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​ = 12 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.75 N·m/A. Find the torque T it develops at an armature current of 12 A.

k = 0.75 N·m/AIa = 12 A
T = N·m
2.Easy
+−240 VRa​M+−E = 230.4 Vn = 900 rpmT

A DC motor turns at 900 rpm, and its back-EMF at that speed is 230.4 V. Find the motor constant k in V·s/rad.

E = 230.4 Vn = 900 rpm
k = V·s/rad
3.Medium
T (N·m)n (rpm)6 N·mn0​nFL​

A DC motor (k = 0.6 V·s/rad, Ra = 0.5 Ω) runs from 48 V. At full load it draws 10 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 48 Vk = 0.6 V·s/radRa = 0.5 ΩIFL = 10 A
SR = %
4.Medium
+−VRa​M+−En = 750 rpmT = ?

A DC motor delivers 2.2 kW of mechanical power at 750 rpm. Find the shaft torque T.

P = 2.2 kWn = 750 rpm
T = N·m
5.Hard
T (N·m)n (rpm)4.8 N·m900 rpm

A DC motor with k = 0.6 V·s/rad and Ra = 1.5 Ω must drive a load of 4.8 N·m at 900 rpm. Find the armature voltage V it needs.

k = 0.6 V·s/radRa = 1.5 ΩT = 4.8 N·mn = 900 rpm
V = V
6.Hard
+−48 VRst​ = ?Ra​ = 0.25 Ω100 AM+−E = 0n = 0T

A 48 V DC motor has an armature resistance of 0.25 Ω. Find the series starting resistance Rst that limits the starting current to 100 A.

V = 48 VRa = 0.25 ΩImax = 100 A
Rst = Ω

Answer key

  1. 1. T = 9 N·m
  2. 2. k = 2.445 V·s/rad
  3. 3. SR = 11.63 %
  4. 4. T = 28.01 N·m
  5. 5. V = 68.55 V
  6. 6. Rst = 0.23 Ω

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