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 110 V with an armature current of 10 A. The armature resistance is 0.5 Ω. Find the developed (electromagnetic) power P_dev.

V = 110 VI_a = 10 AR_a = 0.5 Ω

P_dev = W

2.Easy

A DC motor turns at 1450 rpm, and its back-EMF at that speed is 43.2 V. Find the motor constant k in V·s/rad.

E = 43.2 Vn = 1450 rpm

k = V·s/rad

3.Medium

A DC motor (k = 0.8 V·s/rad, R_a = 2 Ω) runs from 110 V. At full load it draws 5 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 110 Vk = 0.8 V·s/radR_a = 2 ΩI_FL = 5 A

SR = %

4.Medium

A DC motor (k = 0.1 V·s/rad, R_a = 0.4 Ω) runs from 24 V. At full load it draws 5 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 24 Vk = 0.1 V·s/radR_a = 0.4 ΩI_FL = 5 A

SR = %

5.Hard

A DC motor (k = 1 V·s/rad, R_a = 0.25 Ω) on a fixed 240 V supply drives a load of 30 N·m. Find the series resistance R_s that slows it down to 300 rpm.

V = 240 Vk = 1 V·s/radR_a = 0.25 ΩT = 30 N·mn = 300 rpm

R_s = Ω

6.Hard

A separately excited DC motor on 120 V (R_a = 0.4 Ω) runs at 1000 rpm with an armature current of 40 A. The load changes and the current becomes 10 A, with the field unchanged. Find the new speed n₂.

V = 120 VR_a = 0.4 Ωn₁ = 1000 rpmI_a1 = 40 AI_a2 = 10 A

n₂ = rpm

Answer key

1. P_dev = 1050 W
2. k = 0.2845 V·s/rad
3. SR = 10 %
4. SR = 9.091 %
5. R_s = 6.703 Ω
6. n₂ = 1115 rpm

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