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 220 V supply and draws an armature current of 20 A. Its armature resistance is 1.2 Ω. Find the back-EMF E.

V = 220 VI_a = 20 AR_a = 1.2 Ω

E = V

2.Easy

A DC motor with k = 0.12 V·s/rad and an armature resistance of 1.2 Ω runs from 48 V and draws 5 A. Find its speed n in rpm.

V = 48 VR_a = 1.2 ΩI_a = 5 Ak = 0.12 V·s/rad

n = rpm

3.Medium

A DC motor with k = 0.6 N·m/A and R_a = 1 Ω is connected to 110 V. Find its stall torque T_stall, the torque at zero speed.

V = 110 Vk = 0.6 N·m/AR_a = 1 Ω

T_stall = N·m

4.Medium

A DC motor (k = 0.1 V·s/rad, R_a = 1 Ω) runs from 24 V. At full load it draws 2 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 = 1 ΩI_FL = 2 A

SR = %

5.Hard

A separately excited DC motor (k = 0.15 V·s/rad at full field, R_a = 1.5 Ω, V = 48 V) drives a constant load torque of 0.3 N·m. The field flux is reduced to 80 % of its full value. Find the new speed n.

V = 48 Vk = 0.15 V·s/radR_a = 1.5 ΩT = 0.3 N·mΦ = 80 %

n = rpm

6.Hard

A small DC motor has an armature resistance of 1.2 Ω and runs from 48 V. Neglecting rotational losses, find the largest mechanical power P_max it can develop at any speed.

V = 48 VR_a = 1.2 Ω

P_max = W

Answer key

1. E = 196 V
2. n = 3342 rpm
3. T_stall = 66 N·m
4. SR = 9.091 %
5. n = 3521 rpm
6. P_max = 480 W

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