Motor Starters Types of Method

Motor Starters

As their name implies, motor starters apply electric power to a motor to

begin its operation. 

They also remove power to stop the motor. 

Beyond merely switching power on and off starters include overload protection,

as required by the National Electrical Code. 

The code also usually requires a disconnect and short circuit protection on motor branch circuits.

Fused disconnects and circuit breakers provide this and are often incorporated into a motor starter enclosure, resulting in a unit referred to as a combination starter.

Across The Line Starting of Induction Motors

An across the line starter is the least expensive option and is usually

used for induction motors.

All NEMA design induction motors up to 200 HP, and many larger

ones, can withstand full voltage starts.

Manual starters are often used for smaller motors - up to about 10 HP.

They consist of a switch with one set of contacts for each phase and a thermal overload device. 

The starter contacts remain closed if

power is removed from the circuit and the motor restarts when power is reapplied.

Diagram 

Magnetic Starters

Magnetic starters are used with larger motors or where greater control is desired. 

The main element of the starter is the contactor, which is a set of contacts operated by an electromagnetic coil.

 

Energizing the coil causes the contacts A to close, allowing large currents to be initiated and interrupted by a control signal. 

The control voltage need not be the same as the motor supply voltage, and is often low voltage allowing the start and stop controls to be located away from the power circuit. 

Closing the starter button contacts energizes the contactor coil. 

An auxiliary contact, B, on the contactor is wired to seal in the coil circuit. 

The contactor de-energizes if the control circuit is interrupted by operating the stop button or if power is lost.

The overload contacts are arranged so an overload trip on any phase will cause all phases to open.

Contactors are rated for various operating voltages and are sized

according to motor HP and type of duty expected.

Diagram

Reduced Voltage Starters

If the driven load or the power distribution system cannot accept a

full voltage start, some type of reduced voltage or “soft” starting scheme must be used.

Typical reduced voltage starters are: primary resistance starters, 

autotransformers, part winding starters, wye-delta and solid state

starters.

These devices can only be used where low starting torque is acceptable.

Primary Resistance Starters

Closing the contacts at A connects the motor to the supply via resistors which provide a voltage drop to reduce the starting voltage available to the motor.

The resistor’s value is chosen to provide adequate starting torque while minimizing starting current.

Motor inrush current declines during acceleration, reducing the voltage drop across the resistors and providing more motor torque.

This results in smooth acceleration.

After a set period of time, contacts A open and the resistors are shorted out by contacts B, applying full voltage to the motor.

Diagram

Autotransformer Starters

An autotranformer is a single winding transformer on a laminated  core with taps at various points on the winding.

 

The taps are usually expressed as a percentage of the total number of turns and thus percentage of applied voltage output.

Three autotransformers are connected in a wye configuration or two in an open delta configuration, with taps selected to provide adequate starting current.

The motor is first energized at a reduced voltage by closing contacts.

Diagram

After a short time, theautotransformers are switched out of the circuit by opening contacts A and closing contacts B, thus applying full voltage to the motor.

Athe autotransformers need not have high capacity as they are only used for a very short period of time. 

Wye-Delta Starting

Wye-Delta Starting can be used with motors where all six leads of

the stator winding are available (on some motors only three leads

are accessible).

Diagram

By first closing contacts A and B, the windings are connected in a wye configuration which presents 57% of rated voltage to the

motor.

Full voltage is then applied by reconnecting the motor in a delta

configuration by closing contacts C and opening those at A.

The starting current and torque are 33% of their full voltage ratings,

limiting applications to loads requiring very low starting torque.

Part Winding Starters

Part winding starters are sometimes used on motors wound for dual

voltage operation such as a 230/460 V motor. 

These motors have two sets of winding connected in parallel for low voltage, and in series for high voltage operation.

When used on the lower voltage, they can be started by first energizing only one winding, limiting starting current and torque to approximately one half of the full voltage values.

The second winding is then connected normally once the motor nears operating speed.