Starting Methods of Three Phase Induction Motors-Soft starter,VFD

Soft Starter

Principle of operation 

These power electronics starters or soft starters are made using thyristors; an electronic semiconductor which will conduct a continuous current when an input signal pulse is applied to its gate, and they help to regulate the energy consumed by the induction motor.  When the thyristor is turned on initially and if there is an input sinusoidal signal, a voltage will be applied in to the motor where as if the thyristor is turned off the voltage goes to the motor will become zero. The unit is solid state, using a microprocessor to control inverse parallel (back to back) pairs of SCR's.An SCR/thyristor is a semiconductor device that latches when triggered. Once triggered it allows current to flow in one direction only and turns off at zero current. The firing angle of the SCR's are controlled to achieve the desired acceleration of the motor. 
The soft starter incorporates a closed loop torque control system to provide better control over starting & stopping 3 phase induction motors. Conventional voltage ramp control systems typically produce low torque at low speeds. When starting & stopping variable torque loads (e.g Pumps) this can results in a very rapid acceleration or deceleration with a nonlinear change in motor speed 

Circuit/Connection diagrams 

Characteristic curves during starting

Commercial products 

Applications

• Pump applications where pressure surges should be avoided.
•  Fans and other systems with belt drives to avoid belt slipping.
• Conveyor belt systems can be started very smoothly.

  

AC variable speed drive 

Principle of operation 

A variable speed drive is a piece of equipment that regulates the speed and rotational force, or torque output, of an electric motor. Control can mean either manually adjustable - by means of a potentiometer or linear hall effect device, (which is more resistant to dust and grease) or it can also be automatically controlled for example by using a rotational detector such as a Gray code optical encoder. 

In general, a VFD(Variable Frequency Drive) in its most basic configuration controls the speed of an induction or synchronous motor by adjusting the frequency of the power supplied to the motor. 

When changing VFD frequency in standard low-performance variable-torque applications using Volt-per-Hertz (V/Hz) control, the AC motor's voltage-to-frequency ratio can be maintained constant, and its power can be varied, between the minimum and maximum operating frequencies up to a base frequency. Constant voltage operation above base frequency, and therefore with reduced V/Hz ratio, provides reduced torque and constant power capability. 

Regenerative AC drives are a type of AC drive which have the capacity to recover the braking energy of a load moving faster than the motor speed (an overhauling load) and return it to the power system. 

The VFD article provides additional information on electronic speed controls used with various types of AC motors. 

Circuit/Connection diagrams

 Characteristic curves during starting

Commercial products

Applications

• Sewage lift station 

Sewage usually flows through sewer pipes under the force of gravity to a wet well location. From there it is pumped up to a treatment process. When fixed speed pumps are used, the pumps are set to start when the level of the liquid in the wet well reaches some high point and stop when the level has been reduced to a low point. Cycling the pumps on and off results in frequent high surges of electric current to start the motors that results in electromagnetic and thermal stresses in the motors and power control equipment, the pumps and pipes are subjected to mechanical and hydraulic stresses, and the sewage treatment process is forced to accommodate surges in the flow of sewage through the process. When adjustable speed drives are used, the pumps operate continuously at a speed that increases as the wet well level increases. This matches the outflow to the average inflow and provides a much smoother operation of the process 

• Cooling tower fans 
• Dusting fan in steel and iron industry 
• Compressors in oil and gas industry 

Starting Methods of 3 phase IM- Star-Delta,Auto transformer

 Star Delta Starter 

Principle of operation

 Star Delta Starters can be seen in 3 states. 
                 1) Star connected state.
                 2) Open state.
                 3) Delta connected state.
Main and star contactors are normally closed and complete circuit at the starting time. Voltage is reduced to 1/√3 of the Line Voltage through every winding in the star connected state. When motor reaches a better rotational speed, like 90% of full r.p.m ,the starters timer connects the Delta connector after disconnecting Star connector. During the state transition the circuit becomes open and at that time the motor doesn’t stay in any of those two states. This is mentioned as open transition switching because of that. Winding voltage becomes equal to the line voltage in Delta connected state.
At the beginning if the application is light loaded, this starting method is used If the motor is heavily loaded, there will not be enough torque to accelerate the motor up to speed before switching over to the delta position. 

Circuit/Connection diagrams 

 Switching "Start push button" completes Star circuit by switching ON main contactor "K1M" and Star contactor "K2M" through a timer. Now motor keeps on running in Star configuration.   Once circuit is completed and main contactor "K1M" is activated, one of its "NO" contact point, connected parallel to "Start push button", becomes close and keeps circuit complete by providing a continuous holding even after Push button is released
  Motor remains in Star state configuration till "timer" remains close. After a time delay is reached, timer becomes open, thereby de-activating Star contactor. Immediately after that another switch becomes close, thereby activating Delta contactor "K3" to complete Delta circuit. 

Characteristic curves during starting 

                                    Torque Vs Speed 

                                       Current Vs Speed

Commercial products 

The MCW 37kW Open Chassis Star-Delta Starter is manufactured from three 415V coil contactors, a timer, and a thermal overload relay (Trip Class 10). 

Applications 

• Compressors need low torque to accelerate to the full speed 
• Pumps for hydraulic elevators. 
•  Motor applications which need low acceleration

Auto transformer starters 

Principle of operation

The principle of auto-transformer leads to the concept of Auto transformer starter which consists of additional flexibility compared to the other starters. Motor directs line voltage using some relay arrangement method in DOL starter. Starting voltage is a fixed reduced value in star delta starter. The specialty of the auto transformer is the starting voltage is reduced and also can vary in a range which is done by studs connected to different tapings. The versatility to the starter is provided by this method which enables connecting motors with different ratings by controlling the output voltage. This is a result of the adjustable starting voltage at the beginning. 

 

There are some main components of auto transformer starters. 

1) Three phase auto-transformers.

2) Contactor with movable rod. 

3) Handle arrangement. 

 

The line voltage at the primary windings of auto transformers will be connected by moving the operating handle because of the arrangement of movable rod. In addition the secondary windings of the transformer and the motor terminals are connected using that way. Due to the requirements of starting torque and starting current the tapings can be seen varied because the square of the transformation ratio decides this ratio. 

 

Movable handle H controls assembly of all switches and the double throwing switch is connected to every transformer each. All the switches connected with respective contactors and the motor begin running with a low voltage when the handle is moved.  

 

For the maximum performance the motor should fed with full load current after the safe starting of the starter. The handle H should be put into the run position immediately when motor accelerates and get a speed of 80% of its rated value. Full rated voltage is applied at motor terminals and auto-transformer is detached from the circuitry in that way.  

 Circuit/Connection diagrams

Characteristic curves during starting 

Commercial products

Applications 

• For Induction motors which need a smoother startup. 
• Power transmission and Distribution systems
• Railways
• Audio systems  

Starting Methods of Three phase induction Motors

 Starting Methods of induction motors 

Induction motors need certain types of starting methods to start without causing trouble to other utility consumers. When an induction motor begin to work, huge amount of current (6-8 times full load current) draw from the supply. The reason is the high emf induced in rotor bars/wingdings due to high rate of change of magnetic flux. Stator winding can withstand to this large current, but due to this large current receiving end voltage may reduce and other consumers may be affected by this low voltage supply. The increased torque results in sudden mechanical stress on the machine which leads to a reduced service life. To reduce this huge amount of current the voltage should be reduced. In order to do that supply voltage should be reduced. Hence various types of starting methods are used according to the application. 

•  Starting Methods 

1. Direct-on-Line (DoL) starter   

2. Star-Delta Starter  

3. Auto-transformer starter  

4. Soft Starter 

5. AC variable speed drive  

1. DOL Starter

Principle of operation 

In this method, 3-phase power supply is directly connected to the induction motor. Mechanical switch is used to switch power supply. This is the simplest method of starting an induction motor and also the cheapest method. Starters mainly consists of MCB, contactor and overload relay. When the mechanical switch is closed full line voltage is applied to the stator terminals. The contactors are closed against spring action. Very high inrush current will draw from the supply and it will reduce when the speed is increasing. Starting current is depend on motor design and supply voltage and totally independent of the load. Acceleration torque will determine the starting time of the induction motor. An isolator is used to isolate the starter from the supply in maintenance. Safety protections (Over-current, under-voltage, short-circuit, etc.) are provided to the motor to safeguard the motor. 

Characteristic curves during starting

12 Amp Direct On Line Starter Including Overloads METAL  IP65 Enclosure IEC 60947 / EN 60947 / IS 13947 / VDE 0660 Overload Options 0.10 Amps to 13.0 Amps 

Applications 

DOL method is normally limited to smaller cage induction motors, because starting current can be as high as eight times the full load current of the motor. Use of a double –cage rotor requires lower staring current( approximately four times) and use of quick acting A.V.R enables motors of 75 Kw and above to be started direct on line. DOL starter is used to start small water pumps, compressors, fans and conveyor belts.