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​Ia​ = 25 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.15 N·m/A. Find the torque T it develops at an armature current of 25 A.

k = 0.15 N·m/AIa = 25 A
T = N·m
2.Easy
+−48 VRa​ = 0.4 ΩIa​ = 8 AM+−E = ?ωT

A DC motor runs from a 48 V supply and draws an armature current of 8 A. Its armature resistance is 0.4 Ω. Find the back-EMF E.

V = 48 VIa = 8 ARa = 0.4 Ω
E = V
3.Medium
+−VRa​M+−En = 1450 rpmT = ?

A DC motor delivers 0.5 kW of mechanical power at 1450 rpm. Find the shaft torque T.

P = 0.5 kWn = 1450 rpm
T = N·m
4.Medium
T (N·m)n (rpm)18 N·mn0​nFL​

A DC motor (k = 0.6 V·s/rad, Ra = 0.25 Ω) runs from 220 V. At full load it draws 30 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 220 Vk = 0.6 V·s/radRa = 0.25 ΩIFL = 30 A
SR = %
5.Hard
T (N·m)n (rpm)2.5 N·m1200 rpm

A DC motor with k = 0.5 V·s/rad and Ra = 1.5 Ω must drive a load of 2.5 N·m at 1200 rpm. Find the armature voltage V it needs.

k = 0.5 V·s/radRa = 1.5 ΩT = 2.5 N·mn = 1200 rpm
V = V
6.Hard
+−48 VRst​ = ?Ra​ = 1 Ω30 AM+−E = 0n = 0T

A 48 V DC motor has an armature resistance of 1 Ω. Find the series starting resistance Rst that limits the starting current to 30 A.

V = 48 VRa = 1 ΩImax = 30 A
Rst = Ω

Answer key

  1. 1. T = 3.75 N·m
  2. 2. E = 44.8 V
  3. 3. T = 3.293 N·m
  4. 4. SR = 3.529 %
  5. 5. V = 70.33 V
  6. 6. Rst = 0.6 Ω

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