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​ = 1 ΩIa​ = 12 AM+−E = ?ωT

A DC motor runs from a 110 V supply and draws an armature current of 12 A. Its armature resistance is 1 Ω. Find the back-EMF E.

V = 110 VIa = 12 ARa = 1 Ω
E = V
2.Easy
+−220 VRa​ = 2 ΩIa​ = 12 AM+−E = ?ωT

A DC motor runs from a 220 V supply and draws an armature current of 12 A. Its armature resistance is 2 Ω. Find the back-EMF E.

V = 220 VIa = 12 ARa = 2 Ω
E = V
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.3 V·s/rad runs from 120 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 120 Vk = 0.3 V·s/rad
n0 = rpm
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.4 V·s/rad runs from 48 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 48 Vk = 0.4 V·s/rad
n0 = rpm
5.Hard
+−220 VD = ?Ra​ = 1.5 ΩM+−En = 750 rpmT = 0.24 N·m

A DC chopper feeds a motor (k = 0.12 V·s/rad, Ra = 1.5 Ω) from a 220 V DC bus. The average armature voltage is D·220 V. Find the duty cycle D that runs the motor at 750 rpm with a load of 0.24 N·m.

Vdc = 220 Vk = 0.12 V·s/radRa = 1.5 ΩT = 0.24 N·mn = 750 rpm
D = %
6.Hard
T (N·m)n (rpm)9 N·m1000 rpm

A DC motor with k = 0.75 V·s/rad and Ra = 2 Ω must drive a load of 9 N·m at 1000 rpm. Find the armature voltage V it needs.

k = 0.75 V·s/radRa = 2 ΩT = 9 N·mn = 1000 rpm
V = V

Answer key

  1. 1. E = 98 V
  2. 2. E = 196 V
  3. 3. n0 = 3820 rpm
  4. 4. n0 = 1146 rpm
  5. 5. D = 5.648 %
  6. 6. V = 102.5 V

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