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
+−48 VRa​ = 1.5 ΩIa​ = ?M+−E = 42 VωT

A DC motor on a 48 V supply has an armature resistance of 1.5 Ω. At its present speed the back-EMF is 42 V. Find the armature current Ia.

V = 48 VE = 42 VRa = 1.5 Ω
Ia = A
2.Easy
+−240 VRa​ = 1.5 ΩIa​ = 20 AM+−En = ?T

A DC motor with k = 1.5 V·s/rad and an armature resistance of 1.5 Ω runs from 240 V and draws 20 A. Find its speed n in rpm.

V = 240 VRa = 1.5 ΩIa = 20 Ak = 1.5 V·s/rad
n = rpm
3.Medium
T (N·m)n (rpm)4.5 N·mn0​n = ?

A DC motor (k = 0.75 N·m/A, Ra = 0.8 Ω) runs from 120 V and drives a load torque of 4.5 N·m. Find its speed n in rpm.

V = 120 Vk = 0.75 N·m/ARa = 0.8 ΩT = 4.5 N·m
n = rpm
4.Medium
T (N·m)n (rpm)15 N·mn0​nFL​

A DC motor (k = 0.5 V·s/rad, Ra = 0.2 Ω) runs from 120 V. At full load it draws 30 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 120 Vk = 0.5 V·s/radRa = 0.2 ΩIFL = 30 A
SR = %
5.Hard
+−110 VRa​ = 2.5 ΩM+−EωT

A small DC motor has an armature resistance of 2.5 Ω and runs from 110 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 110 VRa = 2.5 Ω
Pmax = W
6.Hard
+−120 VRa​ = 2 ΩM+−EωT

A small DC motor has an armature resistance of 2 Ω and runs from 120 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 120 VRa = 2 Ω
Pmax = W

Answer key

  1. 1. Ia = 4 A
  2. 2. n = 1337 rpm
  3. 3. n = 1467 rpm
  4. 4. SR = 5.263 %
  5. 5. Pmax = 1210 W
  6. 6. Pmax = 1800 W

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