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​ = 0.25 ΩIa​ = 25 AM+−En = ?T

A DC motor with k = 1.5 V·s/rad and an armature resistance of 0.25 Ω runs from 110 V and draws 25 A. Find its speed n in rpm.

V = 110 VRa = 0.25 ΩIa = 25 Ak = 1.5 V·s/rad
n = rpm
2.Easy
+−240 VRa​Ia​ = 25 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.3 N·m/A. Find the torque T it develops at an armature current of 25 A.

k = 0.3 N·m/AIa = 25 A
T = N·m
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.5 N·m/A and Ra = 1.2 Ω is connected to 48 V. Find its stall torque Tstall, the torque at zero speed.

V = 48 Vk = 0.5 N·m/ARa = 1.2 Ω
Tstall = N·m
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 1.2 V·s/rad runs from 240 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 240 Vk = 1.2 V·s/rad
n0 = rpm
5.Hard
+−110 VRs​ = ?Ra​ = 0.8 ΩM+−En = 300 rpmT = 12 N·m

A DC motor (k = 1.5 V·s/rad, Ra = 0.8 Ω) on a fixed 110 V supply drives a load of 12 N·m. Find the series resistance Rs that slows it down to 300 rpm.

V = 110 Vk = 1.5 V·s/radRa = 0.8 ΩT = 12 N·mn = 300 rpm
Rs = Ω
6.Hard
+−48 VRa​ = 0.8 ΩIa​: 5 → 10 AM+−En = ?T

A separately excited DC motor on 48 V (Ra = 0.8 Ω) runs at 1500 rpm with an armature current of 5 A. The load changes and the current becomes 10 A, with the field unchanged. Find the new speed n2.

V = 48 VRa = 0.8 Ωn1 = 1500 rpmIa1 = 5 AIa2 = 10 A
n2 = rpm

Answer key

  1. 1. n = 660.5 rpm
  2. 2. T = 7.5 N·m
  3. 3. Tstall = 20 N·m
  4. 4. n0 = 1910 rpm
  5. 5. Rs = 7.06 Ω
  6. 6. n2 = 1364 rpm

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