So the speed of synchronous motor must be constant and fixed. This speed solely depends upon frequency and no of poles. The American Institute of Electrical Engineers has defined the constant-speed motor as one the speed of which is either constant or does not materially vary; such motors are the synchronous motor, the induction motor with small slip, and the direct-current shunt motor. In the synchronous motor, rotor too have a DC excitation, so it produces a constant flux forming the electromagnetic poles on rotor which initially are stationary.
The rotor can be tested with an ohmmeter for an open winding or a grounded winding. To test the rotor for an open winding, connect one of the ohmmeter leads to each of the slip rings on the rotor shaft, Figure 17—5. They have a constant speed at a given supply frequency. 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. The synchronous speed of an AC motor is determined by the frequency of the source and the number of poles. The armature winding of the synchronous motor is energized from an AC source and its field winding from a DC source. The stator winding of Induction Motor is energized from an AC source.
The induction motor can not run at synchronous speed because synchronous speed is a hypothetical speed not a rela speed. Explanation: Synchronous speed can not be achieved in real world because to achieve that speed a vacuum is required with friction which is total impossible in real world.
Some of the typical applications of high speed synchronous motors are such drives as fans, blowers, dc generators, line shafts, centrifugal pumps, compressors, reciprocating pumps, rubber and paper mills.
Synchronous motors are used to regulate the voltage at the end of transmission lines. Why High Starting Currents: Magnetizing component of current flowing through induction motor is proportional to the applied voltage and is independent of load on the motor similar to transformer.
A load resistance variable is connected in series to the fixed rotor and stator impedance. Hence DC starters are used to limit the starting current ofmotor. For a large rating motor, ranging from 5 HP to 25 HP, oil immersed DOL starters are used which provides insulation against sparking on contact points and hence increases the life of starter. A direct on line starter can be used if the high inrush current of the motor does not cause excessive voltage drop in the supply circuit. The applications of DOL starters are primarily motors where a high inrush current does not cause excessive voltage drop in the supply circuit or where this high voltage drop is acceptable.
Direct on line starters are commonly used to start small water pumps, conveyor belts, fans, and compressors. What is a Three-Point Starter? A three-point starter is an electrical device, used for starting as well as maintaining the DC shunt motor speed. The connection of resistance in this circuit is in series which decreases the initial high current and guards the equipment against any electrical failures.
Which DC motor has got maximum self-loading property? Explanation: A differentially compound DC motor, flux reduces so sharply at small increase in load at higher values of load.
It is advisable that motor should not be used beyond some load value, as it may damage itself by self-loading. A 3 point starter is a device that helps in the starting and running of a DC shunt motor or compound wound DC motor similar to a 4 point starter.
The back emf develops as the motor armature starts to rotate in presence of the magnetic field, by generating action and counters the supply voltage. So, a DC motor is started by using a starter. There are various types of dc motor starters, such as 3 point starter, 4 point starter, no-load release coil starter, thyristor controller starter etc.
The basic concept behind every DC motor starter is adding external resistance to the armature winding during starting.
The most significant difference between the three point and the four-point starter is that in three-point starter the no voltage coil NVC is connected in series with the field winding whereas in four-point starter the NVC is directly connected to the supply voltage.
Hence, a DC shunt motor is able to self-regulate its speed, and can be referred to as a constant speed motor. When a load is applied to a DC shunt motor, its speed decreases, but the motor is able to self-regulate and quickly compensate for the lost speed.
A shunt DC motor has a feedback mechanism that controls its speed. As the armature rotates in a magnetic field, it induces electricity. This EMF is generated in a reverse direction, thus limiting the armature current. Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It only takes a minute to sign up.
Connect and share knowledge within a single location that is structured and easy to search. It is stated in the textbook I'm studying that if we reduce the excitation current after the rotor has reached synchronous speed , its speed will remain constant even if the excitation current becomes zero.
I know that the reason the rotor runs at synchronous speed is because the poles created by the excitation current are attracted to those of the stator. If we reduce the current to 0 though, there are no poles on the rotor to be attracted. Wound rotor synchronous machine tend to have a salient rotor structure. As a result they exhibit a small amount of reluctance torque.
Usually this of little use as it is not enough to facilitate breakout but can be enough to counter drag if unloaded. In my understanding this phenomenon will not change even for wound rotor non salient synchronous machines which produce no reluctance torque. All you need for maintaining magnetic locking is the magnetic flux. This magnetisation will now be adjusted by the reactive power. The machine will draw reactive power from the mains to maintain the magnetic lock. This will be reflected by the change in power factor.
Please check the power factor with and without excitation. Of course the lagging current will increase and as the first observation you will see this change in current for a constant load active power should be the same.
That should automatically explain you the reason behind this behaviour. If we gradually reduce the excitation of a synchronous motor when it is running at no-load, we find that the motor continues to run at synchronous speed even when the exciting current is zero.
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