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​ = 1 ΩIa​ = 6 AM+−EωT

A DC motor runs from 48 V with an armature current of 6 A. The armature resistance is 1 Ω. Find the developed (electromagnetic) power Pdev.

V = 48 VIa = 6 ARa = 1 Ω
Pdev = W
2.Easy
+−240 VRa​ = 0.8 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.8 Ω is switched directly onto a 240 V supply. Its normal full-load current is 40 A. Find the armature current Istart at the moment of starting.

V = 240 VRa = 0.8 Ω
Istart = A
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.25 V·s/rad runs from 24 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 24 Vk = 0.25 V·s/rad
n0 = rpm
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.75 N·m/A and Ra = 1 Ω is connected to 48 V. Find its stall torque Tstall, the torque at zero speed.

V = 48 Vk = 0.75 N·m/ARa = 1 Ω
Tstall = N·m
5.Hard
T (N·m)n (rpm)4.5 N·m750 rpm

A DC motor with k = 0.75 V·s/rad and Ra = 1.5 Ω must drive a load of 4.5 N·m at 750 rpm. Find the armature voltage V it needs.

k = 0.75 V·s/radRa = 1.5 ΩT = 4.5 N·mn = 750 rpm
V = V
6.Hard
+−110 VRa​ = 0.25 ΩM+−En = ?T = 20 N·m

A separately excited DC motor (k = 0.8 V·s/rad at full field, Ra = 0.25 Ω, V = 110 V) drives a constant load torque of 20 N·m. The field flux is reduced to 80 % of its full value. Find the new speed n.

V = 110 Vk = 0.8 V·s/radRa = 0.25 ΩT = 20 N·mΦ = 80 %
n = rpm

Answer key

  1. 1. Pdev = 252 W
  2. 2. Istart = 300 A
  3. 3. n0 = 916.7 rpm
  4. 4. Tstall = 36 N·m
  5. 5. V = 67.9 V
  6. 6. n = 1525 rpm

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