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
+−240 VRa​ = 0.25 ΩIa​ = 50 AM+−En = ?T

A DC motor with k = 0.8 V·s/rad and an armature resistance of 0.25 Ω runs from 240 V and draws 50 A. Find its speed n in rpm.

V = 240 VRa = 0.25 ΩIa = 50 Ak = 0.8 V·s/rad
n = rpm
2.Easy
+−240 VRa​ = 1.2 ΩIa​ = ?M+−E = 210 VωT

A DC motor on a 240 V supply has an armature resistance of 1.2 Ω. At its present speed the back-EMF is 210 V. Find the armature current Ia.

V = 240 VE = 210 VRa = 1.2 Ω
Ia = A
3.Medium
T (N·m)n (rpm)0.6 N·mn0​nFL​

A DC motor (k = 0.3 V·s/rad, Ra = 1 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 ΩIFL = 2 A
SR = %
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.3 N·m/A and Ra = 0.4 Ω is connected to 110 V. Find its stall torque Tstall, the torque at zero speed.

V = 110 Vk = 0.3 N·m/ARa = 0.4 Ω
Tstall = N·m
5.Hard
+−120 VRst​ = ?Ra​ = 0.8 Ω80 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 80 A.

V = 120 VRa = 0.8 ΩImax = 80 A
Rst = Ω
6.Hard
+−110 VRst​ = ?Ra​ = 0.25 Ω15 AM+−E = 0n = 0T

A 110 V DC motor has an armature resistance of 0.25 Ω. Find the series starting resistance Rst that limits the starting current to 15 A.

V = 110 VRa = 0.25 ΩImax = 15 A
Rst = Ω

Answer key

  1. 1. n = 2716 rpm
  2. 2. Ia = 25 A
  3. 3. SR = 9.091 %
  4. 4. Tstall = 82.5 N·m
  5. 5. Rst = 0.7 Ω
  6. 6. Rst = 7.083 Ω

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