WHAT IS THE DIFFERENCE BETWEEN STARTING OF 3 PHASE AND SINGLE PHASE INDUCTION MOTORS?

3 PHASE MOTOR IS SELF STARTING ‎WHEREAS 1 PHASE MOTOR IS NOT.

1-ph induction motor han no starting torque so we use a capacitor in the starting winding to make a phase adifference between the two fluxes of the running and starting windings as T=KI1I2sinO (I1=current of starting winding,I2=current of run winding,sinO=sin of angle bet I1,I2).
where3 -ph motor has a starting torque but we use some techniques to reduce the starting current to a safe values such as :star/delta,autotransformer,starting resistance,soft starter etc......

A single phase induction motor is one designed to run on a single phase supply.
In reality there is some provision for starting, as single phase does not provide a rotating field, but one that reverses 180 degrees.
Starting is usually to split the single phase into two phases separated by 90 degrees, commonly using a capacitor or inductor in series with the starting winding.
The start winding is often switched out once the motor has started.
In a household application they are common in refrigerators and air conditioners.
Fans and other items using smaller motors that don't need a strong starting torque commonly use a shaded pole motor, but these are not as efficient.
It is uncommon for single phase motors to be much more than 5 HP in size, because they are not as suitable as three phase motors.
Usually they are connected directly to the supply for starting, with just two wires.
Three phase induction motors have three phases, so the first requirement is a three phase supply.
The three phases are 120 degrees apart, so that a proper rotating field is produced.
They tend to be smaller and cheaper and run smoother than an equivalent single phase motor.
They can be more efficient than some single phase motors.
As some of these can be large, they can need a specific starting arrangement to reduce the sudden surge at start-up, and perhaps to control the acceleration.
Sometimes a star delta starter is used, and these need a supply with neutral.
There would be 6 wires brought out from the windings for this purpose, one pair for each phase.
Some even have 12 wires, so they can accommodate a range of voltages as well.
Three phase motors can be reversed by reversing the connections to any one phase.
This means that the direction of rotation should always be checked after installation.

The Initial torque of the single phase induction motor is very small so they can not take self start , we use a capacitor to give them a starting torque . while3 ph has self start due to high torque , this high torque can also be improved to avoid current over shoot

3-phase induction motors are self starting due to high stating torque while1-phase induction motors use capacitor to build up starting torque in order to start it. 

Methods of starting (3 PHASE INDUCTION MOTOR) The most usual methods of starting3-phase induction motors are:1.
For slip-ring motors- rotor resistance starting2.
For squirrel-cage motors - direct-on -line starting - star-delta starting - Auto transformer starting.
for single phase : Capacitor Start: Split-phase" or Switch Phase

Hello sir, sir on electric motor ,single phase motor and three phase motor two which one will deliver.thanks

 

Hi

 

Basically , Single Phase motors are not exceeding0.5 HP . It means that single phase motor has no significant Starting Torque .  we use a capacitor in the starting winding to make a phase difference between the two fluxes  of the running and starting winding.

 

Three Phase Motor has different starting torque as per exceeding rating. As the rating will be increased , starting torque will be increase so we reduce the starting torque by using different starter type.

 

For Example :

 

1. Three phase motors not exceeding10 HP to be direct on line (7.46Kw)

2. Three phase motors over10 HP but not exceeding50HP to be automatic star delta magnetic non reversing type with3 pole overload relay and low voltage protection (7.46Kw -37.3 kw) 

3. Three phase motors over50HP to be multiple step auto transformer non reversing magnetic type with3 pole overload relay, low voltage protection and used Soft Starter.