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​ = 0.5 ΩIa​ = 30 AM+−EωT

A DC motor runs from 220 V with an armature current of 30 A. The armature resistance is 0.5 Ω. Find the developed (electromagnetic) power Pdev.

V = 220 VIa = 30 ARa = 0.5 Ω
Pdev = W
2.Easy
+−110 VRa​ = 0.5 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.5 Ω is switched directly onto a 110 V supply. Its normal full-load current is 30 A. Find the armature current Istart at the moment of starting.

V = 110 VRa = 0.5 Ω
Istart = A
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 1.5 N·m/A and Ra = 1 Ω is connected to 220 V. Find its stall torque Tstall, the torque at zero speed.

V = 220 Vk = 1.5 N·m/ARa = 1 Ω
Tstall = N·m
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.6 N·m/A and Ra = 0.4 Ω is connected to 220 V. Find its stall torque Tstall, the torque at zero speed.

V = 220 Vk = 0.6 N·m/ARa = 0.4 Ω
Tstall = N·m
5.Hard
+−240 VD = ?Ra​ = 0.25 ΩM+−En = 500 rpmT = 30 N·m

A DC chopper feeds a motor (k = 1 V·s/rad, Ra = 0.25 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 500 rpm with a load of 30 N·m.

Vdc = 240 Vk = 1 V·s/radRa = 0.25 ΩT = 30 N·mn = 500 rpm
D = %
6.Hard
+−120 VRs​ = ?Ra​ = 0.25 ΩM+−En = 900 rpmT = 20 N·m

A DC motor (k = 0.5 V·s/rad, Ra = 0.25 Ω) on a fixed 120 V supply drives a load of 20 N·m. Find the series resistance Rs that slows it down to 900 rpm.

V = 120 Vk = 0.5 V·s/radRa = 0.25 ΩT = 20 N·mn = 900 rpm
Rs = Ω

Answer key

  1. 1. Pdev = 6150 W
  2. 2. Istart = 220 A
  3. 3. Tstall = 330 N·m
  4. 4. Tstall = 330 N·m
  5. 5. D = 24.94 %
  6. 6. Rs = 1.572 Ω

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