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
+−220 VRa​Ia​ = 15 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.75 N·m/A. Find the torque T it develops at an armature current of 15 A.

k = 0.75 N·m/AIa = 15 A
T = N·m
2.Easy
+−48 VRa​ = 0.25 ΩIa​ = 6 AM+−E = ?ωT

A DC motor runs from a 48 V supply and draws an armature current of 6 A. Its armature resistance is 0.25 Ω. Find the back-EMF E.

V = 48 VIa = 6 ARa = 0.25 Ω
E = V
3.Medium
T (N·m)n (rpm)1 N·mn0​nFL​

A DC motor (k = 0.25 V·s/rad, Ra = 1.2 Ω) runs from 48 V. At full load it draws 4 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 48 Vk = 0.25 V·s/radRa = 1.2 ΩIFL = 4 A
SR = %
4.Medium
+−VRa​M+−En = 3000 rpmT = ?

A DC motor delivers 2.2 kW of mechanical power at 3000 rpm. Find the shaft torque T.

P = 2.2 kWn = 3000 rpm
T = N·m
5.Hard
+−110 VRa​ = 0.25 ΩM+−En = ?T = 36 N·m

A separately excited DC motor (k = 1.2 V·s/rad at full field, Ra = 0.25 Ω, V = 110 V) drives a constant load torque of 36 N·m. The field flux is reduced to 75 % of its full value. Find the new speed n.

V = 110 Vk = 1.2 V·s/radRa = 0.25 ΩT = 36 N·mΦ = 75 %
n = rpm
6.Hard
T (N·m)n (rpm)4.8 N·m500 rpm

A DC motor with k = 0.6 V·s/rad and Ra = 0.25 Ω must drive a load of 4.8 N·m at 500 rpm. Find the armature voltage V it needs.

k = 0.6 V·s/radRa = 0.25 ΩT = 4.8 N·mn = 500 rpm
V = V

Answer key

  1. 1. T = 11.25 N·m
  2. 2. E = 46.5 V
  3. 3. SR = 11.11 %
  4. 4. T = 7.003 N·m
  5. 5. n = 1061 rpm
  6. 6. V = 33.42 V

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