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 permanent-magnet DC motor has the torque constant k = 0.6 N·m/A. Find the torque T it develops at an armature current of 15 A.

k = 0.6 N·m/AI_a = 15 A

T = N·m

2.Easy

A permanent-magnet DC motor has the torque constant k = 0.6 N·m/A. Find the torque T it develops at an armature current of 8 A.

k = 0.6 N·m/AI_a = 8 A

T = N·m

3.Medium

A DC motor (k = 1 V·s/rad, R_a = 0.2 Ω) runs from 110 V. At full load it draws 30 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 110 Vk = 1 V·s/radR_a = 0.2 ΩI_FL = 30 A

SR = %

4.Medium

A DC motor draws 5 A from a 24 V supply. Its armature resistance is 0.5 Ω and its rotational (friction, windage and iron) losses are 3 W. Find its efficiency η.

V = 24 VI_a = 5 AR_a = 0.5 ΩP_rot = 3 W

η = %

5.Hard

A DC motor (k = 0.6 V·s/rad, R_a = 0.2 Ω) on a fixed 110 V supply drives a load of 15 N·m. Find the series resistance R_s that slows it down to 750 rpm.

V = 110 Vk = 0.6 V·s/radR_a = 0.2 ΩT = 15 N·mn = 750 rpm

R_s = Ω

6.Hard

A DC chopper feeds a motor (k = 0.25 V·s/rad, R_a = 1.2 Ω) from a 110 V DC bus. The average armature voltage is D·110 V. Find the duty cycle D that runs the motor at 750 rpm with a load of 0.5 N·m.

V_dc = 110 Vk = 0.25 V·s/radR_a = 1.2 ΩT = 0.5 N·mn = 750 rpm

D = %

Answer key

1. T = 9 N·m
2. T = 4.8 N·m
3. SR = 5.769 %
4. η = 87.08 %
5. R_s = 2.315 Ω
6. D = 20.03 %

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