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
+−220 VRa​ = 0.25 ΩIa​ = 40 AM+−EωT

A DC motor runs from 220 V with an armature current of 40 A. The armature resistance is 0.25 Ω. Find the developed (electromagnetic) power Pdev.

V = 220 VIa = 40 ARa = 0.25 Ω
Pdev = W
2.Easy
+−120 VRa​ = 2 ΩIa​ = ?M+−E = 112 VωT

A DC motor on a 120 V supply has an armature resistance of 2 Ω. At its present speed the back-EMF is 112 V. Find the armature current Ia.

V = 120 VE = 112 VRa = 2 Ω
Ia = A
3.Medium
T (N·m)n (rpm)12 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 10 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 = 10 A
SR = %
4.Medium
+−VRa​M+−En = 3000 rpmT = ?

A DC motor delivers 0.75 kW of mechanical power at 3000 rpm. Find the shaft torque T.

P = 0.75 kWn = 3000 rpm
T = N·m
5.Hard
+−48 VRa​ = 1.5 ΩM+−EωT

A small DC motor has an armature resistance of 1.5 Ω and runs from 48 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 48 VRa = 1.5 Ω
Pmax = W
6.Hard
+−240 VD = ?Ra​ = 1.5 ΩM+−En = 800 rpmT = 3.6 N·m

A DC chopper feeds a motor (k = 0.6 V·s/rad, Ra = 1.5 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 800 rpm with a load of 3.6 N·m.

Vdc = 240 Vk = 0.6 V·s/radRa = 1.5 ΩT = 3.6 N·mn = 800 rpm
D = %

Answer key

  1. 1. Pdev = 8400 W
  2. 2. Ia = 4 A
  3. 3. SR = 6.667 %
  4. 4. T = 2.387 N·m
  5. 5. Pmax = 384 W
  6. 6. D = 24.69 %

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