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​ = 0.5 ΩIa​ = 30 AM+−EωT

A DC motor runs from 240 V with an armature current of 30 A. The armature resistance is 0.5 Ω. Find the developed (electromagnetic) power Pdev.

V = 240 VIa = 30 ARa = 0.5 Ω
Pdev = W
2.Easy
+−110 VRa​Ia​ = 8 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 1 N·m/A. Find the torque T it develops at an armature current of 8 A.

k = 1 N·m/AIa = 8 A
T = N·m
3.Medium
T (N·m)n (rpm)7.2 N·mn0​nFL​

A DC motor (k = 1.2 V·s/rad, Ra = 1.5 Ω) runs from 240 V. At full load it draws 6 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 240 Vk = 1.2 V·s/radRa = 1.5 ΩIFL = 6 A
SR = %
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 1.2 N·m/A and Ra = 0.8 Ω is connected to 120 V. Find its stall torque Tstall, the torque at zero speed.

V = 120 Vk = 1.2 N·m/ARa = 0.8 Ω
Tstall = N·m
5.Hard
+−220 VRs​ = ?Ra​ = 0.25 ΩM+−En = 300 rpmT = 32 N·m

A DC motor (k = 0.8 V·s/rad, Ra = 0.25 Ω) on a fixed 220 V supply drives a load of 32 N·m. Find the series resistance Rs that slows it down to 300 rpm.

V = 220 Vk = 0.8 V·s/radRa = 0.25 ΩT = 32 N·mn = 300 rpm
Rs = Ω
6.Hard
+−48 VRa​ = 1.2 ΩM+−En = ?T = 0.3 N·m

A separately excited DC motor (k = 0.15 V·s/rad at full field, Ra = 1.2 Ω, V = 48 V) drives a constant load torque of 0.3 N·m. The field flux is reduced to 70 % of its full value. Find the new speed n.

V = 48 Vk = 0.15 V·s/radRa = 1.2 ΩT = 0.3 N·mΦ = 70 %
n = rpm

Answer key

  1. 1. Pdev = 6750 W
  2. 2. T = 8 N·m
  3. 3. SR = 3.896 %
  4. 4. Tstall = 180 N·m
  5. 5. Rs = 4.622 Ω
  6. 6. n = 4054 rpm

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