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.

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1.Easy
+−48 VRa​ = 0.2 ΩIa​ = 20 AM+−En = ?T

A DC motor with k = 0.3 V·s/rad and an armature resistance of 0.2 Ω runs from 48 V and draws 20 A. Find its speed n in rpm.

V = 48 VRa = 0.2 ΩIa = 20 Ak = 0.3 V·s/rad
n = rpm
2.Easy
+−48 VRa​Ia​ = 2 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.4 N·m/A. Find the torque T it develops at an armature current of 2 A.

k = 0.4 N·m/AIa = 2 A
T = N·m
3.Medium
+−240 VRa​ = 0.4 ΩIa​ = 20 AM+−EωT

A DC motor draws 20 A from a 240 V supply. Its armature resistance is 0.4 Ω and its rotational (friction, windage and iron) losses are 139 W. Find its efficiency η.

V = 240 VIa = 20 ARa = 0.4 ΩProt = 139 W
η = %
4.Medium
T (N·m)n (rpm)9.6 N·mn0​n = ?

A DC motor (k = 1.2 N·m/A, Ra = 1 Ω) runs from 120 V and drives a load torque of 9.6 N·m. Find its speed n in rpm.

V = 120 Vk = 1.2 N·m/ARa = 1 ΩT = 9.6 N·m
n = rpm
5.Hard
T (N·m)n (rpm)16 N·m1200 rpm

A DC motor with k = 0.8 V·s/rad and Ra = 0.8 Ω must drive a load of 16 N·m at 1200 rpm. Find the armature voltage V it needs.

k = 0.8 V·s/radRa = 0.8 ΩT = 16 N·mn = 1200 rpm
V = V
6.Hard
+−220 VRa​ = 1.2 ΩM+−En = ?T = 40 N·m

A separately excited DC motor (k = 2 V·s/rad at full field, Ra = 1.2 Ω, V = 220 V) drives a constant load torque of 40 N·m. The field flux is reduced to 75 % of its full value. Find the new speed n.

V = 220 Vk = 2 V·s/radRa = 1.2 ΩT = 40 N·mΦ = 75 %
n = rpm

Answer key

  1. 1. n = 1401 rpm
  2. 2. T = 0.8 N·m
  3. 3. η = 93.77 %
  4. 4. n = 891.3 rpm
  5. 5. V = 116.5 V
  6. 6. n = 1197 rpm

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