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 turns at 1800 rpm, and its back-EMF at that speed is 190 V. Find the motor constant k in V·s/rad.

E = 190 Vn = 1800 rpm

k = V·s/rad

2.Easy

A DC motor turns at 1200 rpm, and its back-EMF at that speed is 225 V. Find the motor constant k in V·s/rad.

E = 225 Vn = 1200 rpm

k = V·s/rad

3.Medium

A DC motor draws 12 A from a 48 V supply. Its armature resistance is 0.2 Ω and its rotational (friction, windage and iron) losses are 44 W. Find its efficiency η.

V = 48 VI_a = 12 AR_a = 0.2 ΩP_rot = 44 W

η = %

4.Medium

A DC motor (k = 0.6 V·s/rad, R_a = 1.5 Ω) runs from 220 V. At full load it draws 15 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 220 Vk = 0.6 V·s/radR_a = 1.5 ΩI_FL = 15 A

SR = %

5.Hard

A DC chopper feeds a motor (k = 0.8 V·s/rad, R_a = 1.2 Ω) from a 220 V DC bus. The average armature voltage is D·220 V. Find the duty cycle D that runs the motor at 750 rpm with a load of 4 N·m.

V_dc = 220 Vk = 0.8 V·s/radR_a = 1.2 ΩT = 4 N·mn = 750 rpm

D = %

6.Hard

A separately excited DC motor on 48 V (R_a = 0.8 Ω) runs at 1500 rpm with an armature current of 8 A. The load changes and the current becomes 2 A, with the field unchanged. Find the new speed n₂.

V = 48 VR_a = 0.8 Ωn₁ = 1500 rpmI_a1 = 8 AI_a2 = 2 A

n₂ = rpm

Answer key

1. k = 1.008 V·s/rad
2. k = 1.79 V·s/rad
3. η = 87.36 %
4. SR = 11.39 %
5. D = 31.29 %
6. n₂ = 1673 rpm

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