11. In the biomedical instruments like artificial heart pumps, the motor used is ____________
A. DC shunt motor
B. DC series motor
C. Induction motor
D. BLDC motor
Answer: D
BLDC motors are widely used in various applications in the medical industry. Sensorless BLDC motors and sensor BLDC motors are used because of their easy operation and high reliability compared to conventional motors.
12. Calculate the output power of the Buck converter if the supply voltage is 4 V and the duty cycle value is .1 for a 4 Ω load.
A. 40 mW
B. 20 mW
C. 50 mW
D. 60 mW
Answer: A
The output voltage of the buck converter is Vo = Vin×(D.=.4).
The value of the duty cycle is less than one which makes the Vo < Vin. The buck converter is used to step down the voltage. The output power is
Vo2÷R=40 mW.
13. Calculate the compensator rating required for cos(Φ)=.1.
A. 0.91 P.U
B. 0.99 P.U
C. 0.97 P.U
D. 0.92 P.U
Answer: B
The compensator rating can be calculated using the relation
QP.U=√1-cos2(∅)
=√1-.01=.99.
This per unit value VAR compensator is required to improve the power factor of the system.
14. Calculate the average inductor current of the Boost converter if the load current is 7 A and the duty cycle value is .8.
A. 36 A
B. 35 A
C. 34 A
D. 31 A
Answer: B
The average inductor current of the Boost converter is
Io÷(1-D.=7÷.2=35 A.
The average value of the inductor current can be calculated using the ampere-sec balance method.
15. Calculate the output voltage of the Buck-Boost converter if the supply voltage is 14 V and the duty cycle value is .85.
A. 79.3 V
B. 45.5 V
C. 86.5 V
D. 54.7 V
Answer: A
The output voltage of the buck-boost converter is
Vo = D×Vin ÷ (1-D)
=.85(14)÷.15=79.3 V.
It can step up and step down the voltage depending upon the value of the duty cycle. If the value of the duty cycle is less than .5 it will work as a buck converter and for a duty cycle greater than .5 it will work as a boost converter.
Ques.16. In a Brushless DC (BLDC) motor, the construction of the motor is similar to
Stepper
Universal
DC Motor
Synchronous Motor
Answer.4. Synchronous Motor
Explanation
BLDC motors are one type of synchronous motor. This means the magnetic field generated by the stator and the magnetic field generated by the rotor, rotate at the same frequency. BLDC motors do not produce the “slip” which is normally seen in induction motors. BLDC motors come in single-phase, 2-phase, and 3-phase configurations. Corresponding to its type, the stator has the same number of windings. Out of these, 3-phase motors are the most popular and widely used. A brushless motor is constructed with a permanent magnet rotor and wire-wound stator poles. Electric energy is converted to mechanical energy by the magnetic attractive forces between the permanent magnet rotor and the rotating magnetic field induced in the wound stator poles.
Ques.17. In BLDC motor field winding is kept on
Stator
Rotor
Can be rotor and stator
No field winding on the rotor or stator
Answer.2. Rotor
Explanation
Brushless motors are similar to AC motors in that a moving magnetic field causes rotor movement or rotation. Both motor types use stator windings and have no brushes. Brush-less motors are also similar to permanent magnet (PMDC) motors since they have linear characteristics. Also, both motor types use permanent magnets to generate one field, The brushless motor is, in essence, a hybrid, which combines the best attributes of both the AC and DC motors.
The configuration of the brushless motor most commonly used in contemporary systems is shown in Fig. In this motor, the rotor consists of permanent magnet field winding and the stator consists of windings. These windings are termed “commutation” windings. By passing a current through a winding, a magnetic field is set up with which permanent magnets on the rotor interact. This results in the rotation of the rotor.
Ques.18. Which of the following is not an advantage of BLDC motor over conventional DC motor.
Less maintenance
Long life
No risk of explosion or possibility of RF radiation
Low cost
Answer.4. Low cost
Explanation
Disadvantages of BLDC motor
Cost: Rare-earth magnets are much more expensive than other magnets and result in an increased motor cost. Electronic equipment such as Hall sensor embedded in stator can also increase the cost.
Limited constant power range: A large constant power range is crucial to achieving high vehicle efficiencies. The permanent magnet BLDC motor is incapable of achieving a maximum speed greater than twice the base speed.
Magnet demagnetization: Magnets can be demagnetized by large opposing magnetomotive forces and high temperatures. The critical demagnetization force is different for each magnetic material. Great care must be brought to cooling the motor, especially if it is compactly built.
High-speed capability: The surface-mounted permanent magnet motors cannot reach high speeds because of the limited mechanical strength of the assembly between the rotor yoke and the permanent magnets.
Ques.19. In BLDC motor driver module operation, we may not require ____
SCRs
Power transistors
FETs
Transistors
Answer.4. Transistors
Explanation:-
The schematic diagram of a brushless dc motor driver module is shown in Fig. The stator (armature) and rotor are connected in three-phase. The stator is connected to a variable voltage current source through an inductor and an inverter comprising six SCRs (Si to S6). In place of SCRs, power transistors or FETs could be used according to the power rating of the motor.
Diodes are connected across SCRs to protect these from the L(di/dt) voltage induced in the armature coil undergoing commutation. Position sensors placed on the rotor provide the signal to the sensor decoders and gate drivers which cause the SCRs to be fired in sequence so as to be in synchronism with the rotor’s mechanical position. The stator and rotor fields thus get locked into each other and remain in synchronism at any rotor speed.
Ques.20. Typical brushless motor doesn’t have __________
Commutator
Permanent magnet
Electronic controller
Fixed armature
Answer.1. Commutator
Explanation:-
The brushless DC (BLDC) motor operates in a similar way to a brushed DC motor and has the same torque and speed characteristic curve. As the name implies, BLDC motors do not have mechanical brushes for commutation as traditional DC motors do, but they must be electronically commutated in order to produce rotational torque. Since there are no brushes to wear out and replace, BLDC motors are highly reliable and almost maintenance-free.
