DC motors worksheet with answers

Free printable DC motors worksheet: 6 problems from easy to exam level, with an answer key. Print it or save it as a PDF.

New worksheet Practise online with hints

For teachers: free to print, copy and use in class. Every new worksheet has new values and its own answer key. A link to ExamProblems on your course page is appreciated.

Name: ____________________ Date: ____________

1.Easy

A DC motor runs from a 240 V supply and draws an armature current of 12 A. Its armature resistance is 0.8 Ω. Find the back-EMF E.

V = 240 VI_a = 12 AR_a = 0.8 Ω

E = V

2.Easy

A DC motor runs from 220 V with an armature current of 50 A. The armature resistance is 0.5 Ω. Find the developed (electromagnetic) power P_dev.

V = 220 VI_a = 50 AR_a = 0.5 Ω

P_dev = W

3.Medium

A DC motor draws 4 A from a 48 V supply. Its armature resistance is 1.2 Ω and its rotational (friction, windage and iron) losses are 10 W. Find its efficiency η.

V = 48 VI_a = 4 AR_a = 1.2 ΩP_rot = 10 W

η = %

4.Medium

A DC motor (k = 0.3 V·s/rad, R_a = 1.2 Ω) runs from 110 V. At full load it draws 6 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 110 Vk = 0.3 V·s/radR_a = 1.2 ΩI_FL = 6 A

SR = %

5.Hard

A 120 V DC motor has an armature resistance of 0.5 Ω. Find the series starting resistance R_st that limits the starting current to 30 A.

V = 120 VR_a = 0.5 ΩI_max = 30 A

R_st = Ω

6.Hard

A separately excited DC motor (k = 0.6 V·s/rad at full field, R_a = 1 Ω, V = 110 V) drives a constant load torque of 2.4 N·m. The field flux is reduced to 80 % of its full value. Find the new speed n.

V = 110 Vk = 0.6 V·s/radR_a = 1 ΩT = 2.4 N·mΦ = 80 %

n = rpm

Answer key

1. E = 230.4 V
2. P_dev = 9750 W
3. η = 84.79 %
4. SR = 7.004 %
5. R_st = 3.5 Ω
6. n = 2089 rpm

Full step-by-step solutions: practise this topic on examproblems.com.