Hello friends I hope you all are doing great. In this post, we will have a detailed look at Starting Methods of Single Phase Induction Motor. A Single-phase induction motor is a type of induction motor that operates on a single phase. There are two main types of induction motor first one single a phase induction motor and the second one is a three-phase induction motor.
Here we will cover different types of methods used to start the single-phase induction motors with their circuits and practical uses. So let’s get started.
Starting Methods of Single Phase Induction Motor
- The main feature or can say drawback of single-phase induction motor is that there is no self-starting in this motor. Generally, 3 methods are used to start a single-phase induction motor and according to these techniques single-phase induction motor is classified.
- These methods has different expensive and amount of torque generated and a technician try to use the technique that is less expensive and provides resultant torque.
- These 3 techniques mention here.
- Split-phase windings
- Shaded stator poles
- Capacitor-type windings
- These methods help to provides a stronger field than other so required torque can be obtained.
Split-Phase Windings or a Split Phase Induction motor
- The split-phase motor comprises op 2 windings at stator firs one is main winding and second one auxiliary winding
- The angle for these two windings with the stator is ninety degrees that is an electrical angle and auxiliary windings have such configured that can be removed from the circuitry with the use of a centrifugal switch when the motor gets the required speed.
- The design of auxiliary windings is such that its resistance to reactance ratio R/X is larger than the main windings ratio.
- Due to this cause current is lagging in the main windings than auxiliary windings.
- The small size of auxiliary windings should be kept since it is using to start motor and not taking part in the operation in a continuous basis.
- To know the function of auxiliary windings sees the below figure.
- As there is current is leading in auxiliary than main windings in the main winding,
- The field BA peaks before the main field. As BZ peaks firstly than BM. So there is net anticlockwise rotation in the field. In a simple way, the auxiliary winding creates one of the reverse rotatory stator field greater than the second and offers net starting torque to the motor.
- The typical speed characteristic curve in the above figure.
- ٰIn below figure you can see the cutaway diagram of split phase motor.
- It is simple to observe the main and auxiliary windings and centrifugal switch which remove the auxiliary windings from the circuitry when the motor get the required or rating speed.
- There is moderate torque required with low starting current is needed for a split-phase induction motor.
- SO it preferred such applications where less torque is needed fans, blowers, and centrifugal pumps.
- The rating of this motor is less and cost also less.
- The current in auxiliary windings for this machine always peaks before the field of main windings. The direction of movement of motor is found through space angle of field from the auxiliary windings ninety degrees ahead or ninety degrees lags the angle of main windings.
- So the angle can be carried from ninety degrees ahead to ninety degrees behind through the switching interlinks at the auxiliary windings the direction of movement of a motor can be altered through the switching connections of auxiliary winding but the connection of main windings retaining nonvariable.
Capacitor-Start Induction Motors
- For some applications, torque provided through the split-phase induction motor is not enough to run the load connected at the rotor of motor.
- In such situations, a capacitor start motor can be used.
- In capacitor start, the motor capacitor is connected in series combination tot the auxiliary winding of the motor.\
- Through the good choice of capacitor ratings, the MMF of the starting current in auxiliary windings can be set to equal to the MMF of current in main windings. and phase angle of current in auxiliary windings can be created to lead the current in main windings through ninety degrees.
- As the 2 windings are separated through the ninety degrees of angle the ninety degrees of phase difference in current will result in a single uniform rotating 3 phase power.
- In this situation starting torque of the motor will be three percent greater than the rated value.
- Capacitor start motor is costly than the split-phase induction motor and reffed for such applications where large starting torque is needed.
- Its common applications are compressors, pumps, air conditioners, etc.
Permanent Split-Capacitor and Capacitor-Start, Capacitor-Run Motors
- The starting capacitor does not perform enough work to improve the torque-speed curve of the induction motor that auxiliary windings having a smaller capacitor is used on a permanent basis in the motor.
- If the capacitor rating selected accurately then this motor will have a rotating field at a certain load and will operate like the 3 phase induction motor at a specific location.
- This design configuration is known as a permanent split capacitor or capacitor start and runs the motor.
- Permanent split capacitor motors has a simple circuit configuration than capacitor start motors because in this motor starting switch is not used.
- For normal loads, they are certain and has a large power factor and smooth torque than a normal single-phase induction motor.
- Though a permanent split capacitor motor has less starting torque than a capacitor start motor because the capacitor should has such a rating that balances the current in main and auxiliary windings for normal loads situations.
- As the value of starting current is larger than load current the capacitors that making balance the phases for normal loads left them in unbalance in starting conditions.
- If both largest starting torque and running conditions are required 2 capacitors can use to the auxiliary windings.
- A motor having 2 capacitors is known as capacitor stan, capacitor run or 2 value capacitors motors,
- The larger capacitor exists in the circuitry and for starting and when it make sure that current in main and auxiliary windings are balanced creating high starting torque.
- When the motor gets the required speed the centrifugal switch opens and the permanent capacitor left through itself in the auxiliary windings.
- The permanent capacitor has such value that balances the current for normal motor loads, therefore the motor again runs effectively through large torque and P.F.
- The permanent capacitor in this motor is generally ten to twenty percent of the rating of starting capacitors.
Shaded-Pole Induction Motors
- the shaded pole induction motor is such a motor that comprises of just main windings. In place of auxiliary windings, it uses salient poles and one part of the pole is covered by the short-circuited windings known as the shaded coil.
- Main windings generate time changing flux in the poles.
- When poles flux changes it produces voltage and current in the shading coil that resists the original variation in flux. The restriction retard flux variation in the shaded portion of windings and so generates an imbalance among the 2 oppositely rotating start field. The net rotation in the direction from unshaded to the shaded part of pole faces.
- The torque speed curve of the shaded pole is can see here.
- Shaded pole generates less starting torque than any other category of induction motor starting technique.
- Either it is less efficient but provides high slip than another category of single-phase induction motor. These poles are used in samll rating motors that, provide less starting torque.
- Since shaded pole motors rely at the shading coil for starting torque so its direction can not be changed easily. To get reverse direction it is compulsory to use two shaded windings at every pole face and to choose a short one or anyone from them.
Comparison of Single-Phase Induction Motors
- The single-phase induction motor can be rated from best to worst according to its starting and running characteristics.
- Capacitor-start, capacitor-run motor
- Capacitor-start motor
- Pennant split-capacitor motor
- Split-phase motor
- Shaded-pole motor
That is a detailed post about Starting Methods of Single Phase Induction Motor. I tried to cover every parameter related to this post still if you have any problems ask in the comments. See you in the next post.