Note synchronous speed of 3,000 RPM, no-load speed also about 3,000 RPM, and full load speed 2,932 RPM. Related: What does the speed of regulation mean?įor reference, here is a typical motor characteristic graph. Which synchronous motor will be smallest in size (D) 10 HP, 375 rpm.Explanation: Q15. The only way to change its speed is to vary the supply frequency (because Ns 120 f / P). while running it main-tains a constant speed. Synchronous speed is the speed the motor would spin at if it had absolutely zero load (frictionless bearings and no other losses of any kind.) It runs either at synchronous speed or not at all i.e. Note the motor bearings still impose a nominal load due to friction. No-load speed is the speed of the motor with no load on the shaft. Rated speed is the speed of the motor at rated output (shaft) power. (iv) power angle and (v) maximum or pull-out torque of the motor. Since an induction motor must slip to provide torque, the full-load speed will always be less than synchronous speed.Įven an un-loaded motor with nothing on the shaft still has a decelerating torque, due to the friction of the bearings and so on. Calculate (i) mechanical power developed (ii) armature current. Read the wikipedia articles, or google asynchrnous and synchronous motors.Īn induction motor never spins at synchronous speed.Ī motor spinning at constant speed must be producing enough accelerating torque to overcome the decelerating torque of the load. In some special working regimes this won't be true as the asynchronous motor could operate at for example 900 rotations if you make some adjustments. If you want to find the number of poles round it to the nearest synchronous rotational speed and calculate it normally. Synchronous speed depends on the number of poles of motor and frequency of input voltages The formula of synchronous speed is: Ns 120fe/P In this equation N s represents the synchronous speed f e is frequency and P is no of poles of induction motor. The synchronous motor works on the principle of magnetic locking. It is the speed of the revolving magnetic field in the stator of the induction motor. This is represented by the imaginary stator poles. However, synchronous motors are not used so much because they run at constant speed (i.e., synchronous speed) but because they possess other unique electrical properties. When a three-phase supply is given to the stator of a three-phase wound synchronous motor, a rotating field is set up in the air gap which rotates at synchronous speed (Ns 120f/p). Why is the speed 1460? The stable part of the torque/rotational speed charachteristic for a asynchronous motor is near the synchronous speed. As the name implies, a synchronous motor runs at synchronous speed (Ns 120f/P) i.e., in synchronism with the revolving field produced by the 3-phase supply. The rotational speed is calculated from the difference of the stator and rotor flux frequencies. The asynchrnous motor on the other side has an induced field in the rotor that is dependent on the speed of rotation.Therefore the field rotates in synchronicity with the rotor. The synchronous motor has a constant excitation in the rotor (or has a constant difference between inductances).Lets look at this formula in more detail. Now why is one synchronous and the other asynchronous? Using this simple formula: Ns (120xf) / p, we can calculate the synchronous speed of any synchronous motor. The difference to a synchronous is in the fact that the rotor is usually just a (sqirrel) cage and some iron.įor a synchronous motor the rotor has either a magnet or has geometrically strongly defined(salient) poles and pole gaps. B.A.What you are looking here at is an asynchronous (induction) motor.Electronics and Telecommunication Engineering.Electronics and Communication Engineering.
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