InterviewSolution

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1.	If higher terminal voltage is applied to PMDC what will happen?(a) Speed at 0 torque will be reduced(b) Torque at 0 speed will be reduced(c) Torque at 0 speed will be increased(d) Torque will remain constantThis question was addressed to me in a national level competition.I need to ask this question from PMDC Motors in division DC Machines of DC Machines
Answer» The correct choice is (c) TORQUE at 0 speed will be increased Easy explanation: The straight-line characteristic of PMDC is such that it produces POSITIVE INTERCEPTS on both axes. If terminal voltage is increased both of these intercepts increases, slope remaining constant. Thus, speed and torque at initial conditions both will increase.

2.	PMDC gives better speed regulation than DC shunt motor.(a) True(b) FalseI got this question during an interview.This interesting question is from PMDC Motors in section DC Machines of DC Machines
Answer» Correct choice is (a) True For explanation I would say: PMDC MOTOR exhibits BETTER speed regulation and EFFICIENCY than dc shunt motor. The main problem of dc shunt motor is going to run-away when the field TERMINALS are opened. But in PMDC there is no run-away problem, so it gives PRACTICAL benefit to the industry applications.

3.	PMDC produces high torque on ______________(a) At low speeds(b) Only at maximum speed(c) Torque is constant(d) Cannot sayThis question was addressed to me in final exam.I want to ask this question from PMDC Motors in division DC Machines of DC Machines
Answer» Right choice is (a) At low speeds The best I can explain: PMDC produces high TORQUE even at very low speeds. In DC SHUNT MOTOR torque produced at very low speeds is lower than the torque produced at the same speeds in PMDC motor.

4.	Which of the following is most favoured choice for PMDC motor?(a) Neodymium-iron-boron(b) Iron-nickel(c) Nickel-neodymium(d) Cannot be givenI had been asked this question in semester exam.My doubt stems from PMDC Motors topic in section DC Machines of DC Machines
Answer» The correct choice is (a) Neodymium-iron-boron Explanation: OBVIOUS choice of PMDC MOTOR is neodymium-iron-boron which has high coercivity and high retentivity. Its characteristic is almost a STRAIGHT line. Hence, neodymium-iron-boron COMBINATION is preferred over the other materials.

5.	What is the air gap flux density for PMDC motor?(a) Bg= µ0(tm/lg) Hm(b) Bg= – µ0Hm(c) Bg= – µ0(tm/lg) Hm(d) Bg= – µ0(lg/tm) HmThe question was posed to me during an internship interview.Query is from PMDC Motors topic in portion DC Machines of DC Machines
Answer» Right CHOICE is (c) Bg= – µ0(tm/LG) Hm The explanation: Since, flux crosses the air gap length lg two times a thickness of permanent magnet two times tm. We get, 2lg Hg + 2*tm Hm = 0 and we know that air gap flux density is equal to µ times the MAGNETIC FIELD intensity.

6.	Which speed control method is employed in PMDC?(a) Field control method(b) Armature control method(c) Armature Field control method(d) Cannot be determinedI have been asked this question by my school principal while I was bunking the class.Query is from PMDC Motors topic in chapter DC Machines of DC Machines
Answer» Correct OPTION is (b) Armature control method To explain: UNLIKE in DC MOTORS, field is provided by permanent magnets in PMDC. This implies that field provided in PMDC motor is permanent and can’t be altered unless whole CONSTRUCTION is modified. Thus, armature voltage control method is employed here.

7.	PMDC motors are _________________(a) Smaller than corresponding rated field wound motor(b) Larger than corresponding rated field wound motor(c) Almost same size that of corresponding rated field wound motor(d) Can be smaller or greater than corresponding rated field wound motorThis question was posed to me in a national level competition.I would like to ask this question from PMDC Motors in portion DC Machines of DC Machines
Answer» The correct option is (a) Smaller than corresponding rated field wound motor Easiest explanation: PMDC motors are always of smaller size compare to field wound motors of same rating. Here, field winding is replaced by small block of MAGNET so that power output remains same. PMDC motors are COSTLY compare to DC motor of same rating.

8.	In PMDC magnetic material is placed instead of _____________(a) Armature(b) Rotor(c) Stator(d) Can be placed anywhereI have been asked this question during an internship interview.Enquiry is from PMDC Motors in section DC Machines of DC Machines
Answer» The correct answer is (c) Stator Explanation: In PMDC motors we keep magnetic material or magnetic poles in the place of field WINDING of original DC MACHINE. That is we are changing stator windings with TWO permanent magnetic poles BOUNDED on casing.

9.	PMDC motor offers ___________(a) Cumulative compound type characteristic(b) Differential compound type characteristic(c) Series characteristic(d) Shunt characteristicI have been asked this question in an international level competition.I'd like to ask this question from PMDC Motors topic in section DC Machines of DC Machines
Answer» Right OPTION is (d) Shunt characteristic Easiest explanation: The stator is made of permanent magnet in the PMDC motor hence, there is no NEED for separate FIELD excitation for stator. So, PMDC acts as a shunt motor with FIXED field. This can be seen from the construction also.

10.	In PMDC motors we use magnetic material with ___________(a) Low residual flux(b) Low coercivity(c) High residual flux(d) All magnetic materialsThe question was asked in a national level competition.The doubt is from PMDC Motors topic in division DC Machines of DC Machines
Answer» Correct ANSWER is (c) HIGH residual flux For explanation: In market MAGNETIC materials with various properties are available, but for DC MOTOR application we need magnetic materials with high residual flux in order to get high no load voltage. ALONG with residual flux, coercivity of material should be high.

11.	Speed-torque characteristic of PMDC motors is _______________(a) Starting from zero increasing continuously(b) Starting from some positive value and remaining constant(c) Starting from some positive value and decreasing exponentially(d) Starting from some positive value and decreasing on straight lineThe question was asked in an international level competition.Question is from PMDC Motors topic in division DC Machines of DC Machines
Answer» Right option is (d) Starting from some positive value and decreasing on straight LINE The explanation is: The material chosen for PMDC motor is such that SPEED TORQUE CHARACTERISTIC of a given motor remains straight line for WIDER range of torque, for wider range of armature voltage. The straight line is such that it makes positive intercepts on both axes.