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
+−220 VRa​ = 2 ΩIa​ = 6 AM+−EωT

A DC motor runs from 220 V with an armature current of 6 A. The armature resistance is 2 Ω. Find the developed (electromagnetic) power Pdev.

V = 220 VIa = 6 ARa = 2 Ω
Pdev = W
2.Easy
+−24 VRa​ = 0.2 ΩIa​ = 5 AM+−EωT

A DC motor runs from 24 V with an armature current of 5 A. The armature resistance is 0.2 Ω. Find the developed (electromagnetic) power Pdev.

V = 24 VIa = 5 ARa = 0.2 Ω
Pdev = W
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.8 N·m/A and Ra = 1.5 Ω is connected to 240 V. Find its stall torque Tstall, the torque at zero speed.

V = 240 Vk = 0.8 N·m/ARa = 1.5 Ω
Tstall = N·m
4.Medium
T (N·m)n (rpm)3.6 N·mn0​n = ?

A DC motor (k = 0.3 N·m/A, Ra = 0.8 Ω) runs from 120 V and drives a load torque of 3.6 N·m. Find its speed n in rpm.

V = 120 Vk = 0.3 N·m/ARa = 0.8 ΩT = 3.6 N·m
n = rpm
5.Hard
+−48 VRa​ = 1 ΩM+−EωT

A small DC motor has an armature resistance of 1 Ω and runs from 48 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 48 VRa = 1 Ω
Pmax = W
6.Hard
+−120 VRa​ = 2 ΩM+−En = ?T = 2.5 N·m

A separately excited DC motor (k = 0.5 V·s/rad at full field, Ra = 2 Ω, V = 120 V) drives a constant load torque of 2.5 N·m. The field flux is reduced to 75 % of its full value. Find the new speed n.

V = 120 Vk = 0.5 V·s/radRa = 2 ΩT = 2.5 N·mΦ = 75 %
n = rpm

Answer key

  1. 1. Pdev = 1248 W
  2. 2. Pdev = 115 W
  3. 3. Tstall = 128 N·m
  4. 4. n = 3514 rpm
  5. 5. Pmax = 576 W
  6. 6. n = 2716 rpm

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