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​ = 50 AM+−EωT

A DC motor runs from 240 V with an armature current of 50 A. The armature resistance is 0.5 Ω. Find the developed (electromagnetic) power Pdev.

V = 240 VIa = 50 ARa = 0.5 Ω
Pdev = W
2.Easy
+−48 VRa​Ia​ = 4 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.3 N·m/A. Find the torque T it develops at an armature current of 4 A.

k = 0.3 N·m/AIa = 4 A
T = N·m
3.Medium
+−VRa​M+−En = 1500 rpmT = ?

A DC motor delivers 1.5 kW of mechanical power at 1500 rpm. Find the shaft torque T.

P = 1.5 kWn = 1500 rpm
T = N·m
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.8 N·m/A and Ra = 0.2 Ω is connected to 110 V. Find its stall torque Tstall, the torque at zero speed.

V = 110 Vk = 0.8 N·m/ARa = 0.2 Ω
Tstall = N·m
5.Hard
+−48 VRa​ = 0.8 ΩM+−EωT

A small DC motor has an armature resistance of 0.8 Ω and runs from 48 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 48 VRa = 0.8 Ω
Pmax = W
6.Hard
+−240 VD = ?Ra​ = 0.5 ΩM+−En = 750 rpmT = 1.25 N·m

A DC chopper feeds a motor (k = 0.25 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 750 rpm with a load of 1.25 N·m.

Vdc = 240 Vk = 0.25 V·s/radRa = 0.5 ΩT = 1.25 N·mn = 750 rpm
D = %

Answer key

  1. 1. Pdev = 10750 W
  2. 2. T = 1.2 N·m
  3. 3. T = 9.549 N·m
  4. 4. Tstall = 440 N·m
  5. 5. Pmax = 720 W
  6. 6. D = 9.223 %

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