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Explain, how regenerative braking is done in a 3-phase induction motor. Show
graphically the four-quadrant operation of the motor. What is the slip during
plugging of an induction motor?
Explain, how regenerative braking is done in a 3-phase induction motor. Show graphically the four-quadrant operation of the motor. What is the slip during plugging of an induction motor?
In regenerative braking of a 3-phase induction motor, the rotor speed of the motor must exceed the synchronous speed of the motor. The regenerative braking characteristic is the continuation ofthe motoring characteristic into the upper part of quadrants II/IV. The maximum regenerative braking torque is higher than the maximum motoring torque.
During plugging, the slip is close to 2 because the motor is rotating in the opposite direction to the rotating magnetic field.
The slip is defined as the difference between the synchronous speed and the rotor speed.
How does the braking resistance control the dynamic braking torque in dc
separately excited motor? How to employ dynamic braking in dc series
motors?
How does the braking resistance control the dynamic braking torque in dc separately excited motor? How to employ dynamic braking in dc series motors?
In dynamic braking of d.c. series motor with self-excitation, the supply to the motori is switched off and then the armature circuit including the series field winding is connected across a resistor ensuring that the excitation is not reversed during the change over. The dynamic braking torque is
T = (IR + R_rRn_a)
where the flux ø is dependent on the armature current I_a. When braking is initiated, the current is high, thus, resulting in increased value of flux, and the torque is also high, being approximately proportional to square of the current. The speed torque characteristics for dynamic braking are in the second quadrant as shown below.
To employ dynamic braking in d.c. series motors, the armature of the shunt is disconnected from the supply and is connected across a variable resistance Ras shown in Fig. (b). The field winding is, however, left connected across the supply undisturbed. The braking effect is controlled by varying the series resistance R.
What do you mean by "classes of motor duty"?
What do you mean by "classes of motor duty"?
The selection of the correct motor for an application depends on the type of duty cycle it will experience. This is described by the "classes of motor duty." The classes of motor duty refer to the type of load the motor will experience and the duration of the load. For example, a motor used for a continuous duty application, such as a fan or pump, will need to have a higher power rating than a motor used for an intermittent duty application, such as a crane. The duty cycle is also important to consider. For example, a short time duty motor might operate at high power for short periods with long intervals of rest. A continuous duty motor will operate at a lower power level but for much longer durations. The selection of the right motor is important for ensuring that the motor is not overloaded and that it can operate reliably over the long term.
Study Notes
Electrical Drive
- A device that converts electrical energy into mechanical energy for driving various machines and mechanisms. This includes components like motors, power modulators, control units, and the source.
- Common motor types used in electric drives include DC motors (shunt, series, compound, and permanent magnet), induction motors (squirrel cage, wound rotor, and linear), and synchronous motors (wound field and permanent magnet), as well as others such as brushless dc motors and stepper motors.
Motor Power Rating
- Intermittent duty rating is less than the nameplate rating.
- Continuous duty rating is calculated with TN/97.5η
Speed Control of DC Shunt Motors
- Ward Leonard method: speed control is achieved using a motor-generator set with a generator as the prime mover for the shunt motor. The generator's field is controlled to adjust its output voltage.
- Buck-boost method: uses a constant-speed motor driving a generator. The output voltage from the generator is adjusted to varied voltage to the shunt motor.
Special Type Drives
- Stepper Motor: An incremental motion machine. It operates in discrete steps for each electrical pulse.
- Solar-battery powered drives: Use solar panels as the power source to run DC motors (PM DC and more). The cost of the solar cells and other components is more than the energy used on solar-powered mechanical drives for small applications.
Electrical Traction Drives
- Often use DC series motors for their high starting torque and good efficiency.
- Methods for controlling speed include series-parallel control, tap changing, or dynamic braking.
Starting of Electric Drives
- Energy required to start an induction motor is 1⁄2 Jω², where J is the equivalent moment of inertia of the motor and load.
- If load torque is high relative to the inherent starting torque of the motor, the starting current will be high and may lead to damage. Load torque is typically considered constant.
Dynamics of Electrical Drives
- Load equalization in electric drives: Energy is stored during light-load periods and released during peak load to stabilize load fluctuations and minimize power loss. Flywheels, for instance, can implement this.
- Typical passive loads in industrial applications: Friction, blower, and pump loads.
Four-Quadrant Operation
- A four-quadrant electric motor system allows bidirectional rotation (forward and reverse motion) and regenerating energy.
- The four quadrants represent forward and reverse motoring and braking operations.
Induction Motor Drives
- Speed control methods for induction motors include VVVF, slip power recovery, and rotor resistance control.
- VVVF control maintains a constant ratio of voltage to frequency to keep magnetic flux in the motor nearly constant.
- Slip power recovery: Energy lost during braking/acceleration is recovered and reused, thus improving efficiency.
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Description
Explore the principles of electrical drives, motor power ratings, and speed control methods for DC shunt motors. This quiz covers various motor types, their applications, and techniques used for controlling motor speed. Test your knowledge on this crucial aspect of electrical engineering.
