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For starting an induction motor a motor starter is necessary. That is due to the low impedance of the rotor. The impedance of the rotor depends on the induction motor slip, which is the relative speed from the rotor to the stator. The impedance of the slip varies reverse.
At standstill (rest position), the induction motor's slip is at its highest, i.e. 1, so the impedance is at its lowest, and it draws a large amount of current known as inrush current. The high inrush current magnetizes the air gap between the rotor and the stator, causing the rotor winding to cause an EMF. This EMF causes an electrical current to flow through the rotor winding, creating a magnetic field that generates torque in the rotor. The slip of the motor decreases as the rotor speed increases, and the current drawn by the motor decreases.
The high inrush current is 5-8 times the full load current rated. Such a high current can harm or burn the motor's windings, rendering the machine useless, and it can also cause a significant voltage drop in the supply chain, causing damage to other equipment connected to the same line.
To protect the motor from such a large amount of current, we use a starter that restricts the initial current for a short time at startup and then resumes normal power supply once the motor reaches a certain rpm. During normal service, they also guard against fault conditions such as low voltage and overcurrent.
Although small motors with a power rating of less than one horsepower have a high impedance and can withstand the initial current, they do need an overcurrent protection system, which is given by DOL (Direct On-Line) starters.
A starter is a control system that allows you to manually or automatically turn the motor on and off. Its connections are made or broken to safely regulate the ON/OFF of electrical motors.
motor starter
The manual starter is used for smaller motors with a hand-operated lever that is manually moved to the ON or OFF position (moving the position of the contact). The downside of these starters is that they must be turned on after the electricity has been turned on. In other words, each (ON or OFF) process requires manual control. This operation can cause high currents to flow in the motor winding, potentially causing the motor to burn out. This is why, in most instances, it is not recommended when other motor starters with safety, such as automatic starters, are used.
The automatic starters, which are made up of electromechanical relays and contactors, are used to turn the motor on and off. As current flows through the contactor coils, it energizes them and creates an electric field that pulls or moves the contacts, allowing the motor windings to be connected to the power supply.
Motors can be turned on and off using the start and stop pushbuttons connected to the motor and starter. The contactor coils can be de-energized by pressing the stop button, which causes the coil to become de-energized. As a result of the spring configuration, the contactor contacts return to their natural location, resulting in the motor being switched off. In the event of a power outage or manual switch-off, the motor will not run unless we manually start it by pressing the "start push button." The diagram below depicts how a DOL motor starter works in ON/OFF mode.
For small to medium-sized single- and three-phase motors, manual motor starters are used in so-called full voltage, across-the-line applications. A manual motor starter, which consists of an on/off switch and an overload relay, does not usually allow for motor power disengagement in the event of a power outage, which may be advantageous for small pumps, fans, and other devices that will resume operation once power is restored. Manual motor starters with under-voltage safety de-energize the starter circuit after a power outage, making them ideal for conveyors and other applications where an automatic restart could harm both equipment and staff. Manual motor starters with under-voltage protection are used on machine tools, woodworking equipment, and other applications where the motor must shut down after a power outage for safety reasons. Standard sizes and NEMA and IEC specifications are available for manual motor starts.
Instead of using mechanical latching of on/off switches like manual starters, magnetic motor starters use electromagnets to close and retain contactors. They're used as reduced voltage starters for single- and three-phase motors in across-the-line applications. After a power loss or low voltage condition causes the contactor to drop off, magnetic motor starters with momentary-contact pilot devices (switches, relays, etc.) require a restart. Magnetic motor starters can also be wired to automatically restart motors if the program needs it, such as a remote pump. Magnetic motor starters are available in standard sizes and NEMA and IEC configurations.
Reversing starters have two sets of contactors that provide motors with reversed leads so they can turn in either direction. Electrical and mechanical interlocks are usually used in reversing starters to prevent both sets of contacts from closing at the same time. They come in regular NEMA dimensions.
Soft starters give electro-mechanical starters digital control and allow motors to be brought up to speed sequentially, preventing damage to drive trains, goods, and the electrical distribution system from the high inrush current of medium and large motors starting at full voltage.
Combination starters are usually housed systems that provide disconnects and short circuit safety (in the form of fuses or circuit breakers) in addition to the motor starter's components.
Motor starters are specialized electrical devices that are designed to manage the high electrical current that motors draw when they are started from a standstill while also shielding the motors from excessive heat caused by overloads during normal operation. The motor's starting current can be many times that of its working current. Each start would blast or trip a fuse or circuit breaker if only a fuse or circuit breaker was used.
Instead, when the motor is exposed to a high "inrush" current, thermal or magnetic overload relays are used to add a time delay during startup. If the motor jammed (a so-called locked-rotor scenario), it would continue to draw this inrush current. In this scenario, the overload relays will heat up beyond the time allotted for usual momentary inrush speeds, causing the switch or contactor to fail, and the motor to stop running.
Motor starters may be open setups that are mounted in control panels or standalone systems with their own enclosures that are either NEMA or IEC certified. NEMA standard sizes range from 00 to 9, including a wide range of motor sizes from 1.5 horsepower to 900 horsepower.
Manual motor starters are limited in the size of motors they can start, usually starting at fractional HP levels and increasing to a maximum of 10-15 HP, depending on voltage. They're typically used for equipment that isn't started very often or that runs continuously with few pauses. Beyond that, specifiers should think about magnetic or even soft starters. Application-specific types handle special cases such as reversing or multi-speed operation. Aside from motor size and voltage, other factors to consider include explosion-proof, enclosure ratings, fuse or breaker safety, and so on.
NEMA and IEC ratings are available from most starter manufacturers. NEMA starters are typically larger and more expensive than IEC starters, but they can be specified solely on the basis of horsepower and voltage, whereas IEC starter specifications are more precise. For more details, see the reference below. In general, North American engineer-constructors can define NEMA or IEC applicability, and specifiers can choose from the required vendor offerings in these two ranges when making new purchases. The ability to finely tune the starter to the application, necessitated by the IEC's more elaborate selection requirements, is why machinery builders in North America sometimes use IEC starters in their control panels.
Specifiers can typically choose an enclosure setup, appropriately sized starter and overload relays, control voltages, communications options, and suitable pilot devices (lamps, e-stops, hand/off/auto selector switches, push switches, and so on) when selecting a combination starter. Short circuit safety can be fused or circuit breaker and specifiers can choose between the two. Many manufacturers keep typical units on hand and can ship them out quickly.
Soft starters, which use solid-state electronics to regulate starting currents, are more like AC motor drives than conventional starters. They can also be programmed to monitor the motor's start-up. They are available in both open and enclosed configurations.
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