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​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.5 Ω is switched directly onto a 220 V supply. Its normal full-load current is 15 A. Find the armature current Istart at the moment of starting.

V = 220 VRa = 0.5 Ω
Istart = A
2.Easy
+−24 VRa​ = 0.5 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.5 Ω is switched directly onto a 24 V supply. Its normal full-load current is 6 A. Find the armature current Istart at the moment of starting.

V = 24 VRa = 0.5 Ω
Istart = A
3.Medium
+−VRa​M+−En = 2900 rpmT = ?

A DC motor delivers 7.5 kW of mechanical power at 2900 rpm. Find the shaft torque T.

P = 7.5 kWn = 2900 rpm
T = N·m
4.Medium
+−VRa​M+−En = 1800 rpmT = ?

A DC motor delivers 2.2 kW of mechanical power at 1800 rpm. Find the shaft torque T.

P = 2.2 kWn = 1800 rpm
T = N·m
5.Hard
+−24 VRa​ = 1.5 ΩM+−EωT

A small DC motor has an armature resistance of 1.5 Ω and runs from 24 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 24 VRa = 1.5 Ω
Pmax = W
6.Hard
+−220 VRa​ = 0.2 ΩM+−En = ?T = 60 N·m

A separately excited DC motor (k = 1.5 V·s/rad at full field, Ra = 0.2 Ω, V = 220 V) drives a constant load torque of 60 N·m. The field flux is reduced to 70 % of its full value. Find the new speed n.

V = 220 Vk = 1.5 V·s/radRa = 0.2 ΩT = 60 N·mΦ = 70 %
n = rpm

Answer key

  1. 1. Istart = 440 A
  2. 2. Istart = 48 A
  3. 3. T = 24.7 N·m
  4. 4. T = 11.67 N·m
  5. 5. Pmax = 96 W
  6. 6. n = 1897 rpm

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