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
+−110 VRa​ = 0.25 ΩIa​ = 50 AM+−EωT

A DC motor runs from 110 V with an armature current of 50 A. The armature resistance is 0.25 Ω. Find the developed (electromagnetic) power Pdev.

V = 110 VIa = 50 ARa = 0.25 Ω
Pdev = W
2.Easy
+−240 VRa​ = 1.5 ΩIa​ = ?M+−E = 231 VωT

A DC motor on a 240 V supply has an armature resistance of 1.5 Ω. At its present speed the back-EMF is 231 V. Find the armature current Ia.

V = 240 VE = 231 VRa = 1.5 Ω
Ia = A
3.Medium
+−VRa​M+−En = 2900 rpmT = ?

A DC motor delivers 2.2 kW of mechanical power at 2900 rpm. Find the shaft torque T.

P = 2.2 kWn = 2900 rpm
T = N·m
4.Medium
T (N·m)n (rpm)20 N·mn0​nFL​

A DC motor (k = 1 V·s/rad, Ra = 0.25 Ω) runs from 110 V. At full load it draws 20 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 110 Vk = 1 V·s/radRa = 0.25 ΩIFL = 20 A
SR = %
5.Hard
+−220 VRa​ = 1 ΩIa​: 20 → 8 AM+−En = ?T

A separately excited DC motor on 220 V (Ra = 1 Ω) runs at 1440 rpm with an armature current of 20 A. The load changes and the current becomes 8 A, with the field unchanged. Find the new speed n2.

V = 220 VRa = 1 Ωn1 = 1440 rpmIa1 = 20 AIa2 = 8 A
n2 = rpm
6.Hard
+−48 VRa​ = 1.2 ΩM+−En = ?T = 2.4 N·m

A separately excited DC motor (k = 0.6 V·s/rad at full field, Ra = 1.2 Ω, V = 48 V) drives a constant load torque of 2.4 N·m. The field flux is reduced to 90 % of its full value. Find the new speed n.

V = 48 Vk = 0.6 V·s/radRa = 1.2 ΩT = 2.4 N·mΦ = 90 %
n = rpm

Answer key

  1. 1. Pdev = 4875 W
  2. 2. Ia = 6 A
  3. 3. T = 7.244 N·m
  4. 4. SR = 4.762 %
  5. 5. n2 = 1526 rpm
  6. 6. n = 754.5 rpm

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