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 worksheetPractise 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
+−48 VRa​ = 0.25 ΩIa​ = ?M+−E = 45 VωT

A DC motor on a 48 V supply has an armature resistance of 0.25 Ω. At its present speed the back-EMF is 45 V. Find the armature current Ia.

V = 48 VE = 45 VRa = 0.25 Ω
Ia = A
2.Easy
+−48 VRa​M+−E = 45.6 Vn = 600 rpmT

A DC motor turns at 600 rpm, and its back-EMF at that speed is 45.6 V. Find the motor constant k in V·s/rad.

E = 45.6 Vn = 600 rpm
k = V·s/rad
3.Medium
T (N·m)n (rpm)9.6 N·mn0​n = ?

A DC motor (k = 0.8 N·m/A, Ra = 1.5 Ω) runs from 220 V and drives a load torque of 9.6 N·m. Find its speed n in rpm.

V = 220 Vk = 0.8 N·m/ARa = 1.5 ΩT = 9.6 N·m
n = rpm
4.Medium
T (N·m)n (rpm)1.5 N·mn0​nFL​

A DC motor (k = 0.25 V·s/rad, Ra = 0.5 Ω) runs from 24 V. At full load it draws 6 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 24 Vk = 0.25 V·s/radRa = 0.5 ΩIFL = 6 A
SR = %
5.Hard
T (N·m)n (rpm)20 N·m800 rpm

A DC motor with k = 0.8 V·s/rad and Ra = 0.2 Ω must drive a load of 20 N·m at 800 rpm. Find the armature voltage V it needs.

k = 0.8 V·s/radRa = 0.2 ΩT = 20 N·mn = 800 rpm
V = V
6.Hard
+−24 VRa​ = 1.5 ΩM+−EωT

A small DC motor has an armature resistance of 1.5 Ω and runs from 24 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 24 VRa = 1.5 Ω
Pmax = W

Answer key

  1. 1. Ia = 12 A
  2. 2. k = 0.7257 V·s/rad
  3. 3. n = 2411 rpm
  4. 4. SR = 14.29 %
  5. 5. V = 72.02 V
  6. 6. Pmax = 96 W

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