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​ = 1 ΩIa​ = 15 AM+−EωT

A DC motor runs from 240 V with an armature current of 15 A. The armature resistance is 1 Ω. Find the developed (electromagnetic) power Pdev.

V = 240 VIa = 15 ARa = 1 Ω
Pdev = W
2.Easy
+−120 VRa​ = 0.8 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.8 Ω is switched directly onto a 120 V supply. Its normal full-load current is 10 A. Find the armature current Istart at the moment of starting.

V = 120 VRa = 0.8 Ω
Istart = A
3.Medium
T (N·m)n (rpm)1.2 N·mn0​nFL​

A DC motor (k = 0.6 V·s/rad, Ra = 1.5 Ω) runs from 48 V. At full load it draws 2 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 48 Vk = 0.6 V·s/radRa = 1.5 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 = 0.5 Ω is connected to 110 V. Find its stall torque Tstall, the torque at zero speed.

V = 110 Vk = 0.3 N·m/ARa = 0.5 Ω
Tstall = N·m
5.Hard
+−120 VRs​ = ?Ra​ = 1.2 ΩM+−En = 900 rpmT = 8 N·m

A DC motor (k = 1 V·s/rad, Ra = 1.2 Ω) on a fixed 120 V supply drives a load of 8 N·m. Find the series resistance Rs that slows it down to 900 rpm.

V = 120 Vk = 1 V·s/radRa = 1.2 ΩT = 8 N·mn = 900 rpm
Rs = Ω
6.Hard
+−48 VRs​ = ?Ra​ = 1.2 ΩM+−En = 300 rpmT = 1 N·m

A DC motor (k = 0.5 V·s/rad, Ra = 1.2 Ω) on a fixed 48 V supply drives a load of 1 N·m. Find the series resistance Rs that slows it down to 300 rpm.

V = 48 Vk = 0.5 V·s/radRa = 1.2 ΩT = 1 N·mn = 300 rpm
Rs = Ω

Answer key

  1. 1. Pdev = 3375 W
  2. 2. Istart = 150 A
  3. 3. SR = 6.667 %
  4. 4. Tstall = 66 N·m
  5. 5. Rs = 2.019 Ω
  6. 6. Rs = 14.95 Ω

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