Preventing and protecting your industrial or commercial firm is essential to maintaining it in excellent functioning order. To do this, a variety of devices are used, both to maximize function and to prevent common difficulties and issues that arise during operation. A motor starter is one such tool.
- 1 What is a Motor Starter?
- 2 How A Starter Motor Works
- 3 Motor Starter Types Based on Starting Techniques & Methods
- 4 Types of Motor Starters
- 5 Motor Starter Parts
- 6 Why Do We Need A Motor and A Starter?
- 7 Conclusion
What is a Motor Starter?
A motor starter is an electronic switch that turns on or starts a motor, making it safe to start and stop.
The type of motor determines whether or not a starter is required. Low-power motors, in general, do not require starters, albeit the definition of “low power” is controversial. Small dc motors that run on low voltages (24 V or less), for example, do not require starters. Low-horsepower motors (less than 5 hp) are sometimes considered not to require starters as well.
The amount of current drawn during startup is the most crucial aspect. The current is equal to the applied voltage divided by resistance, according to Ohm’s law. If the motor’s supply voltage is high and the resistance is low, the inrush current can reach 100s of amps, damaging the motor and leading it to fail.
The primary function of a motor starter is;
- To safely start a motor
- To safely stop a motor
- To reverse the direction of a motor
- To safeguard the motor against low voltage and overcurrent.
How A Starter Motor Works
To understand how starters work, we must first look at the two basic components present in all models: an overload protection circuit and an electrical contractor.
When we think of a switch or relay, we usually think of an electrical contractor. This electromechanical device can make or break a circuit connection between two places. As a result, while it’s on, the circuit is completed, and when it’s off, the circuit is disrupted, preventing electricity from flowing.
The overload protection circuit is a device that safeguards the motor’s performance. If an overload occurs, the OPC can detect it and shut off the energy flow, preventing harm to the motor or other appliances connected to the supply.
In a nutshell, an electrical contractor permits starters to function as power switches, while its protective circuit ensures that the motor is protected from power spikes.
Motor Starter Types Based on Starting Techniques & Methods
Induction motors are started using a variety of approaches in industries. Before we go into the many motors, let’s look at some motor starter ways.
Full Voltage or Across the Line Starter
Such starters directly connect the motor with the power line providing the total voltage. The motors attached to these staters have modest power ratings so that the voltage loss in the power line is minimal. They are used in an application where motors have low ratings & need to run in one direction.
Full Voltage Reversing Starter
By switching any two phases, the direction of a three-phase induction motor can be reversed. This starter uses two mechanically interlocked magnetic contactors with swapped phases for forward and backward movement. It’s employed when a motor needs to run in both directions, and contactors are used to control it.
Multi Speed Starter
To vary the speed of an AC motor, you need to vary the AC supply frequency or the number of poles (by reconnecting the windings in some) of the motor. Such starters run the motor at a few pre-selected speeds to meet its applications.
Reduced Voltage Starter
The most common starting approach is to lower the voltage at the motor’s start to prevent inrush current, damaging the motor’s windings and producing a significant voltage drop. These starters are for high-capacity motors.
Types of Motor Starters
We will discuss the following types of motors and their starting methods based on the above methods, with advantages and disadvantages.
- Direct Online Starter (DOL)
- Stator Resistance starter
- Slip Ring Motor Starter
- Autotransformer Starter
- Star Delta Starter
- Soft Starter
- Variable frequency drive (VFD)
There are numerous different types of motor starters; however, they are primarily divided into two categories.
This starter is operated by hand and does not necessitate any prior knowledge. The motor attached to it is turned off and on using a push-button. The button’s mechanism contains a mechanical switch that stops or starts the motor by breaking or making the circuit.
They also defend against overload. On the other hand, these starters lack LVP (low voltage protection), which means they do not break the circuit in the event of a power outage. Because the motor resumes when the power is restored, it can be dangerous for particular applications. As a result, they’re used in low-power motors. A manual starter with overload protection is a Direct On-Line (DOL) starter.
Magnetic starters are the most popular starter form and are typically used with high-powered AC motors. These starters work in the same way that a relay does, by using magnetism to break or form contacts.
It has a reduced and safer starting voltage and protection against low voltage and overcurrent. The magnetic starter instantly breaks the circuit after a power outage. It offers the automatic and remote operation, which eliminates the need for the operator, unlike manual starts.
The magnetic starter consists of two circuits;
a) Power Circuit
The power circuit is in charge of supplying electricity to the motor. It consists of electrical connections that, through an overload relay, turn on and off the power provided from the supply line to the motor.
b) Control Circuit
This circuit controls the contacts of the power circuit to make or interrupt the power supply to the motor. To pull or push the electrical connections, the electromagnetic coil energizes or de-energizes. As a result, the magnetic starter can be controlled remotely.
c) Reversing Motor Starter
Instead of having a single pair of contactors, a reversing motor starter has two sets. The primary function of these double contactors is to allow the system to rotate in either direction, depending on which link is active. To ensure that only one contactor closes at a time, these starters use both mechanical and electrical interlocks.
d) Soft Start Motor
The digital components of a soft start motor starter are the easiest way to identify it. These starters are made to increase the charge and speed provided to the engine gradually. This allows the motor to “warm-up” rather than going at full speed right once. This is usually done to avoid damaging the engine or overtaxing the electrical source.
e) Direct Online (DOL) Starter
The DOL, or Direct Online Starter, is the simplest motor starter since it links the motor to the power supply right away. It consists of a magnetic contactor that connects the motor to the supply line and an overload relay that protects the motor from overcurrent. There is no voltage reduction required to start a motor safely. As a result, the motor utilized in such starts is rated at less than five horsepower. The motor is started and stopped by two simple push buttons.
The coil that pulls the contactors together to shut the circuit is energized when the start button is pressed. The contactor’s coil is de-energized by pushing the stop button, and its contacts are pushed apart, breaking the circuit. The switch used to turn the power supply ON/OFF can be of any type, including rotary, level, float, and so on.
f) Stator Resistance Starter
A stator resistance starter employs the RVS (reduced voltage starter) technique to start a motor. A 3-phase induction motor’s stator has external resistance added in sequence with each phase. The resistor must lower the line voltage applied to the stator, lowering the initial current.
The variable resistor is initially set to its maximum setting, providing maximum resistance. The voltage across the motor is kept to a bare minimum. This is done thanks to the voltage drop across the resistor (safe level). The low stator voltage reduces the inrush current that can harm the motor windings when it starts up. The resistance decreases as the motor speed increases, and the stator phase is directly coupled to the power lines.
Because current is directly proportional to voltage and torque is proportional to the square of the current, a two-fold decrease in voltage results in a four-fold decrease in torque. As a result, the starting torque produced by such a starter is extremely low and must be maintained.
g) Rotor Resistance or Slip Ring Motor Starter
A complete voltage motor starter is used in this sort of motor starter. It is also a slip ring motor starter since it only works on a slip ring induction motor.
The slip ring connects external resistances to the rotor in a star configuration. These resistors limit the rotor current; thus, the torque is increased. As a result, the initial stator current is reduced. It also aids in the enhancement of the power factor.
The resistors are only utilized when the motor is starting, and they are withdrawn after the motor has reached its rated speed.
h) Autotransformer Starter
An autotransformer is utilized as a step-down transformer in this sort of motor starter to reduce the voltage given to the stator during the starting stage. It works with both delta and star motor connections.
Each phase of the motor is connected to the secondary of the autotransformer. The autotransformer’s many tapings give a portion of the rated voltage. The relay is in the start position, i.e., the tap point, which decreases the startup’s voltage. Finally, it connects it to the rated voltage’s maximum.
It provides higher voltage for a specific beginning current than other voltage reduction techniques. It contributes to a higher beginning torque.
i) Star Delta Starter
This is yet another frequent approach for starting huge motors in the industry. The windings are switched from star to delta connection to start a three-phase induction motor.
A triple pole double throw relay starts the induction motor in a star configuration. A factor drops the phase voltage of 1/3 in a star connection, and the starting current and torque are reduced by 1/3 of their typical rated values.
When the motor speeds up, a timer relay converts the stator windings’ star connection to the delta connection, allowing the total voltage to be applied to each winding. The motor is set to its maximum speed.
Motor Starter Parts
A contractor and an overload protector are both included in every motor starter.
For motor starters, a contractor provides current. A small current on a coil opens or closes contacts that allow a larger current to flow across a circuit, similar to how a relay works. A factor drops the phase voltage of 1/3 in a star connection, and the starting current and torque are reduced by 1/3 of their typical rated values. Remote starting is possible, which keeps workers safe by keeping them away from the motor and any potential malfunctions that could result in significant damage.
An overload protector prevents the motor from drawing too much current, which could cause it to overheat and destroy it. It usually guards against a long-term overcurrent problem. In most overload protection units, a current detecting circuit detects the amount of current flowing through the motor. Users can set the maximum current level on certain types of overload protection equipment, such as electronic ones. To avoid nuisance tripping, some manufacturers allow designers to program a small overflow current. Thermal protection units, for example, need the insertion of a thermal element rated for the specified maximum current.
Motor starts can be classified as either manual or magnetic. Manual starts are activated by mechanically pressing a button or toggle device attached to the contactor. On lower voltage loads, manual starts are common. On the other hand, magnetic motor starters have the advantages of remote starting and autonomous operation.
Why Do We Need A Motor and A Starter?
An induction motor requires a motor starter to start. It’s because of the rotor’s low impedance. The rotor impedance of an induction motor is determined by the slip or the difference in speed between the rotor and the stator. The slip has an inverse relationship with the impedance.
At a standstill (rest position), the induction motor’s slip is at its highest, i.e., 1, the impedance is at its lowest, and it draws a large quantity of current known as inrush current. The strong inrush current magnetizes the air gap between the rotor and the stator, causing the rotor winding to create an EMF. An electrical current flows through the rotor winding as a result of the EMF, creating a magnetic field that generates torque in the rotor. The motor’s slip decreases as the rotor speed increases, and the current drawn by the motor decreases.
Inrush current is 5-8 times the rated full load current. Such a high current can damage or burn the motor’s windings, rendering the machine worthless. It can also generate a significant voltage drop in the supply line, causing harm to other appliances connected to the same line.
To safeguard the motor from such a large amount of current, we employ a starter that reduces the initial current for a short time in the beginning and then resumes the regular power supply after the motor reaches a specified speed. They also protect against fault circumstances such as low voltage and overcurrent during regular operation.
Although small motors with a power rating of less than one horsepower have a high impedance and can sustain the initial current, they require an overcurrent protection system provided by DOL (Direct On-Line) starters.
The primary purpose of a motor starter is to make it simple to turn motors on and off as needed. However, they can take on more advanced tasks depending on the starter. They’re also essential pieces of machinery in any industrial setting because of their ability to adjust voltage, turn themselves back on, and reverse a motor. Motor starters appear to be simple at first, but they are among the most dependable instruments available.
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