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​ = 2 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 2 Ω is switched directly onto a 240 V supply. Its normal full-load current is 5 A. Find the armature current Istart at the moment of starting.

V = 240 VRa = 2 Ω
Istart = A
2.Easy
+−24 VRa​Ia​ = 15 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.6 N·m/A. Find the torque T it develops at an armature current of 15 A.

k = 0.6 N·m/AIa = 15 A
T = N·m
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.5 N·m/A and Ra = 1.2 Ω is connected to 48 V. Find its stall torque Tstall, the torque at zero speed.

V = 48 Vk = 0.5 N·m/ARa = 1.2 Ω
Tstall = N·m
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.3 V·s/rad runs from 120 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 120 Vk = 0.3 V·s/rad
n0 = rpm
5.Hard
T (N·m)n (rpm)2.5 N·m1000 rpm

A DC motor with k = 0.5 V·s/rad and Ra = 2 Ω must drive a load of 2.5 N·m at 1000 rpm. Find the armature voltage V it needs.

k = 0.5 V·s/radRa = 2 ΩT = 2.5 N·mn = 1000 rpm
V = V
6.Hard
+−48 VRa​ = 0.4 ΩIa​: 4 → 2.4 AM+−En = ?T

A separately excited DC motor on 48 V (Ra = 0.4 Ω) runs at 1000 rpm with an armature current of 4 A. The load changes and the current becomes 2.4 A, with the field unchanged. Find the new speed n2.

V = 48 VRa = 0.4 Ωn1 = 1000 rpmIa1 = 4 AIa2 = 2.4 A
n2 = rpm

Answer key

  1. 1. Istart = 120 A
  2. 2. T = 9 N·m
  3. 3. Tstall = 20 N·m
  4. 4. n0 = 3820 rpm
  5. 5. V = 62.36 V
  6. 6. n2 = 1014 rpm

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