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
+−240 VRa​ = 0.5 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.5 Ω is switched directly onto a 240 V supply. Its normal full-load current is 25 A. Find the armature current Istart at the moment of starting.

V = 240 VRa = 0.5 Ω
Istart = A
2.Easy
+−220 VRa​ = 1.5 ΩIa​ = 15 AM+−E = ?ωT

A DC motor runs from a 220 V supply and draws an armature current of 15 A. Its armature resistance is 1.5 Ω. Find the back-EMF E.

V = 220 VIa = 15 ARa = 1.5 Ω
E = V
3.Medium
T (N·m)n (rpm)0.2 N·mn0​n = ?

A DC motor (k = 0.1 N·m/A, Ra = 1.2 Ω) runs from 24 V and drives a load torque of 0.2 N·m. Find its speed n in rpm.

V = 24 Vk = 0.1 N·m/ARa = 1.2 ΩT = 0.2 N·m
n = rpm
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 2 N·m/A and Ra = 1 Ω is connected to 240 V. Find its stall torque Tstall, the torque at zero speed.

V = 240 Vk = 2 N·m/ARa = 1 Ω
Tstall = N·m
5.Hard
+−110 VRa​ = 1 ΩM+−EωT

A small DC motor has an armature resistance of 1 Ω and runs from 110 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 110 VRa = 1 Ω
Pmax = W
6.Hard
T (N·m)n (rpm)0.48 N·m600 rpm

A DC motor with k = 0.08 V·s/rad and Ra = 0.4 Ω must drive a load of 0.48 N·m at 600 rpm. Find the armature voltage V it needs.

k = 0.08 V·s/radRa = 0.4 ΩT = 0.48 N·mn = 600 rpm
V = V

Answer key

  1. 1. Istart = 480 A
  2. 2. E = 197.5 V
  3. 3. n = 2063 rpm
  4. 4. Tstall = 480 N·m
  5. 5. Pmax = 3025 W
  6. 6. V = 7.427 V

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