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​ = 15 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.75 N·m/A. Find the torque T it develops at an armature current of 15 A.

k = 0.75 N·m/AIa = 15 A
T = N·m
2.Easy
+−110 VRa​ = 0.8 ΩIa​ = ?M+−E = 105.2 VωT

A DC motor on a 110 V supply has an armature resistance of 0.8 Ω. At its present speed the back-EMF is 105.2 V. Find the armature current Ia.

V = 110 VE = 105.2 VRa = 0.8 Ω
Ia = A
3.Medium
+−VRa​M+−En = 3000 rpmT = ?

A DC motor delivers 7.5 kW of mechanical power at 3000 rpm. Find the shaft torque T.

P = 7.5 kWn = 3000 rpm
T = N·m
4.Medium
T (N·m)n (rpm)1.6 N·mn0​n = ?

A DC motor (k = 0.4 N·m/A, Ra = 2 Ω) runs from 110 V and drives a load torque of 1.6 N·m. Find its speed n in rpm.

V = 110 Vk = 0.4 N·m/ARa = 2 ΩT = 1.6 N·m
n = rpm
5.Hard
+−120 VRa​ = 0.5 ΩIa​: 8 → 3.2 AM+−En = ?T

A separately excited DC motor on 120 V (Ra = 0.5 Ω) runs at 1440 rpm with an armature current of 8 A. The load changes and the current becomes 3.2 A, with the field unchanged. Find the new speed n2.

V = 120 VRa = 0.5 Ωn1 = 1440 rpmIa1 = 8 AIa2 = 3.2 A
n2 = rpm
6.Hard
+−240 VRst​ = ?Ra​ = 0.4 Ω20 AM+−E = 0n = 0T

A 240 V DC motor has an armature resistance of 0.4 Ω. Find the series starting resistance Rst that limits the starting current to 20 A.

V = 240 VRa = 0.4 ΩImax = 20 A
Rst = Ω

Answer key

  1. 1. T = 11.25 N·m
  2. 2. Ia = 6 A
  3. 3. T = 23.87 N·m
  4. 4. n = 2435 rpm
  5. 5. n2 = 1470 rpm
  6. 6. Rst = 11.6 Ω

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