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
+−24 VRa​ = 0.5 ΩIa​ = 6 AM+−E = ?ωT

A DC motor runs from a 24 V supply and draws an armature current of 6 A. Its armature resistance is 0.5 Ω. Find the back-EMF E.

V = 24 VIa = 6 ARa = 0.5 Ω
E = V
2.Easy
+−110 VRa​ = 1.2 ΩIa​ = 4 AM+−EωT

A DC motor runs from 110 V with an armature current of 4 A. The armature resistance is 1.2 Ω. Find the developed (electromagnetic) power Pdev.

V = 110 VIa = 4 ARa = 1.2 Ω
Pdev = W
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 1 V·s/rad runs from 110 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 110 Vk = 1 V·s/rad
n0 = rpm
4.Medium
+−VRa​M+−En = 1800 rpmT = ?

A DC motor delivers 15 kW of mechanical power at 1800 rpm. Find the shaft torque T.

P = 15 kWn = 1800 rpm
T = N·m
5.Hard
+−48 VRa​ = 0.5 ΩM+−EωT

A small DC motor has an armature resistance of 0.5 Ω and runs from 48 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 48 VRa = 0.5 Ω
Pmax = W
6.Hard
+−220 VRa​ = 1 ΩM+−En = ?T = 25 N·m

A separately excited DC motor (k = 1 V·s/rad at full field, Ra = 1 Ω, V = 220 V) drives a constant load torque of 25 N·m. The field flux is reduced to 75 % of its full value. Find the new speed n.

V = 220 Vk = 1 V·s/radRa = 1 ΩT = 25 N·mΦ = 75 %
n = rpm

Answer key

  1. 1. E = 21 V
  2. 2. Pdev = 420.8 W
  3. 3. n0 = 1050 rpm
  4. 4. T = 79.58 N·m
  5. 5. Pmax = 1152 W
  6. 6. n = 2377 rpm

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