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THEME : DC MOTORS Exercise 1:

Explain clearly the effect of the back emf of a shunt motor. What precautions must be

taken when starting a shunt motor?

A four pole dc motor is connected to a 500V dc supply and takes an armature current

of 80A. The resistance of the armature circuit 0.4Ω. The armature is wave wound

with 522 conductors and the useful flux per pole is 0.025 Wb. Calculate: (a) the back

emf of the motor; (b) the speed of the motor; (c) the torque developed by the

armature.

Exercise 2:

A dc motor takes an armature current of 110A at 480V. The resistance of the

armature circuit is 0.2Ω. The machine has 6 poles and the armature is lap connected

with 864 conductors. The flux per pole is 0.05Wb. Calculate (a) the speed and (b) the

gross torque developed by the armature.

Exercise 3:

A six pole dc motor has a wave connected armature with 87 slots, each containing 6

conductors. The flux per pole is 20mWb and the armature has a resistance of 0.13Ω.

Calculate the speed when the motor is running off a 240V supply and taking an

armature current of 80 A. Calculate also the torque developed by the armature.

Exercise 4:

Explain the necessity for using a starter with a dc motor.

A 240V dc shunt motor has an armature of resistance of 0.2Ω. Calculate: (a) the

value of resistance which most be introduced into the armature circuit to limit the

starting current to 40A; (b) the emf generated when the motor is running at a constant

speed with this additional resistance in circuit and with an armature current of 30A.

Exercise 5:

Calculate the torque developed by a dc motor having an armature resistance 0.25Ω

and running at 750rev/min when taking an armature current of 60A from a 480V

supply.

TRAINING QUESTIONS IN ELECTRICAL MACHINES/F36, GTHS K’BO PAPER TWO: JANUARY 2010

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Exercise 6:

A six pole, lap wound, 220V, shunt excited dc machine takes an armature current of

2.5A when unloaded at 950rev/min. When loaded, it takes an armature current of

54A from the supply and runs at 950rev/min. The resistance of the armature circuit is

0.18Ω and there are 1044 armature conductors.

For the loaded condition, calculate: (a) the generated emf; (b) the useful flux per pole;

(c) the useful torque developed by the machine.

Exercise 7:

State Fleming’s left hand rule and explain how you would use it to determine the

direction of a current induced in a conductor.

Briefly describe and state the functions of each of the following: (a) armature; (b)

commutator; (c) brushes; (d) interpoles.

A series motor taking 120A is adjusted so that its field current is reduced to 90A. If

the resistance of the field is 0.05Ω, calculate the value of the resistance to be

connected in parallel with the field. Also calculate the value of the power being

wasted in the additional resistance.

Exercise 7:

A motor name plate shows the following rating: 10hp, 230V, 1350rpm, 37.5A, shunt.

When the field current is 0.75A and the armature resistance is 0.38Ω, calculate the

following:

a) The full-load efficiency

b) The terminal torque at rated load

c) The internal torque and speed developed when the line current is 18A and

the field current remains at 0.75A.

Exercise 8:

A 400V shunt connected dc motor has a full load output of 20kW at an efficiency of

85%. The shunt field resistance is 200Ω and the armature resistance is 0.15Ω.

Calculate for this load:

a) The input current

b) The armature current

c) The back emf

d) The total copper losses

e) The rotational losses.

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Exercise 9:

A perfectly compensated shunt motor is supplying an effective mechanical power of

15kW from a voltage 240V supply. The total joule losses represent 9% of the total

power absorbed. The excitation current is 3.125A. The motor efficiency is 0.8.

Calculate:

a) The total power absorbed

b) The joule losses in the stator

c) The joule losses in the armature

d) The current in the armature

Exercise 10:

A separately excited dc motor has the following characteristics.

• Supply voltage U = 250V

• Armature current Ia = 50A

• Excitation current J = 1.5A

• Armature resistance Ra = 0.3Ω

A no load test as a generator gave the following results:

Uo = 308V for I = 1.5A and N = 1200rpm

A no load test as a motor gave the following results: P0 = 1200W and V = 250V.

In the whole problem, U and I are constant.

1. What is the speed of the motor on no load?

2. Determine the no load torque T0.

3. Determine the speed of the motor at normal load and hence deduce the

electromagnetic torque Te of the motor.

4. Show that the electromagnetic torque is proportional to the current I.

Exercise 11:

The starter of a Peugeot 504 (a series motor) takes 200A under a voltage of 12V.

Under these conditions, it runs at 1000rpm and delivers a useful power of 1500W.

The constant losses are estimated at 100W. Calculate:

1. The power absorbed by the starter.

2. Its efficiency.

3. Its useful torque.

4. The joule losses.

5. The total resistance (armature + field winding)

e) The back emf of the motor

f) The useful torque

g) The constant losses

h) The effective electrical power

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6. The back emf of the motor.

7. Determine the direct starting current.

8. What is the value of the resistance to be connected in series with the motor to

limit the starting current to 240A?

Exercise 12:

A 4 pole, long shunt, compound dc generator supplies 100A at a terminal voltage of

500V. if armature resistance is 0.02Ω, series resistance is 0.04Ω and shunt field

resistance 100Ω, find the generated emf. Take the drop per brush as 1V.

Exercise 13:

1) Calculate the torque in Nm developed by a 440V dc motor having an armature

resistance of 0.25Ω and running at 750rpm when taking a current of 60A.

2) A 4 pole lap connected dc motor has 576 armature conductors and draws an

armature current of 10A. Calculate the flux per pole if the torque is 18.3N.m.

3) A shunt motor running on no load takes 5A at 200V. The resistance of the field

and armature are 150Ω and 0.1Ω respectively. Determine the output and

efficiency of the motor when the input current is 120A.

4) Calculate the value of a series resistance with the armature of the motor in

question 3 if the starting current must not exceed the full load value.

5) The following information refers to a 4 pole, lap wound dc series motor with

200 armature conductors. Terminal voltage = 400V; Power output = 51kW;

Efficiency = 85%; Speed = 500rpm; Field resistance = 0.06Ω; Armature

resistance = 0.08Ω. Find: (a) the back emf, (b) the power developed in the

armature, (c) the armature torque (d) the shaft torque (e) the useful flux per

pole.

6) The following information was recorded on a brake test in a dc motor.

Diameter of pulley = 0.3m; Net brake load = 350N; Supply voltage = 250V;

Line current = 32A; Speed = 20rps. Calculate (a) the output power (b) the

motor efficiency.

Answers for exercise 13:

1) 325Nm; 2) 0.02Wb; 3) 20.52kW, 65.5%; 4) 1.57Ω; 5) 379V, 56850W,

1091.5Nm, 974Nm, 0.228Wb; 6) 6594W, 82.43%.

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TECHNOLOGY

1) For a dc motor, assume that:

U = Armature supply voltage

r = Armature resistance

I = Armature current

a) Show that φN

rIUn

−=

b) At no load, deduce that NKI

Un = Where K is the constant of the field circuit

and I is the field current.

c) Give the role of the excitation resistance.

d) Explain Why the DC motor turn in high speed when the field flux is cancel.

e) Indicate the two precautions to be taken to avoid the DC motor to turn in high

speed.

f) Explain why the DC series motor turns in high speed at no load

2) For series, shunt and compound dc motor, give the following characteristics:

a) N = f(Ia) characteristics

b) Tm = f(Ia) characteristics

c) N = f (Tm) characteristics

3) Give 2 suitable applications of dc series motors, justify your answer.

4) Give 2 suitable applications of dc shunt motors, justify your answer.

5) Give 3 starting methods of dc motors.

6) Why is the full voltage starting dangerous for the motor?

7) Give 2 braking method of dc motor.

8) Give the 3 main electric braking methods.

9) What’s the drawback of the friction braking method?

10) What’s the Ward-Leonard system?

Instructor: Mr. NGOUNE Jean – Paul,

Teacher in the Electrical Department, GTHS K’BO

N = number of armature conductors

Ф = Flux under a pole

N = speed of the armature