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
+−48 VRa​ = 0.4 ΩIa​ = 12 AM+−E = ?ωT

A DC motor runs from a 48 V supply and draws an armature current of 12 A. Its armature resistance is 0.4 Ω. Find the back-EMF E.

V = 48 VIa = 12 ARa = 0.4 Ω
E = V
2.Easy
+−110 VRa​ = 1 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 1 Ω is switched directly onto a 110 V supply. Its normal full-load current is 4 A. Find the armature current Istart at the moment of starting.

V = 110 VRa = 1 Ω
Istart = A
3.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
4.Medium
+−VRa​M+−En = 1200 rpmT = ?

A DC motor delivers 11 kW of mechanical power at 1200 rpm. Find the shaft torque T.

P = 11 kWn = 1200 rpm
T = N·m
5.Hard
+−120 VRa​ = 2.5 ΩM+−EωT

A small DC motor has an armature resistance of 2.5 Ω and runs from 120 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 120 VRa = 2.5 Ω
Pmax = W
6.Hard
T (N·m)n (rpm)0.24 N·m1200 rpm

A DC motor with k = 0.12 V·s/rad and Ra = 1.2 Ω must drive a load of 0.24 N·m at 1200 rpm. Find the armature voltage V it needs.

k = 0.12 V·s/radRa = 1.2 ΩT = 0.24 N·mn = 1200 rpm
V = V

Answer key

  1. 1. E = 43.2 V
  2. 2. Istart = 110 A
  3. 3. T = 39.79 N·m
  4. 4. T = 87.54 N·m
  5. 5. Pmax = 1440 W
  6. 6. V = 17.48 V

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