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​ = 0.25 ΩIa​ = 25 AM+−EωT

A DC motor runs from 110 V with an armature current of 25 A. The armature resistance is 0.25 Ω. Find the developed (electromagnetic) power Pdev.

V = 110 VIa = 25 ARa = 0.25 Ω
Pdev = W
2.Easy
+−240 VRa​ = 0.8 ΩIa​ = 40 AM+−EωT

A DC motor runs from 240 V with an armature current of 40 A. The armature resistance is 0.8 Ω. Find the developed (electromagnetic) power Pdev.

V = 240 VIa = 40 ARa = 0.8 Ω
Pdev = W
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.3 N·m/A and Ra = 1.2 Ω is connected to 120 V. Find its stall torque Tstall, the torque at zero speed.

V = 120 Vk = 0.3 N·m/ARa = 1.2 Ω
Tstall = N·m
4.Medium
+−VRa​M+−En = 1450 rpmT = ?

A DC motor delivers 7.5 kW of mechanical power at 1450 rpm. Find the shaft torque T.

P = 7.5 kWn = 1450 rpm
T = N·m
5.Hard
+−48 VRa​ = 1.5 ΩM+−EωT

A small DC motor has an armature resistance of 1.5 Ω and runs from 48 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 48 VRa = 1.5 Ω
Pmax = W
6.Hard
+−110 VRst​ = ?Ra​ = 0.4 Ω50 AM+−E = 0n = 0T

A 110 V DC motor has an armature resistance of 0.4 Ω. Find the series starting resistance Rst that limits the starting current to 50 A.

V = 110 VRa = 0.4 ΩImax = 50 A
Rst = Ω

Answer key

  1. 1. Pdev = 2594 W
  2. 2. Pdev = 8320 W
  3. 3. Tstall = 30 N·m
  4. 4. T = 49.39 N·m
  5. 5. Pmax = 384 W
  6. 6. Rst = 1.8 Ω

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