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 = 205 VωT

A DC motor on a 220 V supply has an armature resistance of 0.5 Ω. At its present speed the back-EMF is 205 V. Find the armature current Ia.

V = 220 VE = 205 VRa = 0.5 Ω
Ia = A
2.Easy
+−240 VRa​ = 1.5 ΩIa​ = 5 AM+−E = ?ωT

A DC motor runs from a 240 V supply and draws an armature current of 5 A. Its armature resistance is 1.5 Ω. Find the back-EMF E.

V = 240 VIa = 5 ARa = 1.5 Ω
E = V
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 240 V. Find its stall torque Tstall, the torque at zero speed.

V = 240 Vk = 0.6 N·m/ARa = 0.4 Ω
Tstall = N·m
4.Medium
+−VRa​M+−En = 3000 rpmT = ?

A DC motor delivers 2.2 kW of mechanical power at 3000 rpm. Find the shaft torque T.

P = 2.2 kWn = 3000 rpm
T = N·m
5.Hard
T (N·m)n (rpm)0.8 N·m400 rpm

A DC motor with k = 0.2 V·s/rad and Ra = 1 Ω must drive a load of 0.8 N·m at 400 rpm. Find the armature voltage V it needs.

k = 0.2 V·s/radRa = 1 ΩT = 0.8 N·mn = 400 rpm
V = V
6.Hard
+−24 VRa​ = 0.2 ΩIa​: 8 → 12 AM+−En = ?T

A separately excited DC motor on 24 V (Ra = 0.2 Ω) runs at 1440 rpm with an armature current of 8 A. The load changes and the current becomes 12 A, with the field unchanged. Find the new speed n2.

V = 24 VRa = 0.2 Ωn1 = 1440 rpmIa1 = 8 AIa2 = 12 A
n2 = rpm

Answer key

  1. 1. Ia = 30 A
  2. 2. E = 232.5 V
  3. 3. Tstall = 360 N·m
  4. 4. T = 7.003 N·m
  5. 5. V = 12.38 V
  6. 6. n2 = 1389 rpm

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