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
+−240 VRa​ = 1 ΩIa​ = 12 AM+−E = ?ωT

A DC motor runs from a 240 V supply and draws an armature current of 12 A. Its armature resistance is 1 Ω. Find the back-EMF E.

V = 240 VIa = 12 ARa = 1 Ω
E = V
2.Easy
+−220 VRa​M+−E = 200 Vn = 600 rpmT

A DC motor turns at 600 rpm, and its back-EMF at that speed is 200 V. Find the motor constant k in V·s/rad.

E = 200 Vn = 600 rpm
k = V·s/rad
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.3 V·s/rad runs from 48 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 48 Vk = 0.3 V·s/rad
n0 = rpm
4.Medium
+−VRa​M+−En = 3000 rpmT = ?

A DC motor delivers 0.5 kW of mechanical power at 3000 rpm. Find the shaft torque T.

P = 0.5 kWn = 3000 rpm
T = N·m
5.Hard
+−120 VRa​ = 2.5 ΩM+−EωT

A small DC motor has an armature resistance of 2.5 Ω and runs from 120 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 120 VRa = 2.5 Ω
Pmax = W
6.Hard
T (N·m)n (rpm)0.3 N·m300 rpm

A DC motor with k = 0.15 V·s/rad and Ra = 1 Ω must drive a load of 0.3 N·m at 300 rpm. Find the armature voltage V it needs.

k = 0.15 V·s/radRa = 1 ΩT = 0.3 N·mn = 300 rpm
V = V

Answer key

  1. 1. E = 228 V
  2. 2. k = 3.183 V·s/rad
  3. 3. n0 = 1528 rpm
  4. 4. T = 1.592 N·m
  5. 5. Pmax = 1440 W
  6. 6. V = 6.712 V

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