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
+−120 VRa​ = 2 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 2 Ω is switched directly onto a 120 V supply. Its normal full-load current is 6 A. Find the armature current Istart at the moment of starting.

V = 120 VRa = 2 Ω
Istart = A
2.Easy
+−240 VRa​ = 0.8 ΩIa​ = 15 AM+−EωT

A DC motor runs from 240 V with an armature current of 15 A. The armature resistance is 0.8 Ω. Find the developed (electromagnetic) power Pdev.

V = 240 VIa = 15 ARa = 0.8 Ω
Pdev = W
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 1 V·s/rad runs from 240 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 240 Vk = 1 V·s/rad
n0 = rpm
4.Medium
T (N·m)n (rpm)8 N·mn0​nFL​

A DC motor (k = 1 V·s/rad, Ra = 1.2 Ω) runs from 110 V. At full load it draws 8 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 110 Vk = 1 V·s/radRa = 1.2 ΩIFL = 8 A
SR = %
5.Hard
+−240 VRa​ = 0.8 ΩM+−En = ?T = 37.5 N·m

A separately excited DC motor (k = 1.5 V·s/rad at full field, Ra = 0.8 Ω, V = 240 V) drives a constant load torque of 37.5 N·m. The field flux is reduced to 90 % of its full value. Find the new speed n.

V = 240 Vk = 1.5 V·s/radRa = 0.8 ΩT = 37.5 N·mΦ = 90 %
n = rpm
6.Hard
+−110 VRs​ = ?Ra​ = 0.4 ΩM+−En = 500 rpmT = 18 N·m

A DC motor (k = 1.5 V·s/rad, Ra = 0.4 Ω) on a fixed 110 V supply drives a load of 18 N·m. Find the series resistance Rs that slows it down to 500 rpm.

V = 110 Vk = 1.5 V·s/radRa = 0.4 ΩT = 18 N·mn = 500 rpm
Rs = Ω

Answer key

  1. 1. Istart = 60 A
  2. 2. Pdev = 3420 W
  3. 3. n0 = 2292 rpm
  4. 4. SR = 9.562 %
  5. 5. n = 1540 rpm
  6. 6. Rs = 2.222 Ω

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