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
+−24 VRa​Ia​ = 10 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.08 N·m/A. Find the torque T it develops at an armature current of 10 A.

k = 0.08 N·m/AIa = 10 A
T = N·m
2.Easy
+−48 VRa​ = 0.25 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.25 Ω is switched directly onto a 48 V supply. Its normal full-load current is 20 A. Find the armature current Istart at the moment of starting.

V = 48 VRa = 0.25 Ω
Istart = A
3.Medium
T (N·m)n (rpm)12 N·mn0​n = ?

A DC motor (k = 1 N·m/A, Ra = 0.4 Ω) runs from 110 V and drives a load torque of 12 N·m. Find its speed n in rpm.

V = 110 Vk = 1 N·m/ARa = 0.4 ΩT = 12 N·m
n = rpm
4.Medium
T (N·m)n (rpm)0.5 N·mn0​n = ?

A DC motor (k = 0.1 N·m/A, Ra = 0.2 Ω) runs from 24 V and drives a load torque of 0.5 N·m. Find its speed n in rpm.

V = 24 Vk = 0.1 N·m/ARa = 0.2 ΩT = 0.5 N·m
n = rpm
5.Hard
+−120 VRst​ = ?Ra​ = 0.25 Ω80 AM+−E = 0n = 0T

A 120 V DC motor has an armature resistance of 0.25 Ω. Find the series starting resistance Rst that limits the starting current to 80 A.

V = 120 VRa = 0.25 ΩImax = 80 A
Rst = Ω
6.Hard
+−220 VRst​ = ?Ra​ = 1 Ω40 AM+−E = 0n = 0T

A 220 V DC motor has an armature resistance of 1 Ω. Find the series starting resistance Rst that limits the starting current to 40 A.

V = 220 VRa = 1 ΩImax = 40 A
Rst = Ω

Answer key

  1. 1. T = 0.8 N·m
  2. 2. Istart = 192 A
  3. 3. n = 1005 rpm
  4. 4. n = 2196 rpm
  5. 5. Rst = 1.25 Ω
  6. 6. Rst = 4.5 Ω

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