When the motor is excited with three-phase supply,three-phase stator winding produce a rotating magnetic field with 120 displacements at constant magnitude which rotates at synchronous speed. This changing magnetic field cuts the rotor conductors and induces a current in them according to the principle of Faraday’s laws of electromagnetic induction. As these rotor conductors are shorted, the current starts to flow through these conductors.
In the presence of magnetic field of stator, rotor conductors are placed, and therefore, according to the Lorenz force principle, a mechanical force acts on the rotor conductor. Thus, all the rotor conductors force, i.e., the sum of the mechanical forces produces torque in the rotor which tends to move it in the same direction of rotating magnetic field.
This rotor conductor’s rotation can also be explained by Lenz’s law which tells that the induced currents in the rotor oppose the cause for its production, here this opposition is rotating magnetic field. This result the rotor starts rotating in the same direction of the stator rotating magnetic field.
If the rotor speed more than stator speed,then no current will induce in the rotor because the reason for rotor rotation is the relative speed of the rotor and stator magnetic fields. This stator and the rotor fields difference is called as slip. This how 3-phase motor is called as asynchronous machine due to this relative speed difference between the stator and the rotors.
As we discussed above, the relative speed between the stator field and the rotor conductors causes to rotate the rotor in a particular direction. Hence, for producing the rotation, the rotor speed Nr must always be less than the stator field speed Ns, and the difference between these two parameters depends on the load on the motor.
The difference of speed or the slip of the AC induction motor is given as
When the stator is stationary, Nr=0; so the slip becomes 1 or 100%.
When Nr is at synchronous speed, the slip becomes zero; so the motor never runs at synchronous speed.
The slip in the 3 phase induction motor from no load to full load is about 0.1% to 3%; that’s why the induction motors are called as constant-speed motors.
SVPWM Control of 3 Phase Induction Motor
Most commonly for controlling the induction motors, PWM inverter based drives are used. As compared with fixed frequency drives, these PWM dives controls the both magnitude of voltage and frequency of the current as well as voltage applied to induction motor. By changing the PWM signals applied to the power switch gates, the amount of power delivered by these drives is also varied so that the three phase induction motor speed control is achieved.
A number of Pulse width modulation (PWM) schemes are used for controlling three phase motor drives. But most widely Sine PWM (SPWM) and space vector PWM (SVPWM) are used. As compared with SPWM, SVPWM control gives higher level of fundamental voltage and reduced harmonic content. Here we have given a practical implementation of this SVPWM control using 8051 microcontroller.
In the below circuit, three-level voltage inverter is used for obtaining three output voltages depends on DC bus voltage. Single phase supply is rectified to supply DC power to both microcontroller circuit and inverter circuits.8051 Microcontroller is programmed to produce SVPWM signals which are given to gate driver IC.
The inverter circuit comprises of six MOSFETs to produce variable three phase supply, for each phase two MOSFETs are deployed. These MOSFETs gates are connected to gate driver IC. Upon receiving the PWM signals from microcontroller gate driver switches the MOSFETs so that the variable AC output voltage is produced. Therefore, this variable AC with change in voltage and frequency varies speed of the motor.
This is basic information on AC induction motor with construction and working principle. In addition to this, SVPWM technique of controlling speed of the motor has many advantages than other PWM techniques as we seen above. If you have doubts on programming microcontroller for implementing SVPWM technique in it, you can contact us by commenting below.
