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 30 A. Find the armature current Istart at the moment of starting.

V = 110 VRa = 0.4 Ω
Istart = A
2.Easy
+−120 VRa​ = 1.5 ΩIa​ = ?M+−E = 108 VωT

A DC motor on a 120 V supply has an armature resistance of 1.5 Ω. At its present speed the back-EMF is 108 V. Find the armature current Ia.

V = 120 VE = 108 VRa = 1.5 Ω
Ia = A
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.6 N·m/A and Ra = 0.4 Ω is connected to 220 V. Find its stall torque Tstall, the torque at zero speed.

V = 220 Vk = 0.6 N·m/ARa = 0.4 Ω
Tstall = N·m
4.Medium
+−VRa​M+−En = 1450 rpmT = ?

A DC motor delivers 1.5 kW of mechanical power at 1450 rpm. Find the shaft torque T.

P = 1.5 kWn = 1450 rpm
T = N·m
5.Hard
+−24 VRa​ = 2.5 ΩM+−EωT

A small DC motor has an armature resistance of 2.5 Ω and runs from 24 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 24 VRa = 2.5 Ω
Pmax = W
6.Hard
+−120 VRa​ = 1 ΩIa​: 10 → 5 AM+−En = ?T

A separately excited DC motor on 120 V (Ra = 1 Ω) runs at 1800 rpm with an armature current of 10 A. The load changes and the current becomes 5 A, with the field unchanged. Find the new speed n2.

V = 120 VRa = 1 Ωn1 = 1800 rpmIa1 = 10 AIa2 = 5 A
n2 = rpm

Answer key

  1. 1. Istart = 275 A
  2. 2. Ia = 8 A
  3. 3. Tstall = 330 N·m
  4. 4. T = 9.879 N·m
  5. 5. Pmax = 57.6 W
  6. 6. n2 = 1882 rpm

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