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​ = 50 AM+−EωT

A DC motor runs from 220 V with an armature current of 50 A. The armature resistance is 0.5 Ω. Find the developed (electromagnetic) power Pdev.

V = 220 VIa = 50 ARa = 0.5 Ω
Pdev = W
2.Easy
+−120 VRa​ = 0.25 ΩIa​ = 40 AM+−En = ?T

A DC motor with k = 0.6 V·s/rad and an armature resistance of 0.25 Ω runs from 120 V and draws 40 A. Find its speed n in rpm.

V = 120 VRa = 0.25 ΩIa = 40 Ak = 0.6 V·s/rad
n = rpm
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.1 N·m/A and Ra = 1 Ω is connected to 24 V. Find its stall torque Tstall, the torque at zero speed.

V = 24 Vk = 0.1 N·m/ARa = 1 Ω
Tstall = N·m
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.4 V·s/rad runs from 110 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 110 Vk = 0.4 V·s/rad
n0 = rpm
5.Hard
T (N·m)n (rpm)3.2 N·m300 rpm

A DC motor with k = 0.8 V·s/rad and Ra = 1.5 Ω must drive a load of 3.2 N·m at 300 rpm. Find the armature voltage V it needs.

k = 0.8 V·s/radRa = 1.5 ΩT = 3.2 N·mn = 300 rpm
V = V
6.Hard
+−220 VD = ?Ra​ = 0.8 ΩM+−En = 400 rpmT = 0.5 N·m

A DC chopper feeds a motor (k = 0.25 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 400 rpm with a load of 0.5 N·m.

Vdc = 220 Vk = 0.25 V·s/radRa = 0.8 ΩT = 0.5 N·mn = 400 rpm
D = %

Answer key

  1. 1. Pdev = 9750 W
  2. 2. n = 1751 rpm
  3. 3. Tstall = 2.4 N·m
  4. 4. n0 = 2626 rpm
  5. 5. V = 31.13 V
  6. 6. D = 5.487 %

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