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​ = 1 ΩIa​ = 25 AM+−E = ?ωT

A DC motor runs from a 220 V supply and draws an armature current of 25 A. Its armature resistance is 1 Ω. Find the back-EMF E.

V = 220 VIa = 25 ARa = 1 Ω
E = V
2.Easy
+−48 VRa​Ia​ = 15 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.4 N·m/A. Find the torque T it develops at an armature current of 15 A.

k = 0.4 N·m/AIa = 15 A
T = N·m
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 1 V·s/rad runs from 120 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 120 Vk = 1 V·s/rad
n0 = rpm
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.4 N·m/A and Ra = 1 Ω is connected to 48 V. Find its stall torque Tstall, the torque at zero speed.

V = 48 Vk = 0.4 N·m/ARa = 1 Ω
Tstall = N·m
5.Hard
+−220 VD = ?Ra​ = 0.8 ΩM+−En = 1200 rpmT = 0.24 N·m

A DC chopper feeds a motor (k = 0.12 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 1200 rpm with a load of 0.24 N·m.

Vdc = 220 Vk = 0.12 V·s/radRa = 0.8 ΩT = 0.24 N·mn = 1200 rpm
D = %
6.Hard
+−120 VRa​ = 0.4 ΩM+−En = ?T = 15 N·m

A separately excited DC motor (k = 1.5 V·s/rad at full field, Ra = 0.4 Ω, V = 120 V) drives a constant load torque of 15 N·m. The field flux is reduced to 90 % of its full value. Find the new speed n.

V = 120 Vk = 1.5 V·s/radRa = 0.4 ΩT = 15 N·mΦ = 90 %
n = rpm

Answer key

  1. 1. E = 195 V
  2. 2. T = 6 N·m
  3. 3. n0 = 1146 rpm
  4. 4. Tstall = 19.2 N·m
  5. 5. D = 7.582 %
  6. 6. n = 817.4 rpm

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