How do we reach the relation between mechanical angle(\$\theta m\$) and electrical angle(\$\theta e\$) as: \$\theta e = (P/2)\theta m\$ ?
; where P is the number of poles.
Induction Motor – Electrical and Mechanical Angle Explained
induction motor
Related Solutions
The problem is that the rotor and stator must have spaces called slots, in the iron for the conductors. The conductors and air in the slots have a lower reluctance than the reluctance of the iron. There are more slots than there are poles in the stator. The windings are distributed among the slots to form distributed poles. Their is a different numbers of slots used for the stator and rotor. Those two features prevent the rotor from becoming locked or "cogged" in the minimum reluctance position that would otherwise exist.
Even with proper selection of the number of stator and rotor slots, there can be reluctance variations at different rotor angles that cause torque fluctuations and inflections in the torque vs. speed curve. Skewing the rotor slots mitigates that effect.
Here is a picture of the rotor and stator of a 4-pole, permanent-split capacitor, single-phase induction motor.
During half of a cycle of the power frequency, the position of a rotor pole moves from one stator pole to the opposite pole of the same pair. If the motor has only two poles, that rotor pole moves back to the original position during the next half cycle. So there is one revolution per cycle of the power frequency. If there are more than two poles, the rotor moves from one pair of poles to another pair of poles during one cycle of the power frequency. Thus Speed(RPM) = 120 f(Hz) / P(poles).
Slip
In an induction motor, the mechanical speed of the rotor is less than the speed of the magnetic fields described above. The difference allows the stator to induce current in the rotor. The difference is very small when the motor has no external mechanical load. For the most common 3-phase motor designs, the slip is about 2 or 3 percent at full-load torque. For single-phase motors, the full-load slip is a little higher.
Power is speed multiplied by torque. Therefore a motor of a given power rating will have a higher torque rating if the speed rating is lower.
The physical size, weight and cost of a motor are more closely related to its torque rating than to power its power rating.
Best Answer
An electrical machine is simply an electrical<>mechanical energy conversion device which utilises magnetic fields as the exchange medium.
When an electrical machine is operating as a motor, the idea is to create a traveling, rotating magnetic field, via the stator and "hope" 1 that this moving flux attracts the rotor (be it via an equivalent magnetic field or via an affinity to reduce the reluctance)
So a stator:
A 3 phase, single pole pair topology. If the stator was now unrolled
you can now see that for 360degree electrical we have equally achieved 360degree mechanical.
As the number of pole-pairs is increased, the point at which an electrical cycle is completed becomes a fraction of the main mechanical cycle & this factor is the pole-pair count
1 hope is used because there are some prerequisites that must be met that are machine specific. freq, voltage, load, supply ...