Starting methods for three-phase AC motors

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Electric motor knowledge: starting methods for AC 3 phase

Starting methods for AC 3 phase motors are divided into  2 types:

1. Direct motor starting ( Direct on line starting) 
means starting at full rated voltage ( Full-Voltage Starting)   This method is widely used for small motors. The motor is connected through a starter directly to the power supply, immediately receiving the supply voltage. The starting current can reach approximately 600 % of the full rated value, potentially harming the motor or other circuits sharing the motor supply.

A direct motor starting circuit connects the power supply to start ( start) the motor directly, without any voltage-reduction equipment or method   before the motor. This is starting at full rated voltage ( Full-VoltageStarting) This widely used method is suitable for small motors connected through a starter directly to the power supply. The motor therefore starts with voltage   equal to the supply voltage immediately, causing starting current of approximately 600 % of the full rated value.

Components of a direct motor starting circuit

1. The operating circuit ( Schematic Diagram) 
of a direct motor starter is divided into 3 parts. It cuts current to the contactor's magnetic coil and stops the motor. The Stop button is normally positioned above the Start

2. Starting circuit ( Start Circuit) 
supplies current to the magnetic coil, causing the contactor to deliver power to the motor.

3. Holding circuit ( Holding or Maintaining Circuit) 
keeps the contactor energised after the starting circuit opens.

4. Motor protection circuit ( Protection Circuit) 
comprises fuses and overload protection, protecting the motor against overload and short circuits.

2. Reduced-voltage starting  (Reduced Voltage Starting) 
Voltage is reduced during starting to prevent a dangerously high starting current; this also reduces the starting current. There are several reduced-voltage starting methods, including:  

2.1   Using an autotransformer ( Auto-Transformer Reduced-Voltage Starter) 
This starting method uses an autotransformer with multiple winding taps providing various voltage levels, such as 50%, 65% or 80%  of the supply voltage.

2.2 Part-winding starting ( Part-Winding Starter) 
This method uses 2 separate winding sets connected in parallel inside the stator. It can reduce starting current to 20%-35 %  of the full rated current.

2.3   Primary resistance starting ( Primary Resistance Starter)
This is the simplest method, connecting resistance in series with each motor phase winding. Starting voltage drops across the resistance, and each winding receives approximately 70%-80% of the supply voltage. After the motor has run for a while, the resistance is disconnected and the motor receives supply voltage directly.

2.4 Primary inductance starting ( Primary Reactance Starter)
This motor starting method is similar to the resistance method in item 2.3 but substitutes inductors for resistors. Its advantage over resistance is reduced electrical energy loss as heat.

2.5   Star-delta starting ( Star-Delta Starter)
Star-delta starting is widely used because its design is simple and suitable for three-phase induction motors. It is used with motors whose internal windings have 6 external leads. The motor's rated delta voltage must safely match the supply voltage. For a supply system rated at 3 phase 380 V the motor nameplate is normally marked 380/660 V During starting, the motor is connected in star ( Star or Y)   which reduces starting voltage. After it has run for a while, it is connected in delta ( Delta or D) The source indicates that further details are covered in later modules.

2.6   Slip-ring motor starting ( Slip ring motor starter)
A slip-ring motor has a wound rotor and slip rings ( Slip ring) for connecting external resistance to the rotor winding at startup, with the aim of reducing starting current.  

2.7   Solid-state starting ( Solid State motor starter)
This method starts the motor using solid-state electronic devices, normally SCRs ( Silicon Control Rectifier; SCR) As the motor accelerates, the solid-state devices control voltage and current appropriately. SCRs can switch rapidly, producing smooth starting torque without abrupt shocks.  

Reducing starting voltage with star-delta starting
Starting 3 phase motors larger than 5 kilowatts cannot, according to the source, use direct starting ( Direc Staart) because the starting current is very high, normally approximately 5 - 7 times the motor's normal rated current. A method that reduces starting current is therefore needed. Otherwise, starting a large motor can adversely affect the electrical system in several ways:   such as
1. Lights may dim or flicker.
2. Undervoltage protection may operate.
3. The factory power supply, such as its transformer, may be overloaded.
4. High-voltage supply fuses may blow.
5. Other motors in the factory operating under overload may stop because of the voltage drop.

Thus, the source says motors larger than 5 kilowatts require a current-reducing starting technique. There are 3 methods:
1. Star-delta starting
2. Reduced-current starting using resistors  
3. Starting with a step-down transformer
This section covers only star-delta starting. The motor starts in star, then when it reaches 75% of rated speed it must change to delta.

Star-delta starting can be implemented by 2 methods:
1. using a star-delta switch
2. using contactors
Circuit and operating principles of star-delta starting