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
+−110 VRa​ = 0.4 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.4 Ω is switched directly onto a 110 V supply. Its normal full-load current is 25 A. Find the armature current Istart at the moment of starting.

V = 110 VRa = 0.4 Ω
Istart = A
2.Easy
+−240 VRa​ = 0.8 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.8 Ω is switched directly onto a 240 V supply. Its normal full-load current is 25 A. Find the armature current Istart at the moment of starting.

V = 240 VRa = 0.8 Ω
Istart = A
3.Medium
+−220 VRa​ = 0.8 ΩIa​ = 20 AM+−EωT

A DC motor draws 20 A from a 220 V supply. Its armature resistance is 0.8 Ω and its rotational (friction, windage and iron) losses are 245 W. Find its efficiency η.

V = 220 VIa = 20 ARa = 0.8 ΩProt = 245 W
η = %
4.Medium
+−VRa​M+−En = 1800 rpmT = ?

A DC motor delivers 7.5 kW of mechanical power at 1800 rpm. Find the shaft torque T.

P = 7.5 kWn = 1800 rpm
T = N·m
5.Hard
+−120 VRa​ = 0.5 ΩM+−En = ?T = 15 N·m

A separately excited DC motor (k = 0.75 V·s/rad at full field, Ra = 0.5 Ω, V = 120 V) drives a constant load torque of 15 N·m. The field flux is reduced to 80 % of its full value. Find the new speed n.

V = 120 Vk = 0.75 V·s/radRa = 0.5 ΩT = 15 N·mΦ = 80 %
n = rpm
6.Hard
+−110 VRa​ = 0.4 ΩIa​: 25 → 37.5 AM+−En = ?T

A separately excited DC motor on 110 V (Ra = 0.4 Ω) runs at 1800 rpm with an armature current of 25 A. The load changes and the current becomes 37.5 A, with the field unchanged. Find the new speed n2.

V = 110 VRa = 0.4 Ωn1 = 1800 rpmIa1 = 25 AIa2 = 37.5 A
n2 = rpm

Answer key

  1. 1. Istart = 275 A
  2. 2. Istart = 300 A
  3. 3. η = 87.16 %
  4. 4. T = 39.79 N·m
  5. 5. n = 1711 rpm
  6. 6. n2 = 1710 rpm

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