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
+−48 VRa​ = 1 ΩIa​ = 4 AM+−EωT

A DC motor runs from 48 V with an armature current of 4 A. The armature resistance is 1 Ω. Find the developed (electromagnetic) power Pdev.

V = 48 VIa = 4 ARa = 1 Ω
Pdev = W
2.Easy
+−120 VRa​ = 0.5 ΩIa​ = 8 AM+−E = ?ωT

A DC motor runs from a 120 V supply and draws an armature current of 8 A. Its armature resistance is 0.5 Ω. Find the back-EMF E.

V = 120 VIa = 8 ARa = 0.5 Ω
E = V
3.Medium
T (N·m)n (rpm)0.96 N·mn0​nFL​

A DC motor (k = 0.12 V·s/rad, Ra = 0.4 Ω) runs from 48 V. At full load it draws 8 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 48 Vk = 0.12 V·s/radRa = 0.4 ΩIFL = 8 A
SR = %
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.75 N·m/A and Ra = 2 Ω is connected to 48 V. Find its stall torque Tstall, the torque at zero speed.

V = 48 Vk = 0.75 N·m/ARa = 2 Ω
Tstall = N·m
5.Hard
+−220 VD = ?Ra​ = 1.5 ΩM+−En = 400 rpmT = 6 N·m

A DC chopper feeds a motor (k = 1.2 V·s/rad, Ra = 1.5 Ω) from a 220 V DC bus. The average armature voltage is D·220 V. Find the duty cycle D that runs the motor at 400 rpm with a load of 6 N·m.

Vdc = 220 Vk = 1.2 V·s/radRa = 1.5 ΩT = 6 N·mn = 400 rpm
D = %
6.Hard
+−220 VD = ?Ra​ = 0.8 ΩM+−En = 300 rpmT = 14.4 N·m

A DC chopper feeds a motor (k = 1.2 V·s/rad, Ra = 0.8 Ω) from a 220 V DC bus. The average armature voltage is D·220 V. Find the duty cycle D that runs the motor at 300 rpm with a load of 14.4 N·m.

Vdc = 220 Vk = 1.2 V·s/radRa = 0.8 ΩT = 14.4 N·mn = 300 rpm
D = %

Answer key

  1. 1. Pdev = 176 W
  2. 2. E = 116 V
  3. 3. SR = 7.143 %
  4. 4. Tstall = 18 N·m
  5. 5. D = 26.26 %
  6. 6. D = 21.5 %

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