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
+−110 VRa​ = 0.5 ΩIa​ = 30 AM+−EωT

A DC motor runs from 110 V with an armature current of 30 A. The armature resistance is 0.5 Ω. Find the developed (electromagnetic) power Pdev.

V = 110 VIa = 30 ARa = 0.5 Ω
Pdev = W
2.Easy
+−48 VRa​ = 0.5 ΩIa​ = 6 AM+−E = ?ωT

A DC motor runs from a 48 V supply and draws an armature current of 6 A. Its armature resistance is 0.5 Ω. Find the back-EMF E.

V = 48 VIa = 6 ARa = 0.5 Ω
E = V
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.75 V·s/rad runs from 240 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 240 Vk = 0.75 V·s/rad
n0 = rpm
4.Medium
T (N·m)n (rpm)1.2 N·mn0​nFL​

A DC motor (k = 0.3 V·s/rad, Ra = 0.4 Ω) runs from 24 V. At full load it draws 4 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 24 Vk = 0.3 V·s/radRa = 0.4 ΩIFL = 4 A
SR = %
5.Hard
T (N·m)n (rpm)1.6 N·m750 rpm

A DC motor with k = 0.4 V·s/rad and Ra = 1 Ω must drive a load of 1.6 N·m at 750 rpm. Find the armature voltage V it needs.

k = 0.4 V·s/radRa = 1 ΩT = 1.6 N·mn = 750 rpm
V = V
6.Hard
+−110 VRa​ = 0.25 ΩM+−En = ?T = 9 N·m

A separately excited DC motor (k = 0.6 V·s/rad at full field, Ra = 0.25 Ω, V = 110 V) drives a constant load torque of 9 N·m. The field flux is reduced to 85 % of its full value. Find the new speed n.

V = 110 Vk = 0.6 V·s/radRa = 0.25 ΩT = 9 N·mΦ = 85 %
n = rpm

Answer key

  1. 1. Pdev = 2850 W
  2. 2. E = 45 V
  3. 3. n0 = 3056 rpm
  4. 4. SR = 7.143 %
  5. 5. V = 35.42 V
  6. 6. n = 1977 rpm

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