What is the typical frequency of a synchronous motor drive for a ball mills in a cement plant?
The motor operates with typical frequencies from zero to about 6 Hz. In general, the bigger the mill diameter, the lower are the mill operating and critical speeds. Thus, for larger mills the number of poles will increase.
What is the power factor of a synchronous motor?
Synchronous motors are designed to operate at unity (1.0) power factor or 0.8 leading power factor. By varying the DC excitation of the motor, the power factor of the motor can be varied widely. Overexcited synchronous motors operate at leading power factor and provide reactive kVAR-like capacitors.
What is the efficiency of a synchronous motor?
Leading power factor synchronous motors have efficiencies approximately 0.5 to 1.0% lower.
How can a synchronous motor be made to have a leading power factor?
Once the synchronous motor is synchronized, the field poles on the rotor are in line with the rotating magnetic poles of the stator. … To increase the line voltage, the motor will supply ac magnetizing current to all “magnets” on the system to increase their magnetic flux. This is leading power factor.
What are the different types of drives we use in paper mills and cement mills?
The following types of drives are used for rolling mills:
- (i) DC motors,
- (ii) AC slip ring motors with speed control.
- (i) Slip ring induction motor.
- (ii) Three phase shunt wound commutator motor.
- (iii) Cascade controlled AC motor.
- (iv) Ward Leonard controlled DC motor.
- (v) DC motor with transformer step switch control.
What is synchronous speed?
: a definite speed for an alternating-current machine that is dependent on the frequency of the supply circuit because the rotating member passes one pair of poles for each alternation of the alternating current.
Why synchronous motors are 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 synchronous speed and it’s formula?
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. … Actual speed of the induction motor will be less than the synchronous speed.
What is the main disadvantage of synchronous motors?
Disadvantages or Demerits:
Synchronous motors requires dc excitation which must be supplied from external sources. Synchronous motors are inherently not self starting motors and needs some arrangement for its starting and synchronizing. The cost per kW output is generally higher than that of induction motors.
How does synchronous motor change speed?
Changing the number of poles is not easy, so we do not use that method. However, with the invention of solid-state devices, the frequency of the current fed to the synchronous motor can be varied. We can control the speed of the synchronous motor by changing the frequency of the supply to the motor.
When a synchronous motor is running at synchronous speed?
When a synchronous motor is running at synchronous speed, the damper winding produces. damping torqe. no torque. both 1 and 2.
Does the excitation affect the synchronous motor speed?
why speed decreases as field excitation increases in synchronous machine. why the speed of the synchronous motor remains constant on any load. why with the change of excitation of a synchronous motor the magnitude of armature current first decrease and then increase.
How do you control the speed of a synchronous motor?
Speed of synchronous motors can be controlled using two methods called open loop and close loop control. Open loop contol is the simplest scalar control method where motor speed is controlled by independent frequency control of the converter.
What is excitation EMF?
The process of generating a magnetic field by means of an electric current is called excitation. Field coils yield the most flexible form of magnetic flux regulation and de-regulation, but at the expense of a flow of electric current.