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
+−48 VRa​ = 0.25 ΩIa​ = 15 AM+−EωT

A DC motor runs from 48 V with an armature current of 15 A. The armature resistance is 0.25 Ω. Find the developed (electromagnetic) power Pdev.

V = 48 VIa = 15 ARa = 0.25 Ω
Pdev = W
2.Easy
+−220 VRa​ = 2 ΩIa​ = 6 AM+−En = ?T

A DC motor with k = 1.5 V·s/rad and an armature resistance of 2 Ω runs from 220 V and draws 6 A. Find its speed n in rpm.

V = 220 VRa = 2 ΩIa = 6 Ak = 1.5 V·s/rad
n = rpm
3.Medium
+−VRa​M+−En = 1450 rpmT = ?

A DC motor delivers 1.5 kW of mechanical power at 1450 rpm. Find the shaft torque T.

P = 1.5 kWn = 1450 rpm
T = N·m
4.Medium
+−VRa​M+−En = 1450 rpmT = ?

A DC motor delivers 4 kW of mechanical power at 1450 rpm. Find the shaft torque T.

P = 4 kWn = 1450 rpm
T = N·m
5.Hard
+−48 VRa​ = 4 ΩM+−EωT

A small DC motor has an armature resistance of 4 Ω and runs from 48 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 48 VRa = 4 Ω
Pmax = W
6.Hard
+−110 VRa​ = 1.2 ΩM+−En = ?T = 2.4 N·m

A separately excited DC motor (k = 0.3 V·s/rad at full field, Ra = 1.2 Ω, V = 110 V) drives a constant load torque of 2.4 N·m. The field flux is reduced to 70 % of its full value. Find the new speed n.

V = 110 Vk = 0.3 V·s/radRa = 1.2 ΩT = 2.4 N·mΦ = 70 %
n = rpm

Answer key

  1. 1. Pdev = 663.8 W
  2. 2. n = 1324 rpm
  3. 3. T = 9.879 N·m
  4. 4. T = 26.34 N·m
  5. 5. Pmax = 144 W
  6. 6. n = 4378 rpm

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