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
+−120 VRa​ = 0.8 ΩIa​ = 10 AM+−En = ?T

A DC motor with k = 0.75 V·s/rad and an armature resistance of 0.8 Ω runs from 120 V and draws 10 A. Find its speed n in rpm.

V = 120 VRa = 0.8 ΩIa = 10 Ak = 0.75 V·s/rad
n = rpm
2.Easy
+−24 VRa​Ia​ = 5 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 5 A.

k = 0.15 N·m/AIa = 5 A
T = N·m
3.Medium
T (N·m)n (rpm)0.6 N·mn0​nFL​

A DC motor (k = 0.3 V·s/rad, Ra = 1.5 Ω) runs from 24 V. At full load it draws 2 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 24 Vk = 0.3 V·s/radRa = 1.5 ΩIFL = 2 A
SR = %
4.Medium
+−VRa​M+−En = 3000 rpmT = ?

A DC motor delivers 11 kW of mechanical power at 3000 rpm. Find the shaft torque T.

P = 11 kWn = 3000 rpm
T = N·m
5.Hard
+−120 VRst​ = ?Ra​ = 0.8 Ω100 AM+−E = 0n = 0T

A 120 V DC motor has an armature resistance of 0.8 Ω. Find the series starting resistance Rst that limits the starting current to 100 A.

V = 120 VRa = 0.8 ΩImax = 100 A
Rst = Ω
6.Hard
+−240 VD = ?Ra​ = 1.2 ΩM+−En = 750 rpmT = 0.5 N·m

A DC chopper feeds a motor (k = 0.25 V·s/rad, Ra = 1.2 Ω) from a 240 V DC bus. The average armature voltage is D·240 V. Find the duty cycle D that runs the motor at 750 rpm with a load of 0.5 N·m.

Vdc = 240 Vk = 0.25 V·s/radRa = 1.2 ΩT = 0.5 N·mn = 750 rpm
D = %

Answer key

  1. 1. n = 1426 rpm
  2. 2. T = 0.75 N·m
  3. 3. SR = 14.29 %
  4. 4. T = 35.01 N·m
  5. 5. Rst = 0.4 Ω
  6. 6. D = 9.181 %

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