Introduction
Motor starters are one of the major inventions for motor control applications. As the name suggests, a starter is an
electrical device which controls the electrical power for starting a motor. These electrical devices are also used for
the purpose of stopping, reversing and protecting electric motors. The following are the two major components of a
starter:
THE BEST ELECTRICAL CONTROLS BUSINESS ON THE PLANET!
Unmatched Service | Superior Product Quality | Advantage Pricing
Page 1 of 9
1. Contactor: The main function of the contactor is to control the electric current to the motor. A contactor can
make or break power to the circuit.
2. Overload Relay: Overheating and drawing too much current can cause the motor to burn out and become
practically useless. Overload relays prevent this from happening and protect the motor from any potential
danger.
A starter is an assembly of these two components, which allows it to turn on or off an electric motor or motor
controlled electrical equipment. The starter also provides the necessary overload protection to the circuit.
Types of Motor Starters
There are several types of motor starters. However, the two most basic types of these electrical devices are:
1. Manual Starters
Manual starters are devices that are operated manually. These starters are extremely easy and
straightforward to operate and do not require expert intervention. The starter includes a button (or rotary
knob) which enables a user to turn the connected equipment on or off. The buttons feature mechanical
linkages, which make the contacts open or close, starting or stopping the motor. The following features of
a manual starter make it a preferred choice over other types:
• These starters deliver a safe, as well as economical operation.
• The compact size of these devices make them suitable for a wide range of applications.
• They provide overload protection to the motor, protecting it from any potential damage.
• These devices come with a vast choice of enclosures.
• The initial cost of the manual starter is low.
2. Magnetic Motor Starters
This is the other main type of motor starter. It is operated electromagnetically. It means that the motor
load connected to the motor starter is typically started and stopped using a lower and safer voltage than
the motor voltage. Just like other motor starters, the magnetic starter also has an electrical contactor and
overload relay to protect the device from too much current or overheating. 1. Power Circuit:
The power circuit connects the line to the motor. It provides transmission of electricity through the
starter contacts, overload relay, and then to the motor. The motor current is carried by the power
(main) contacts of the contactor.
2. Control Circuit:
This is the other motor starter circuit, which operates the contactor to turn it on or off. The contactor
main contacts are responsible for allowing or interrupting the flow of current to the motor. To do
this, the contacts in the control circuit are either opened or closed. The control circuit energizes
the contactor coil, which creates an electromagnetic field. The power contacts are pulled by this
electromagnetic field to a closed position. This completes the circuit between the motor and the line.
This way, remote operations are made possible by the control circuit. The control circuit can be wired
in the following two ways:
• Method 1: One of the most widely used methods employed for wiring the control circuit is
referred to as the ”Two-wire method”. A maintained contact type of pilot device like presence
sensor, thermostat, or float switch is used in the two-wire method of wiring the control circuit.
• Method 2: Unlike the two-wire method, the “Three-wire method” of wiring the control circuit
uses a holding circuit contact and the momentary contact pilot devices.
The control circuit can derive the power from either of the following three ways:
• Common Control: This type of control is when the power source of the control circuit is the
same as the motor.
• Separate Control: This is the most popular type of control. As the name suggests, the control
circuit derives power from a separate source in this arrangement. Generally, the power derived
is lower in voltage as compared to the motor’s power source.
• Transformer Control: As the name suggests, the control circuit derives power from a control
circuit transformer. Generally, the power derived is lower in voltage as compared to the motor’s
power source.
Types of Magnetic Motor Starters:-
The direct-on-line starter is the simplest form of motor starter, other than a manual starter. The controller
of this starter is typically a simple push button (but could be a selector switch, limit switch, float switch,
etc.). Pressing the start button closes the contactor (by energizing the contactor coil) connected to the
main supply and motor. This provides the supply current to the motor. To turn the motor off, a stop button
is provided. To protect it from over current, the control circuit is wired through a normally closed auxiliary
contact of the overload relay. When the overload relay trips, the normally closed auxiliary contact opens,
and de-energizes the contactor coil, and the contactor main contacts open.
The Advantages of Using Direct-On-Line Motor Starters:
• They have a compact design.
• They are cost-effective.
• They have a simple construction.
Features of Motor Starters:-
Today, motor starters are used on a large scale due to their list of beneficial features. The following are some
features of these highly useful electrical devices:
1. They facilitate the starting and stopping of the motor.
2. The starters are rated by power (horsepower, kilowatt) and current (amperes).
3. They provide the necessary overload protection for the motor.
4. The electrical device facilitates remote on/off control feature.
5. These devices allow you to make and break current rapidly (plugging and jogging).
Fundamental Functions of Motor Starters:-
1. Control: The control function is mainly carried out by the contactor component of a starter. It is controlling
the opening and closing of the power electrical circuit. The switching is done by the main contacts (poles) of
the contactor. An electromagnetic coil is energized, which open or close the contacts. This electromagnetic
coil has a nominal control voltage, and can either be an AC or DC voltage.
2. Short-Circuit Protection: In industrial applications, normal load current can be up to thousands of amperes.
In the case of a short-circuit fault, the fault current can go over 100,000 amperes. This can cause severe
damage to the equipment. The short-circuit protection disconnects the supply and prevents the potential
damage in a safe manner. Short circuit protection is provided by fuses or circuit breakers in a Combination
Motor Controller.
3. Overload Protection: When a motor draws more current than it is designed to, an overload condition
is caused. The main objective of an overload relay is to detect the excess currents. When an overload is
detected, the auxiliary contact of the overload relay opens the circuit and prevents the motor from burning
out or overheating. Electronic or electromechanical overload relays are used in combination with a contactor
to provide the required overload protection.
4. Disconnecting and Breaking: In order to prevent an unintended restart, it is required to disconnect the
motor from the main power circuit. In order to safely perform maintenance on a motor or starter, a motor
must be able to switch off and be isolated from the power. The disconnect switch of the circuit provides this
function. Disconnecting and breaking is provided by a disconnect switch or circuit breaker in a Combination
Motor Controller (or can be installed remotely from the starter).
Standards and Ratings:-
. There are many factors involved in motor starter ratings, such as thermal current, continuous current, motor voltage,
and power.
The thermal current is dependent on the thermal conductivity (k), which is the property indicating the heat conducting
ability of a material. It means that thermal current is directly proportional to the thermal conductivity.
The continuous current, which is also commonly referred to as the continuous ampere rating is a measure of the
capability of the motor control starter to handle current for a continuous time.
The power rating of the motor starter is based on the type of motor used. DC motor starters have ratings for DC
horsepower. On the other hand, AC motor starters have a single-phase power and three-phase power rating.
The rating of the motor starter is based on the size and type of load for which it was designed. Starters conform to
the standards and ratings from Underwriters Laboratories (UL), Canadian Standards Association (CSA), International
Electrotechnical Commission (IEC) and National Electrical Manufacturers Association (NEMA).
Comments
Post a Comment