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​ = 12 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.25 N·m/A. Find the torque T it develops at an armature current of 12 A.

k = 0.25 N·m/AIa = 12 A
T = N·m
2.Easy
+−48 VRa​ = 0.25 ΩIa​ = 12 AM+−En = ?T

A DC motor with k = 0.15 V·s/rad and an armature resistance of 0.25 Ω runs from 48 V and draws 12 A. Find its speed n in rpm.

V = 48 VRa = 0.25 ΩIa = 12 Ak = 0.15 V·s/rad
n = rpm
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.1 N·m/A and Ra = 0.4 Ω is connected to 24 V. Find its stall torque Tstall, the torque at zero speed.

V = 24 Vk = 0.1 N·m/ARa = 0.4 Ω
Tstall = N·m
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.3 N·m/A and Ra = 1 Ω is connected to 48 V. Find its stall torque Tstall, the torque at zero speed.

V = 48 Vk = 0.3 N·m/ARa = 1 Ω
Tstall = N·m
5.Hard
+−48 VRst​ = ?Ra​ = 0.5 Ω25 AM+−E = 0n = 0T

A 48 V DC motor has an armature resistance of 0.5 Ω. Find the series starting resistance Rst that limits the starting current to 25 A.

V = 48 VRa = 0.5 ΩImax = 25 A
Rst = Ω
6.Hard
+−120 VRs​ = ?Ra​ = 1.2 ΩM+−En = 750 rpmT = 3 N·m

A DC motor (k = 0.5 V·s/rad, Ra = 1.2 Ω) on a fixed 120 V supply drives a load of 3 N·m. Find the series resistance Rs that slows it down to 750 rpm.

V = 120 Vk = 0.5 V·s/radRa = 1.2 ΩT = 3 N·mn = 750 rpm
Rs = Ω

Answer key

  1. 1. T = 3 N·m
  2. 2. n = 2865 rpm
  3. 3. Tstall = 6 N·m
  4. 4. Tstall = 14.4 N·m
  5. 5. Rst = 1.42 Ω
  6. 6. Rs = 12.26 Ω

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