Table of Contents
Rotor Current Frequency (ii) As the rotor picks up speed, the relative speed between the rotating flux and the rotor decreases. Consequently, the slip s and hence rotor current frequency decreases.
How is rotor frequency affected by slip speed?
Once the operating slip frequency reaches its maximum value, the rotor current will decrease and, in turn, the output torque will decrease in proportion to 1 / ω s 2 as the stator frequency increases as shown in Fig. 3.51.
How does frequency affect motor speed?
Varying the voltage frequency affects both the motor speed and the strength of the magnetic field. When the frequency is increased (for higher motor speed), the magnetic field decreases, and lower torque is produced. In order to keep the magnetic flux constant, the V/Hz ratio must remain constant.
What are the effect of variation of rotor resistance on speed of rotor?
The torque speed characteristic for different values of rotor reactance. Speed control is achieved from rated speed to lower speed in a downward direction. The speed control is smoother if external resistance and reactance addition has large no.
How does the rotor speed change with wind speed?
The higher the rotor speed, the more the blades are turned out of the wind. The blades are turned back into the wind when the rotor speed falls. In general, fixed speed turbines use stall control for technical reasons, while variable speed turbines are usually equipped with pitch control [17, 20].
What is the impact of slip on rotor current?
When the rotor is stationary, s = 1. Under these conditions, the per phase rotor e.m.f. E2 has a frequency equal to that of supply frequency f. At any slip s, the relative speed between stator field and the rotor is decreased.
What is standstill rotor EMF?
When rotor is stationary i.e. s = 1, the frequency of rotor e.m.f. is the same as that of the stator supply frequency. The value of e.m.f. induced in the rotor at standstill is maximum because the relative speed between the rotor and the revolving stator flux is maximum.
What happens when frequency changes?
The number of complete wavelengths in a given unit of time is called frequency (f). As a wavelength increases in size, its frequency and energy (E) decrease. From these equations you may realize that as the frequency increases, the wavelength gets shorter. As the frequency decreases, the wavelength gets longer.
What will happen if frequency is increased?
For example, if there’s more demand for electricity than there is supply, frequency will fall. If the frequency rises, the turbine reduces its steam flow. If it falls it will increase, changing the electrical output – a change that needs to happen in seconds.
Does changing frequency affect voltage?
In an inductance, the lower the frequency, the lower its impedance. So having constant voltage over it, the current will rise if the frequency gets lower.
What is the formula for rotor turns per phase?
8. What is the formula for rotor turns per phase? Explanation: Firstly, the winding factor for stator is obtained along with the winding factor for stator. Next the ratio of the rotor voltage per phase to the stator voltage per phase.
What is rotor resistance control?
Rotor resistance control is one among the various methods for the speed control of induction motor. In this method of speed control, the rotor circuit resistance is varied by connecting a variable external resistance. This method is only applicable for slip ring or wound rotor induction motor (WRIM).
What are the advantages and disadvantages of rotor resistance control?
Because of low cost and high torque capability at low speeds, rotor resistance control is employed in cranes, Ward Leonard Ilgener Drives, and other intermittent load applications. Major disadvantage is low efficiency due to additional losses in resistor connected in the rotor circuit.
How do you control rotor speed?
Speed control from rotor side: Rotor rheostat control. This method is similar to that of armature rheostat control of DC shunt motor. Cascade operation. In this method of speed control, two motors are used. By injecting EMF in rotor circuit.
Which type of rotor runs at faster speed?
Rotor speeds typically lie in the 120–210 m/s range, but mostly between 150 and 190 m/s, with a tendency to be higher with a smaller rotor diameter [6]. Currently the smallest rotor diameter used industrially is 28 mm with rotor speed up to 150,000 rpm, though some machines can reach 160,000 rpm.
What is the difference between rotor speed and synchronous speed?
Induction motor with squirrel cage rotor has two different speeds. The synchronous speed refers to the stator rotating magnetic field, which depends on the number of poles and frequency. The other speed is the rotor’s. The rotor speed will be always slower than the stator speed, we call it slip.
What causes slip in induction motor?
“Slip” in an AC induction motor is defined as: As the speed of the rotor drops below the stator speed, or synchronous speed, the rotation rate of the magnetic field in the rotor increases, inducing more current in the rotor’s windings and creating more torque.
How Slip affects the rotor frequency of the three phase induction motor?
The frequency of this induced e.m.f. at start is same as that of supply frequency. Thus frequency of rotor induced e.m.f. in running condition (fr) is slip times the supply frequency (f). As slip of the induction motor is in the range 0.01 to 0.05, rotor frequency is very small in the running condition.
What are the effects of slip in the slip ring induction motors?
The windings are connected to external resistance through slip rings, which helps to control the torque/speed of a motor.Difference Between Squirrel Cage and Slip Ring Induction Motor. Slip Ring Motor Squirrel Cage Motor Starting torque is high Torque is low.
What happens when slip is zero?
Zero slip means that rotor speed is equal to synchronous speed. If rotor is rotating at synchronous speed in the direction of rotating magnetic field the, there will be no flux cutting action, no emf in the rotor conductors, no current in rotor bar conductor and hence no development of electromagnetic torque.
What is frequency of rotor emf?
The frequency of the induced emf in rotor is? Explanation: The induced frequency will be f = P(Ns-Nr)/2. f = 4(1500-1450)/2 = 100 Hz.
What is slip at standstill?
At standstill, induction motor slip s is 1, hence the frequency of induced EMF in the rotor of the induction motor is same as that of supply frequency and reduces with increase in speed (due to the reduction in slip).
