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​ = 0.4 ΩIa​ = 15 AM+−EωT

A DC motor runs from 48 V with an armature current of 15 A. The armature resistance is 0.4 Ω. Find the developed (electromagnetic) power Pdev.

V = 48 VIa = 15 ARa = 0.4 Ω
Pdev = W
2.Easy
+−120 VRa​ = 0.5 ΩIa​ = ?M+−E = 112.5 VωT

A DC motor on a 120 V supply has an armature resistance of 0.5 Ω. At its present speed the back-EMF is 112.5 V. Find the armature current Ia.

V = 120 VE = 112.5 VRa = 0.5 Ω
Ia = A
3.Medium
T (N·m)n (rpm)1.2 N·mn0​nFL​

A DC motor (k = 0.3 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 = 0.3 V·s/radRa = 1.2 ΩIFL = 4 A
SR = %
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.08 V·s/rad runs from 24 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 24 Vk = 0.08 V·s/rad
n0 = rpm
5.Hard
+−110 VRs​ = ?Ra​ = 0.4 ΩM+−En = 400 rpmT = 15 N·m

A DC motor (k = 1 V·s/rad, Ra = 0.4 Ω) on a fixed 110 V supply drives a load of 15 N·m. Find the series resistance Rs that slows it down to 400 rpm.

V = 110 Vk = 1 V·s/radRa = 0.4 ΩT = 15 N·mn = 400 rpm
Rs = Ω
6.Hard
+−220 VRa​ = 0.5 ΩM+−En = ?T = 48 N·m

A separately excited DC motor (k = 1.2 V·s/rad at full field, Ra = 0.5 Ω, V = 220 V) drives a constant load torque of 48 N·m. The field flux is reduced to 70 % of its full value. Find the new speed n.

V = 220 Vk = 1.2 V·s/radRa = 0.5 ΩT = 48 N·mΦ = 70 %
n = rpm

Answer key

  1. 1. Pdev = 630 W
  2. 2. Ia = 15 A
  3. 3. SR = 11.11 %
  4. 4. n0 = 2865 rpm
  5. 5. Rs = 4.141 Ω
  6. 6. n = 2176 rpm

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