn the motor windings and the repair work may be heavy on the pockets. Different techniques are used to control and reduce the starting current. High starting current may result in fluctuations and voltage dip in the system.
1. DOL (Direct Online Starter) Method
This method is used for low power motors whose high starting current does not affect the connected system. Generally, motors less than 75kW can be operated on DOL, but a study of the system must be carried out to know the amount of inrush current the system can handle.
In the DOL method:
- A motor is started through a contactor
- A circuit breaker or isolator is used at the start of the line.
- It is used to isolate the circuit in case of a fault or maintenance.
- After the circuit breaker, 3-phase NO (Normally Open) power contacts of the contactor are connected. The contactor switches the NO (Normally Open) contacts to NC (Normally closed) once power is supplied to the supply coil through the control circuit.
- An overload relay is used to protect the circuit from overload current.
2. Stat Delta Starter Method
The motor in delta connection takes 3 times more current than a motor connected in star.
To reduce the starting current taken by a motor, it is connected in star connection while starting and switched to delta connection in normal operation.
- 03 contactors are used i.e.
- Main Contactor: It is used to turn on/ off the 3-phase supply to the motor. This contactor is controlled directly by the control circuit.
- Star Contactor: This contactor is used to start the motor in star connection.
- Delta Contactor: This contactor is used to operate the motor in delta connection during normal operation.
- Timer: A timer is used in the control circuit. Its NC (Normally Closed) contact is connected to the power contacts of star contactor and NO (Normally Open) contact is connected to the power contacts of the delta contactor. Initially, supply is provided to the motor through star contactor. After the set time, the timer changes its contacts (i.e. NO to NC and NC to NO), and power is supplied through delta contactor for normal operation.
- Thermal Overload: A thermal overload relay is used in the control circuit. It disconnects the circuit in case of overload. As the current increases, temperature also increases and the bimetallic strip in the thermal overload detects it.
3. Autotransformer Motor Starting Method
In this method, an autotransformer is used to reduce the starting voltage. A decrease in voltage reduces the input current.
- The taps of an autotransformer can provide three levels of current depending on the rating of the motor connected to it. According to NEMA standards, these taps are usually 50%, 65%, and 80%.
- Initially, the transformer contactor is connected and a reduced current is supplied through the autotransformer.
- After achieving 80% to 95% of the rated speed, the transformer connection is opened and power is supplied through the DOL method.
4. Variable Frequency Drive (VFD)
A Variable Frequency Drive (VFD) is an inverter that is used to operate an induction motor on variable speed. They are also called AC Drives.
- For an induction motor, it is very important to keep the V/f (Voltage/ Frequency) ratio constant. Hence, as the frequency is decreased, the voltage decreases as well.
- Not only are VFDs used for controlling the speed of a motor according to its application, but they can also be used to limit the inrush current at the start. This is achieved by slowly ramping up the frequency up to 50/ 60 Hz on starting a motor. This reduces the starting voltage and current as well.
- It consists of a rectifier that converts AC into pulsating DC. For this diodes are used.
- A capacitor is used to filter out the irregularities and a constant DC is achieved.
- Solid-state devices such as transistors, IGBTs are used as a switching device to convert DC into AC of the required frequency. This is the inverter circuit. For this, a technique called pulse width modulation is used to control the frequency and resulting voltage. This allows an induction motor to be operated on variable speed.
Controlling an induction motor effectively allows it to be used in a wide range of industrial and domestic applications. Although it increases the complexity of a system, the advantages are far greater to overlook the disadvantages. The future is exciting for motor control technologies, as more work and effort are being put to bring forward compact, low cost, efficient, and user-friendly motor control technologies with greater flexibility for the future.