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​ = 1.2 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 1.2 Ω is switched directly onto a 120 V supply. Its normal full-load current is 12 A. Find the armature current Istart at the moment of starting.

V = 120 VRa = 1.2 Ω
Istart = A
2.Easy
+−48 VRa​Ia​ = 4 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.8 N·m/A. Find the torque T it develops at an armature current of 4 A.

k = 0.8 N·m/AIa = 4 A
T = N·m
3.Medium
+−VRa​M+−En = 1500 rpmT = ?

A DC motor delivers 4 kW of mechanical power at 1500 rpm. Find the shaft torque T.

P = 4 kWn = 1500 rpm
T = N·m
4.Medium
T (N·m)n (rpm)4 N·mn0​n = ?

A DC motor (k = 1 N·m/A, Ra = 1 Ω) runs from 110 V and drives a load torque of 4 N·m. Find its speed n in rpm.

V = 110 Vk = 1 N·m/ARa = 1 ΩT = 4 N·m
n = rpm
5.Hard
+−220 VRst​ = ?Ra​ = 0.5 Ω80 AM+−E = 0n = 0T

A 220 V DC motor has an armature resistance of 0.5 Ω. Find the series starting resistance Rst that limits the starting current to 80 A.

V = 220 VRa = 0.5 ΩImax = 80 A
Rst = Ω
6.Hard
+−220 VRst​ = ?Ra​ = 1.2 Ω30 AM+−E = 0n = 0T

A 220 V DC motor has an armature resistance of 1.2 Ω. Find the series starting resistance Rst that limits the starting current to 30 A.

V = 220 VRa = 1.2 ΩImax = 30 A
Rst = Ω

Answer key

  1. 1. Istart = 100 A
  2. 2. T = 3.2 N·m
  3. 3. T = 25.46 N·m
  4. 4. n = 1012 rpm
  5. 5. Rst = 2.25 Ω
  6. 6. Rst = 6.133 Ω

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