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
+−220 VRa​ = 0.25 ΩIa​ = 50 AM+−En = ?T

A DC motor with k = 0.8 V·s/rad and an armature resistance of 0.25 Ω runs from 220 V and draws 50 A. Find its speed n in rpm.

V = 220 VRa = 0.25 ΩIa = 50 Ak = 0.8 V·s/rad
n = rpm
2.Easy
+−48 VRa​M+−E = 44 Vn = 3000 rpmT

A DC motor turns at 3000 rpm, and its back-EMF at that speed is 44 V. Find the motor constant k in V·s/rad.

E = 44 Vn = 3000 rpm
k = V·s/rad
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.5 N·m/A and Ra = 0.25 Ω is connected to 48 V. Find its stall torque Tstall, the torque at zero speed.

V = 48 Vk = 0.5 N·m/ARa = 0.25 Ω
Tstall = N·m
4.Medium
+−VRa​M+−En = 1200 rpmT = ?

A DC motor delivers 5.5 kW of mechanical power at 1200 rpm. Find the shaft torque T.

P = 5.5 kWn = 1200 rpm
T = N·m
5.Hard
+−24 VRa​ = 1.2 ΩM+−EωT

A small DC motor has an armature resistance of 1.2 Ω and runs from 24 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 24 VRa = 1.2 Ω
Pmax = W
6.Hard
+−48 VRa​ = 2 ΩM+−EωT

A small DC motor has an armature resistance of 2 Ω and runs from 48 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 48 VRa = 2 Ω
Pmax = W

Answer key

  1. 1. n = 2477 rpm
  2. 2. k = 0.1401 V·s/rad
  3. 3. Tstall = 96 N·m
  4. 4. T = 43.77 N·m
  5. 5. Pmax = 120 W
  6. 6. Pmax = 288 W

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