Table of Contents
An induction motor can’t development torque at synchronous speed because an induction motor can’t run at synchronous speed because if it runs at synchronous speed the relative speed between the rotor and stator R.M.F will be zero so the torque will be zero.
Why Cannot an induction motor run at synchronous speed?
Due to this difference in flux, the rotor will experience a torque and will start to rotate at speed less than synchronous speed due to lagging. So, theoretically, the Induction motor can never run at synchronous speed. This will result in no torque on the rotor, and it will stop moving due to this.
How we can run an induction motor at synchronous speed?
We can run an induction motor at synchronous speed by using rotor emf injection method. Rotor emf injection method: For below-rated speeds: In this method, injected emf has the same frequency of rotor slip frequency and that emf is 180° out of phase with rotor emf. Here, the value of rotor emf becomes less.
What is cogging in 3 phase induction?
The phenomenon of Magnetic Locking between the stator and the rotor teeth is called Cogging or Teeth Locking. Even after applying full voltage to the stator winding, the rotor of a 3 phase induction motor fails to start. Thus, a locking is created between the stator and rotor teeth.
Why the synchronous motor is not self starting?
Above a certain size, synchronous motors are not self-starting motors. This property is due to the inertia of the rotor; it cannot instantly follow the rotation of the magnetic field of the stator. Once the rotor nears the synchronous speed, the field winding is excited, and the motor pulls into synchronization.
What is standstill rotor EMF?
When rotor is stationary i.e. s = 1, the frequency of rotor e.m.f. is the same as that of the stator supply frequency. The value of e.m.f. induced in the rotor at standstill is maximum because the relative speed between the rotor and the revolving stator flux is maximum.
Why does the speed of induction motor always less than its synchronous speed?
The induction motor always runs at speed less than its synchronous speed. Due to the lag between the flux current in the rotor and the flux current in the stator, the rotor will never reach its rotating magnetic field speed (i.e. the synchronous speed).
What will happen if induction motor speed reaches synchronous speed?
An induction motor can’t development torque at synchronous speed because an induction motor can’t run at synchronous speed because if it runs at synchronous speed the relative speed between the rotor and stator R.M.F will be zero so the torque will be zero.
What is 120 in motor speed formula?
The equation for synchronous speed – the speed of the magnetic field inside the motor stator is 120 x F / P where F is the applied frequency in Hz, and P is the number of poles (always a multiple of 2) in the motor winding. 120 is a constant that gives your answer in revolutions per minute.
Is it possible to run an induction machine as a motor at speed greater than synchronous speed?
The speed of an induction motor of conventional type (not the doubly fed type) is always below synchronous speed, and the increase of voltage can not change this fact.
How does an induction motor start?
A three-phase Induction Motor is Self Starting. When the supply is connected to the stator of a three-phase induction motor, a rotating magnetic field is produced, and the rotor begins rotating and the induction motor starts.
What would happen if synchronous speed is equal to rotor speed?
If the rotor reached synchronous speed, there would be no movement of stator field relative to the rotor conductors, and no induced voltage, current or torque. In order that the no-load losses can be met, the rotor attains a speed less than synchronous.
What is the formula of synchronous speed?
The synchronous speed of an AC motor is determined by the frequency of the source and the number of poles. The RPM is calculated by multiplying the frequency times 60 and dividing by the number of pairs of poles. This was explained in Chapter 6. Some motors are designed to operate at synchronous speed.
What is the difference between synchronous speed and rotor speed?
Induction motor with squirrel cage rotor has two different speeds. The synchronous speed refers to the stator rotating magnetic field, which depends on the number of poles and frequency. The other speed is the rotor’s. The rotor speed will be always slower than the stator speed, we call it slip.
What are the types of induction motor?
Types of Induction Motor: A Concise and Useful Guild
- Types of Single-Phase Induction Motors. Split Phase Induction Motors. 2&3. Capacitor Start and Capacitor Start Capacitor Run Induction Motors. Shaded Pole Induction Motors.
- Types of Three-Phase Induction Motors. Squirrel Cage Induction Motor. Slip-ring or Wound Rotor Induction Motor.
What is normal slip of an induction motor?
Because of this speed difference, the motor rotates at a speed slightly slower than the synchronous speed. Slip is normally expressed in percentage. Slip of a power induction motor is 2 to 3% when the motor is operated under the rated load.
What is synchronous speed of motor?
The synchronous speed is the speed of the revolution of the magnetic field in the stator winding of the motor. It is the speed at which the electromotive force is produced by the alternating machine. A machine that runs at synchronous speed is called a synchronous machine.
How do you calculate the full load speed of a motor?
A reasonable full-load speed for a 60 Hz, four-pole motor would be 1,725 RPM.For a 60 Hz system with four poles, the calculations to determine RPM would be:
- (Hz x 60 x 2) / number of poles = no-load RPM.
- (60 x 60 x 2) / 4.
- 7,200 / 4 = 1,800 RPM.
Why can an AC induction motor not operate at 0% slip?
In motors the number of rotor slots and stator slots are different and thus cogging (magnetic locking) can’t happen. And thus The slip motor can’t run at rated speed. Torque developed by induction motor is proportional to the slip. At synchronous speed,slip is zero and hence torque developed by the motor is zero.
