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​ = 0.8 ΩIa​ = 12 AM+−EωT

A DC motor runs from 110 V with an armature current of 12 A. The armature resistance is 0.8 Ω. Find the developed (electromagnetic) power Pdev.

V = 110 VIa = 12 ARa = 0.8 Ω
Pdev = W
2.Easy
+−240 VRa​ = 1 ΩIa​ = 8 AM+−En = ?T

A DC motor with k = 1.5 V·s/rad and an armature resistance of 1 Ω runs from 240 V and draws 8 A. Find its speed n in rpm.

V = 240 VRa = 1 ΩIa = 8 Ak = 1.5 V·s/rad
n = rpm
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.12 N·m/A and Ra = 0.5 Ω is connected to 24 V. Find its stall torque Tstall, the torque at zero speed.

V = 24 Vk = 0.12 N·m/ARa = 0.5 Ω
Tstall = N·m
4.Medium
T (N·m)n (rpm)30 N·mn0​n = ?

A DC motor (k = 0.75 N·m/A, Ra = 0.25 Ω) runs from 220 V and drives a load torque of 30 N·m. Find its speed n in rpm.

V = 220 Vk = 0.75 N·m/ARa = 0.25 ΩT = 30 N·m
n = rpm
5.Hard
+−120 VRst​ = ?Ra​ = 2 Ω40 AM+−E = 0n = 0T

A 120 V DC motor has an armature resistance of 2 Ω. Find the series starting resistance Rst that limits the starting current to 40 A.

V = 120 VRa = 2 ΩImax = 40 A
Rst = Ω
6.Hard
+−110 VRa​ = 0.2 ΩIa​: 20 → 30 AM+−En = ?T

A separately excited DC motor on 110 V (Ra = 0.2 Ω) runs at 1800 rpm with an armature current of 20 A. The load changes and the current becomes 30 A, with the field unchanged. Find the new speed n2.

V = 110 VRa = 0.2 Ωn1 = 1800 rpmIa1 = 20 AIa2 = 30 A
n2 = rpm

Answer key

  1. 1. Pdev = 1205 W
  2. 2. n = 1477 rpm
  3. 3. Tstall = 5.76 N·m
  4. 4. n = 2674 rpm
  5. 5. Rst = 1 Ω
  6. 6. n2 = 1766 rpm

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