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​ = 2 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 2 N·m/A. Find the torque T it develops at an armature current of 2 A.

k = 2 N·m/AIa = 2 A
T = N·m
2.Easy
+−120 VRa​ = 1 ΩIa​ = ?M+−E = 105 VωT

A DC motor on a 120 V supply has an armature resistance of 1 Ω. At its present speed the back-EMF is 105 V. Find the armature current Ia.

V = 120 VE = 105 VRa = 1 Ω
Ia = A
3.Medium
T (N·m)n (rpm)60 N·mn0​n = ?

A DC motor (k = 1.5 N·m/A, Ra = 0.25 Ω) runs from 120 V and drives a load torque of 60 N·m. Find its speed n in rpm.

V = 120 Vk = 1.5 N·m/ARa = 0.25 ΩT = 60 N·m
n = rpm
4.Medium
T (N·m)n (rpm)6.4 N·mn0​nFL​

A DC motor (k = 0.8 V·s/rad, Ra = 2 Ω) runs from 240 V. At full load it draws 8 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 240 Vk = 0.8 V·s/radRa = 2 ΩIFL = 8 A
SR = %
5.Hard
+−220 VRst​ = ?Ra​ = 1.5 Ω30 AM+−E = 0n = 0T

A 220 V DC motor has an armature resistance of 1.5 Ω. Find the series starting resistance Rst that limits the starting current to 30 A.

V = 220 VRa = 1.5 ΩImax = 30 A
Rst = Ω
6.Hard
+−110 VRa​ = 0.5 ΩM+−EωT

A small DC motor has an armature resistance of 0.5 Ω and runs from 110 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 110 VRa = 0.5 Ω
Pmax = W

Answer key

  1. 1. T = 4 N·m
  2. 2. Ia = 15 A
  3. 3. n = 700.3 rpm
  4. 4. SR = 7.143 %
  5. 5. Rst = 5.833 Ω
  6. 6. Pmax = 6050 W

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