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
+−24 VRa​Ia​ = 10 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.15 N·m/A. Find the torque T it develops at an armature current of 10 A.

k = 0.15 N·m/AIa = 10 A
T = N·m
2.Easy
+−120 VRa​ = 0.2 ΩIa​ = ?M+−E = 114 VωT

A DC motor on a 120 V supply has an armature resistance of 0.2 Ω. At its present speed the back-EMF is 114 V. Find the armature current Ia.

V = 120 VE = 114 VRa = 0.2 Ω
Ia = A
3.Medium
+−220 VRa​ = 0.8 ΩIa​ = 10 AM+−EωT

A DC motor draws 10 A from a 220 V supply. Its armature resistance is 0.8 Ω and its rotational (friction, windage and iron) losses are 64 W. Find its efficiency η.

V = 220 VIa = 10 ARa = 0.8 ΩProt = 64 W
η = %
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.2 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 = 0.2 N·m/ARa = 1 Ω
Tstall = N·m
5.Hard
+−120 VRa​ = 0.5 ΩM+−En = ?T = 25 N·m

A separately excited DC motor (k = 1 V·s/rad at full field, Ra = 0.5 Ω, V = 120 V) drives a constant load torque of 25 N·m. The field flux is reduced to 90 % of its full value. Find the new speed n.

V = 120 Vk = 1 V·s/radRa = 0.5 ΩT = 25 N·mΦ = 90 %
n = rpm
6.Hard
T (N·m)n (rpm)0.16 N·m900 rpm

A DC motor with k = 0.08 V·s/rad and Ra = 1.2 Ω must drive a load of 0.16 N·m at 900 rpm. Find the armature voltage V it needs.

k = 0.08 V·s/radRa = 1.2 ΩT = 0.16 N·mn = 900 rpm
V = V

Answer key

  1. 1. T = 1.5 N·m
  2. 2. Ia = 30 A
  3. 3. η = 93.45 %
  4. 4. Tstall = 9.6 N·m
  5. 5. n = 1126 rpm
  6. 6. V = 9.94 V

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