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​ = 0.2 ΩIa​ = 30 AM+−EωT

A DC motor runs from 120 V with an armature current of 30 A. The armature resistance is 0.2 Ω. Find the developed (electromagnetic) power Pdev.

V = 120 VIa = 30 ARa = 0.2 Ω
Pdev = W
2.Easy
+−110 VRa​Ia​ = 6 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 6 A.

k = 1 N·m/AIa = 6 A
T = N·m
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 1.2 N·m/A and Ra = 1 Ω is connected to 110 V. Find its stall torque Tstall, the torque at zero speed.

V = 110 Vk = 1.2 N·m/ARa = 1 Ω
Tstall = N·m
4.Medium
T (N·m)n (rpm)20 N·mn0​nFL​

A DC motor (k = 1 V·s/rad, Ra = 0.4 Ω) runs from 220 V. At full load it draws 20 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 220 Vk = 1 V·s/radRa = 0.4 ΩIFL = 20 A
SR = %
5.Hard
T (N·m)n (rpm)0.4 N·m1500 rpm

A DC motor with k = 0.2 V·s/rad and Ra = 2 Ω must drive a load of 0.4 N·m at 1500 rpm. Find the armature voltage V it needs.

k = 0.2 V·s/radRa = 2 ΩT = 0.4 N·mn = 1500 rpm
V = V
6.Hard
+−240 VD = ?Ra​ = 0.5 ΩM+−En = 900 rpmT = 24 N·m

A DC chopper feeds a motor (k = 1.2 V·s/rad, Ra = 0.5 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 900 rpm with a load of 24 N·m.

Vdc = 240 Vk = 1.2 V·s/radRa = 0.5 ΩT = 24 N·mn = 900 rpm
D = %

Answer key

  1. 1. Pdev = 3420 W
  2. 2. T = 6 N·m
  3. 3. Tstall = 132 N·m
  4. 4. SR = 3.774 %
  5. 5. V = 35.42 V
  6. 6. D = 51.29 %

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