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​ = 1.5 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 1.5 Ω is switched directly onto a 240 V supply. Its normal full-load current is 10 A. Find the armature current Istart at the moment of starting.

V = 240 VRa = 1.5 Ω
Istart = A
2.Easy
+−220 VRa​Ia​ = 5 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.12 N·m/A. Find the torque T it develops at an armature current of 5 A.

k = 0.12 N·m/AIa = 5 A
T = N·m
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 2 N·m/A and Ra = 0.8 Ω is connected to 220 V. Find its stall torque Tstall, the torque at zero speed.

V = 220 Vk = 2 N·m/ARa = 0.8 Ω
Tstall = N·m
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 1 V·s/rad runs from 240 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 240 Vk = 1 V·s/rad
n0 = rpm
5.Hard
+−240 VRst​ = ?Ra​ = 2 Ω30 AM+−E = 0n = 0T

A 240 V DC motor has an armature resistance of 2 Ω. Find the series starting resistance Rst that limits the starting current to 30 A.

V = 240 VRa = 2 ΩImax = 30 A
Rst = Ω
6.Hard
+−24 VRa​ = 0.4 ΩM+−En = ?T = 0.4 N·m

A separately excited DC motor (k = 0.1 V·s/rad at full field, Ra = 0.4 Ω, V = 24 V) drives a constant load torque of 0.4 N·m. The field flux is reduced to 90 % of its full value. Find the new speed n.

V = 24 Vk = 0.1 V·s/radRa = 0.4 ΩT = 0.4 N·mΦ = 90 %
n = rpm

Answer key

  1. 1. Istart = 160 A
  2. 2. T = 0.6 N·m
  3. 3. Tstall = 550 N·m
  4. 4. n0 = 2292 rpm
  5. 5. Rst = 6 Ω
  6. 6. n = 2358 rpm

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