EducationSingle phase induction motor - Construction, operation and applications

Single phase induction motor – Construction, operation and applications

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Single phase induction motor are squirrel-cage rotors with scattered or concentrated stator winds specifically designed to supply power in a single phase.

They are frequently employed in electric drives of household appliances, as well as low-power technology equipment used in agriculture and industry and also in industrial drives that are auxiliary.

single phase inductor motor field

The AC voltage applied on the single-phase stator’s winding generates an alternating magnetic field Ph that is stationary in space, that can be broken down in two fields, Ph1 as well as Ph2, which rotate in opposite directions, thus:

Φ = Φ1 + Φ2

The magnetic fields Φ1 and Φ2 create electromotive forces within the rotor’s windings through which electric currents flow through the rotor’s windings. Therefore, the rotor cage also produces two magnetic fields which rotate at opposite angles.

In an interaction between the magnetic fields of the stator with the magnetic fields of the rotor two components moments M1 and M2 result. The combination of these moments provides an electromagnetic torque that is characteristic that is produced by an uni-phase induction motor.

single phase inductor motor characteristics

Based on the properties of the magnetic torque is evident that a single phase induction motor doesn’t generate an initial torque (at n = 0 (n = 0; Mr = 0.) It is unable to start its operation by itself and does not have a particular direction of rotation. It is however enough to provide the rotor of a single phase motor an starting speed, in any direction until the point at which the magnetic torque exceeds the load torque from the outside which means that the motor can begin to spin and then reach a sub-synchronous speed within the stable portion of the force characteristic of M.

Therefore the single-phase induction motor is regarded as two motors working on a common shaft where the fluxes spin in opposing directions. To attain a beginning torque, most single-phase motor models are fitted with additional starting windingsthat are that are to capacitors.

Single-phase low-power induction motors are generally constructed as motors:

  • with an auxiliary phase that is short-circuited, with a magnetic field created by a current inside a coil , or in coils that are mounted on poles;
  • with the magnetic field created by current in staggered winds which are placed in grooves
  • which has a resistive start phase A two-phase winding is installed within the stator (divided into the appropriate proportion into the main phase and beginning phase).

The term “single-phase motors” implies that they’re powered by single-phase AC grid. They have two separate windings on the stator- the main winding that has terminals that are labeled U1 and U2 as well as the secondary winding that has terminals that are labeled Z1 and Z2 which are offset by 90 degrees in phase. In assessing the resistance of both windings, you will see one characteristic feature: it is approximately two times less than that of the winding that is auxiliary.

To enable the motor to show a significant beginning torque, for modern motors a capacitor must be connected in series to the winding of the auxiliary.

Here is the procedure for joining the motor terminals in order to achieve “right” or “left” rotation. The diagram of connection is typically located on the reverse on the motor terminal board cover.

The direction of rotation for a single phase motor is accomplished by changing the direction of flow of current within each phase.

single phase induction motor

Principle of operation of a single-phase induction motor

The magnetic field created by the stator windings spins around the stationary the rotor. In the result the field cutting the cage bars of the rotor the rotor cage bars, a voltage is generated within these bars (hence the term “induction motor”) and the current starts to flow through the cages. (See the phenomena in the phenomenon of electromagnetic induction). The current flow in the magnetic field generates Electrodynamic forces (see the electrodynamic phenomenon force) which is tangentially linked to the rotor circuit and consequently an electromagnetic moment also gets generated. If the amount of the torque is greater that the torque of load the rotor is able to begin to speed up. In addition, the increased speed of the rotor causes the cage’s bars being crossed by magnetic fields at a increasing speed, which results in a decrease in magnitude of the electromotive force, and a reduction in the amount of current that flows through the cage’s bars and, consequently, the magnitude of the electromagnetic torque decreases. When this torque falls to a level equivalent to the load force, then the rotor ceases in its accelerated state and continues to move at a constant rate. If there was any load torque present, then the rotor will achieve a speed equivalent to the the speed of rotation that the field is at, and thus its synchronous rate. In this case the rotor’s field would be stationary in relation to the stator’s field, which means that the cutting of cage bar by the field of the stator would stop and no currents will be flowing through the cage bars, and no electromagnetism would have been generated. But, this scenario cannot be achieved in the real motor as there will always be some form of load moment, even if it’s there is only a moment of friction in the bearings , or the air resistance (unless the rotor is mechanically driven by a different motor). The rotor is then able to reach this speed (usually not significantly less than the synchronous speed) that all electromagnetic forces of both the motor as well as the load have the same magnitude. Because this isn’t synchronous speed, it has to be the case that it is synchronous, and to this the induction motor is owed its second name “asynchronous motor.”

Construction of single phase induction motor

An induction motor comprises two primary components:

  • The stator stationary
  • The moving(rotating) the rotor.
Construction of single phase induction motor

Special grooves, commonly referred to as grooves, are formed on the inner core of the stator and on the outside of the rotor core within which the windings are positioned. The core area between adjacent grooves is referred to as teeth. The teeth and grooves may differ in their shapes, and generally their number in the rotor and stator is distinct. There’s as little of an air gap as is possible between the rotor and stator.

single phase induction motor construction

The winding of the stator is constructed of insulated wirethat has been which is impregnated with a brace that is firmly anchored in order to minimize the chance of damage due to mechanical vibration.

Speed control of single phase induction motor

In real-world applications for electric drives, it’s typically required to change how the rotor is turned as well as control the rate of rotation for the induction motor. Based on the relation to determine the rate of the rotor’s rotation:

The speed that is produced by an induction motor is determined using the formula:

Therefore, the velocity of the induction motor is able to be controlled by making changes to:

  • The power supply’s frequency,
  • The amount of poles in magnetic fields.
  • slip.

Altering the quantity of pair of poles affects the speed of the ma-gnetic field. For two poles, the velocity is 3000rpm and for four poles it’s 1500 rpm. One motor is able to be designed for two different pole pairs. The speed control process involves switching the stator windings in order that pole pair count change. The change from a smaller to a higher quantity of pairs of poles needs to be set to be in sync with what speed is being generated by the motor. If the switch is made too fast then the motor could enter the zone of generator operation, which could result in damages for the motor.

The slip adjustment is possible by two methods. The voltage supply to the stator is possible , without altering the frequency. The motor torque is influenced by the proportion of the stator’s supply voltage. Modifications to the rotor circuit could be accomplished by adding resistors to the circuit for the rotor or by adding more voltage to the circuit. The control of the rotor may occur only in the instance of ring motors because of being able to access the windings.

The speed can be altered by increasing the loss of power in the rotating. Sub-synchronous cascades can be employed to control the rate of speed for an Asynchronous motor. This method is where the rotor circuits connect via slip rings in addition to a DC generator or rectifier bridge that is controlled. Modifying the intensity of input voltage alters the magnetic field of the motor and the speed.

Frequency control is possible to control your speed motor with the least energy loss. Speed of magnetic field is affected by frequency. To ensure that the torque remains constant the voltage that supplies the power source will change in accordance with frequency.


Electronics and Telecommunications engineer with Electro-energetics Master degree graduation. Lightning designer experienced engineer. Currently working in IT industry.