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
+−120 VRa​M+−E = 110 Vn = 600 rpmT

A DC motor turns at 600 rpm, and its back-EMF at that speed is 110 V. Find the motor constant k in V·s/rad.

E = 110 Vn = 600 rpm
k = V·s/rad
2.Easy
+−120 VRa​ = 0.8 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.8 Ω is switched directly onto a 120 V supply. Its normal full-load current is 10 A. Find the armature current Istart at the moment of starting.

V = 120 VRa = 0.8 Ω
Istart = A
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.3 V·s/rad runs from 110 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 110 Vk = 0.3 V·s/rad
n0 = rpm
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 1.2 N·m/A and Ra = 0.5 Ω is connected to 220 V. Find its stall torque Tstall, the torque at zero speed.

V = 220 Vk = 1.2 N·m/ARa = 0.5 Ω
Tstall = N·m
5.Hard
+−240 VD = ?Ra​ = 2 ΩM+−En = 300 rpmT = 3.6 N·m

A DC chopper feeds a motor (k = 0.6 V·s/rad, Ra = 2 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 300 rpm with a load of 3.6 N·m.

Vdc = 240 Vk = 0.6 V·s/radRa = 2 ΩT = 3.6 N·mn = 300 rpm
D = %
6.Hard
+−48 VRst​ = ?Ra​ = 1.5 Ω25 AM+−E = 0n = 0T

A 48 V DC motor has an armature resistance of 1.5 Ω. Find the series starting resistance Rst that limits the starting current to 25 A.

V = 48 VRa = 1.5 ΩImax = 25 A
Rst = Ω

Answer key

  1. 1. k = 1.751 V·s/rad
  2. 2. Istart = 150 A
  3. 3. n0 = 3501 rpm
  4. 4. Tstall = 528 N·m
  5. 5. D = 12.85 %
  6. 6. Rst = 0.42 Ω

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