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
+−240 VRa​ = 1.2 ΩIa​ = 10 AM+−E = ?ωT

A DC motor runs from a 240 V supply and draws an armature current of 10 A. Its armature resistance is 1.2 Ω. Find the back-EMF E.

V = 240 VIa = 10 ARa = 1.2 Ω
E = V
2.Easy
+−120 VRa​Ia​ = 8 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.4 N·m/A. Find the torque T it develops at an armature current of 8 A.

k = 0.4 N·m/AIa = 8 A
T = N·m
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.8 N·m/A and Ra = 1.2 Ω is connected to 240 V. Find its stall torque Tstall, the torque at zero speed.

V = 240 Vk = 0.8 N·m/ARa = 1.2 Ω
Tstall = N·m
4.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
5.Hard
+−48 VRa​ = 0.4 ΩM+−En = ?T = 3.2 N·m

A separately excited DC motor (k = 0.4 V·s/rad at full field, Ra = 0.4 Ω, V = 48 V) drives a constant load torque of 3.2 N·m. The field flux is reduced to 90 % of its full value. Find the new speed n.

V = 48 Vk = 0.4 V·s/radRa = 0.4 ΩT = 3.2 N·mΦ = 90 %
n = rpm
6.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

Answer key

  1. 1. E = 228 V
  2. 2. T = 3.2 N·m
  3. 3. Tstall = 160 N·m
  4. 4. T = 7.958 N·m
  5. 5. n = 1179 rpm
  6. 6. Pmax = 144 W

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