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​ = 0.5 ΩIa​ = ?M+−E = 106 VωT

A DC motor on a 110 V supply has an armature resistance of 0.5 Ω. At its present speed the back-EMF is 106 V. Find the armature current Ia.

V = 110 VE = 106 VRa = 0.5 Ω
Ia = A
2.Easy
+−220 VRa​ = 1 ΩIa​ = 20 AM+−E = ?ωT

A DC motor runs from a 220 V supply and draws an armature current of 20 A. Its armature resistance is 1 Ω. Find the back-EMF E.

V = 220 VIa = 20 ARa = 1 Ω
E = V
3.Medium
+−VRa​M+−En = 2000 rpmT = ?

A DC motor delivers 0.75 kW of mechanical power at 2000 rpm. Find the shaft torque T.

P = 0.75 kWn = 2000 rpm
T = N·m
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.6 N·m/A and Ra = 1.2 Ω is connected to 240 V. Find its stall torque Tstall, the torque at zero speed.

V = 240 Vk = 0.6 N·m/ARa = 1.2 Ω
Tstall = N·m
5.Hard
T (N·m)n (rpm)9 N·m600 rpm

A DC motor with k = 1.5 V·s/rad and Ra = 1.5 Ω must drive a load of 9 N·m at 600 rpm. Find the armature voltage V it needs.

k = 1.5 V·s/radRa = 1.5 ΩT = 9 N·mn = 600 rpm
V = V
6.Hard
+−24 VRst​ = ?Ra​ = 0.4 Ω20 AM+−E = 0n = 0T

A 24 V DC motor has an armature resistance of 0.4 Ω. Find the series starting resistance Rst that limits the starting current to 20 A.

V = 24 VRa = 0.4 ΩImax = 20 A
Rst = Ω

Answer key

  1. 1. Ia = 8 A
  2. 2. E = 200 V
  3. 3. T = 3.581 N·m
  4. 4. Tstall = 120 N·m
  5. 5. V = 103.2 V
  6. 6. Rst = 0.8 Ω

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