A motor starter is an electrical device that is used to start & stop a motor safely. Like a relay, the motor starter switches the power ON/OFF, but unlike a relay, it also provides thermal overcurrent protection.
The main function of a motor starter is:
- To safely start or stop a motor.
- To reverse the direction of a motor.
- To protect the motor from low voltage & overcurrent.
A motor starter consists of 2 main components:
- Contactor: The main function of the contactor is to make or break the current to an electric motor.
- Overload Relay: Overheating and drawing too much current can cause damage to the motor. Overload relays prevent this from happening and protect the motor from any potential danger.
A motor starter typically has two circuits wired to it:
- Power Circuit: The power circuit passes the main voltage to the motor through the starter contacts and overload relay. The motor current is carried by the power contacts of the contactor.
- Control Circuit: This circuit operates the contactor coil, which creates an electromagnetic field. The power contacts are pulled by this electromagnetic field to a closed position. This allows power to flow to the motor from the main voltage. This way, remote operations are made possible by the control circuit. The control circuit can be wired two different ways:
- Two-wire method: Utilizes maintained contacts such as selector switches, sensors, thermostats, float switches, etc.
- Three-wire method: Uses a holding circuit contact, and momentary contact pilot devices such as pushbuttons.
The control circuit can derive power three ways:
- Common Control: When the incoming voltage of the control circuit is the same as the motor.
- Independent Control: The control circuit power comes from a separate exterior voltage.
- Transformer Control: The incoming voltage is reduced by a control circuit transformer to a lower, safer control voltage.
Types of Motor Starters
Below you'll find more detailed information on different types of motor starters. If you are unsure which type of motor starter is best suited to your application or have further questions, our experts would be happy to discuss your specific needs with you.
Manual Motor Starters: The starter includes a button (or rotary knob) which allows a user to turn the motor on or off. The mechanism behind the button includes a switch that breaks or makes the circuit to stop or start the motor. This type of starter does not break a control circuit in the event of power loss. It can be dangerous for some applications because the motor restarts automatically when the incoming voltage is restored.
Features of a manual motor starter:
- East and economical install cost
- Compact
- Reliability
- Overload protection to the motor
Magnetic motor starters rely on energizing a coil to close and hold the contacts. During a power failure the magnetic starter automatically breaks the control circuit, and the holding contacts open removing power to the motor. A magnetic motor starter includes protection against low voltage & overcurrent. A combination motor starter is an enclosed unit that incorporates disconnects and short circuit protection (in the form of fuses or circuit breakers) together with the components of the motor starter.
Magnetic motor starters are often referred to as Direct Online (DOL).
This type of magnetic starter has a coil to pull in the contacts but operates the coil with the main incoming voltage. There is no voltage reduction for safe starting a motor, therefore the motor used is 5 HP and below. It has two simple push buttons that start and stop the motor. The overload relay provides protection from overheating and overcurrent.
Advantages of a Direct Online motor starter:
- Easy and economical install cost
- Compact
- Reliable
- Overload protection to the motor
Disadvantages of a Direct Online motor starter:
- Limited to 5 HP and below
- High inrush current can cause damage to the motor windings and potentially a voltage drop in the power line.
- Lifespan of motor can decrease
They use momentary contact pilot devices such as switches, pushbuttons, relays, float switches, etc. These starters require a restart after a power loss or low voltage condition causes the contactor to drop off. This motor starter can also be wired to restart motors automatically if the application requires it (e.g., a remote pump). A control circuit transformer is used to step incoming power into a lower, safer control circuit voltage.
Advantages of a ATL / FVNR motor starter:
- Easy and economical install cost
- Compact
- Reliability
- Safer lower control circuit voltage
- Provides overload protection to the motor.
Disadvantages of a ATL / FVNR motor starter:
- High inrush current can cause damage to the motor windings and potentially a voltage drop in the power line.
FVR starters are used in an applications where the motor needs to run in both directions. Reversing can be achieved with pushbuttons with holding contacts, or a maintained selector switch.
This starter incorporates two mechanically interlocked magnetic contactors, which prevent both contactors from closing at the same time. Phases are swapped for forward & reverse direction.
Advantages of a FVR motor starter:
- Easy and economical install cost
- Reverses a motor
- Multiple reversing methods (with pilot devices)
- Safer lower control circuit voltage
- Provides overload protection to the motor.
Disadvantages of a FVR motor starter:
- High inrush current can cause damage to the motor windings and potentially a voltage drop in the power line.
Reduced Voltage Auto Transformer starting is used widely in starting large industrial motors due to its advantage of providing the highest starting torque with lowest starting line current. An Autotransformer Starter is suitable for both star and delta connected motors. This type of motor starting involves inserting an autotransformer in between the motor and the source. The incoming voltage to the autotransformer input is reduced in proportion to the ‘tap’ setting of auto transformer at the output.
Thermal overload relays protect from overcurrent and overheating.
Auto Transformer provide a better starting torque by offering high voltage for a specific starting current.
Advantages of an Autotransformer Starter
- Provides a better starting torque.
- Used for starting large motors with a significant load.
- Manual speed control.
- Flexibility in starting characteristics.
Disadvantages of an Autotransformer Starter
- Large size
- Complex and costly circuitry
There are three main contactors that play critical role in auto transformer reduced voltage starting. They are the start (S) contactor, main (M) contactor, and the run (R) contactor. The start contactor connects all three phases of the system together to create a neutral point. This ‘neutral’ will be internal to the starter and will be at zero voltage potential.
Wye-Delta / Star-Delta starters are the most common reduced voltage starters. They reduce the start current applied to the motor during startup as a means of reducing the voltage drop on the electrical supply line. Voltage reduction during Wye-Delta, also known as Star-Delta, starting is achieved by physically reconfiguring the motor windings. During the startup of the motor, windings are connected in Wye / Star configuration, and this reduces the voltage across each winding. This also reduces the torque by a factor of three.
After a timing period the winding are reconnected with a delta configuration and runs this way until a stop command is given.
Advantages of a Wye-Delta Starter
- Inexpensive
- Simple and rugged design
- Good torque/current performance
- Draws twice the normal starting current of the motor connected
Disadvantages of a Wye-Delta Starter
- Open transition has a break in supply voltage potential transient current.
- Requires a six-terminal motor and six motor cables to starter.
- Low starting torque
There are two methods for transitioning from Wye/Star connection to Delta connection:
Open transition: There is an open state between the Wye/Star state and the delta state. In open transition the power is disconnected from the motor while the windings are reconfigured via external switching. If not properly transitioned can cause a changeover transient current which can be damaging to the equipment.
Closed transition: This requires the use of a fourth contactor and a set of three resistors. The resistors are sized to dissipate considerable current allowing a safe transition of power to the motor.
Reduced Voltage Soft Starter ( RVSS )
A soft starter is another reduced voltage method. During the typical ramp up phase of a motor, a smooth acceleration of power, instead of a sudden and violent burst of power, reduces the strain put on the motor that could potentially cause damage to the motor and the machine it is operating. Certain applications require a smooth operation that other types of reduced voltage starters cannot provide.
Soft starters use a series of silicon controlled rectifiers (SCR), or thyristors, to limit the voltage to a more manageable amount for the motor as it begins to start up. These SCRs have an ON state, where they allow the current to flow, and an OFF state, where they control and limit the electrical current. When you power up your machine, these SCRs activate, restrict the voltage, and then relax as the machine reaches full power. This keeps the motor’s heat down and reduces overall strain.
Advantages of a Soft Starter
- Fast investment payback/inexpensive
- Simple and rugged design
- Easy to install
- Built in protection
- Programmable current and torque limits
- Programmable acceleration and deceleration ramp curves
- Smooth ramp up and down
- Motors run cooler than other types of reduced voltage starters/ Which extends motor and equipment life.
- Many start and stops per hour
Applications for Soft Starters
- Fans
- Pumps
- Conveyors
- Compressors
- Mixers
- Stone Crushers
- Reduced energy uses
- Applications that require high starts and stops per/hr.
Disadvantages of a Soft Starter
- Cannot vary the motor speed. 100% motor rated RPM’s
A Variable Frequency Drive ( VFD ) is an electronic device used to vary the frequency of an AC voltage to adjust the speed of an AC motor. VFDs also provide start and stop control, acceleration and deceleration, and overload protection.
Benefits of using a VFD
- Saves energy.
- Reduce peak energy demand.
- Reduces power when not required to run full speed.
- Controlled starting, stopping, and acceleration.
- Provides smooth motion.
- Maintains speed of equipment.
- Fully adjustable speed
- Easy to apply to many types of industrial equipment.
- Self-diagnostics/communications on more advanced VFD’s
- Built in overload protection
- PLC-like functionality and software programming on some of the more advanced versions of VFD’s
Digital inputs/outputs (DI/DO)
Analog inputs/outputs (AI/AO)
Relay outputs
Disadvantages of a VFD
- Heat dissipation using fans of air conditioning in certain environments.
- Expensive
Applications for VFD’s
- Fans
- Pumps
- Conveyors
- Grinders
- Mixers
- Stone Crushers
- Elevator/Escalator
A VFD is comprised of three main components: A Rectifier, a Filter and DC Buss link, and an Inverter. What are their functions?
- AC supply: Comes from the facility power network (typically 208V, 230V, or 480V, 60 Hz AC)
- Rectifier: Converts network AC power to DC power
- Filter and DC bus: Work together to smooth the rectified DC
power and to provide clean, low ripple DC power to the inverter. - Inverter: Uses DC power from the DC bus and filter to invert an output that resembles an AC sine wave. This is achieved by using a pulse width modulation (PWM) technique. This AC voltage is then sent to the motor.
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