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​Ia​ = 8 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.75 N·m/A. Find the torque T it develops at an armature current of 8 A.

k = 0.75 N·m/AIa = 8 A
T = N·m
2.Easy
+−48 VRa​ = 0.4 ΩIa​ = 12 AM+−EωT

A DC motor runs from 48 V with an armature current of 12 A. The armature resistance is 0.4 Ω. Find the developed (electromagnetic) power Pdev.

V = 48 VIa = 12 ARa = 0.4 Ω
Pdev = W
3.Medium
+−VRa​M+−En = 1200 rpmT = ?

A DC motor delivers 1 kW of mechanical power at 1200 rpm. Find the shaft torque T.

P = 1 kWn = 1200 rpm
T = N·m
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.75 N·m/A and Ra = 2 Ω is connected to 220 V. Find its stall torque Tstall, the torque at zero speed.

V = 220 Vk = 0.75 N·m/ARa = 2 Ω
Tstall = N·m
5.Hard
+−220 VRa​ = 0.4 ΩM+−En = ?T = 24 N·m

A separately excited DC motor (k = 1.2 V·s/rad at full field, Ra = 0.4 Ω, V = 220 V) drives a constant load torque of 24 N·m. The field flux is reduced to 90 % of its full value. Find the new speed n.

V = 220 Vk = 1.2 V·s/radRa = 0.4 ΩT = 24 N·mΦ = 90 %
n = rpm
6.Hard
+−110 VRa​ = 0.8 ΩM+−En = ?T = 12 N·m

A separately excited DC motor (k = 1.2 V·s/rad at full field, Ra = 0.8 Ω, V = 110 V) drives a constant load torque of 12 N·m. The field flux is reduced to 80 % of its full value. Find the new speed n.

V = 110 Vk = 1.2 V·s/radRa = 0.8 ΩT = 12 N·mΦ = 80 %
n = rpm

Answer key

  1. 1. T = 6 N·m
  2. 2. Pdev = 518.4 W
  3. 3. T = 7.958 N·m
  4. 4. Tstall = 82.5 N·m
  5. 5. n = 1867 rpm
  6. 6. n = 994.7 rpm

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