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
+−240 VRa​Ia​ = 50 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.8 N·m/A. Find the torque T it develops at an armature current of 50 A.

k = 0.8 N·m/AIa = 50 A
T = N·m
2.Easy
+−240 VRa​ = 2 ΩIa​ = 10 AM+−EωT

A DC motor runs from 240 V with an armature current of 10 A. The armature resistance is 2 Ω. Find the developed (electromagnetic) power Pdev.

V = 240 VIa = 10 ARa = 2 Ω
Pdev = W
3.Medium
T (N·m)n (rpm)22.5 N·mn0​nFL​

A DC motor (k = 0.75 V·s/rad, Ra = 0.5 Ω) runs from 240 V. At full load it draws 30 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 240 Vk = 0.75 V·s/radRa = 0.5 ΩIFL = 30 A
SR = %
4.Medium
T (N·m)n (rpm)1.5 N·mn0​n = ?

A DC motor (k = 0.25 N·m/A, Ra = 0.4 Ω) runs from 24 V and drives a load torque of 1.5 N·m. Find its speed n in rpm.

V = 24 Vk = 0.25 N·m/ARa = 0.4 ΩT = 1.5 N·m
n = rpm
5.Hard
+−110 VRa​ = 1 ΩM+−En = ?T = 4 N·m

A separately excited DC motor (k = 0.5 V·s/rad at full field, Ra = 1 Ω, V = 110 V) drives a constant load torque of 4 N·m. The field flux is reduced to 90 % of its full value. Find the new speed n.

V = 110 Vk = 0.5 V·s/radRa = 1 ΩT = 4 N·mΦ = 90 %
n = rpm
6.Hard
T (N·m)n (rpm)0.8 N·m300 rpm

A DC motor with k = 0.2 V·s/rad and Ra = 0.4 Ω must drive a load of 0.8 N·m at 300 rpm. Find the armature voltage V it needs.

k = 0.2 V·s/radRa = 0.4 ΩT = 0.8 N·mn = 300 rpm
V = V

Answer key

  1. 1. T = 40 N·m
  2. 2. Pdev = 2200 W
  3. 3. SR = 6.667 %
  4. 4. n = 825.1 rpm
  5. 5. n = 2146 rpm
  6. 6. V = 7.883 V

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