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
+−24 VRa​M+−E = 23 Vn = 600 rpmT

A DC motor turns at 600 rpm, and its back-EMF at that speed is 23 V. Find the motor constant k in V·s/rad.

E = 23 Vn = 600 rpm
k = V·s/rad
2.Easy
+−48 VRa​ = 0.8 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.8 Ω is switched directly onto a 48 V supply. Its normal full-load current is 6 A. Find the armature current Istart at the moment of starting.

V = 48 VRa = 0.8 Ω
Istart = A
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.2 N·m/A and Ra = 1.2 Ω is connected to 48 V. Find its stall torque Tstall, the torque at zero speed.

V = 48 Vk = 0.2 N·m/ARa = 1.2 Ω
Tstall = N·m
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.2 N·m/A and Ra = 0.25 Ω is connected to 24 V. Find its stall torque Tstall, the torque at zero speed.

V = 24 Vk = 0.2 N·m/ARa = 0.25 Ω
Tstall = N·m
5.Hard
T (N·m)n (rpm)25 N·m1200 rpm

A DC motor with k = 1 V·s/rad and Ra = 0.4 Ω must drive a load of 25 N·m at 1200 rpm. Find the armature voltage V it needs.

k = 1 V·s/radRa = 0.4 ΩT = 25 N·mn = 1200 rpm
V = V
6.Hard
+−48 VRa​ = 1.5 ΩIa​: 2 → 0.5 AM+−En = ?T

A separately excited DC motor on 48 V (Ra = 1.5 Ω) runs at 1000 rpm with an armature current of 2 A. The load changes and the current becomes 0.5 A, with the field unchanged. Find the new speed n2.

V = 48 VRa = 1.5 Ωn1 = 1000 rpmIa1 = 2 AIa2 = 0.5 A
n2 = rpm

Answer key

  1. 1. k = 0.3661 V·s/rad
  2. 2. Istart = 60 A
  3. 3. Tstall = 8 N·m
  4. 4. Tstall = 19.2 N·m
  5. 5. V = 135.7 V
  6. 6. n2 = 1050 rpm

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