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​M+−E = 207.5 Vn = 1000 rpmT

A DC motor turns at 1000 rpm, and its back-EMF at that speed is 207.5 V. Find the motor constant k in V·s/rad.

E = 207.5 Vn = 1000 rpm
k = V·s/rad
2.Easy
+−110 VRa​ = 0.5 ΩIa​ = 12 AM+−En = ?T

A DC motor with k = 0.75 V·s/rad and an armature resistance of 0.5 Ω runs from 110 V and draws 12 A. Find its speed n in rpm.

V = 110 VRa = 0.5 ΩIa = 12 Ak = 0.75 V·s/rad
n = rpm
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.5 N·m/A and Ra = 0.4 Ω is connected to 110 V. Find its stall torque Tstall, the torque at zero speed.

V = 110 Vk = 0.5 N·m/ARa = 0.4 Ω
Tstall = N·m
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.5 N·m/A and Ra = 1 Ω is connected to 110 V. Find its stall torque Tstall, the torque at zero speed.

V = 110 Vk = 0.5 N·m/ARa = 1 Ω
Tstall = N·m
5.Hard
+−120 VRa​ = 0.5 ΩIa​: 30 → 18 AM+−En = ?T

A separately excited DC motor on 120 V (Ra = 0.5 Ω) runs at 1200 rpm with an armature current of 30 A. The load changes and the current becomes 18 A, with the field unchanged. Find the new speed n2.

V = 120 VRa = 0.5 Ωn1 = 1200 rpmIa1 = 30 AIa2 = 18 A
n2 = rpm
6.Hard
T (N·m)n (rpm)24 N·m400 rpm

A DC motor with k = 0.8 V·s/rad and Ra = 0.4 Ω must drive a load of 24 N·m at 400 rpm. Find the armature voltage V it needs.

k = 0.8 V·s/radRa = 0.4 ΩT = 24 N·mn = 400 rpm
V = V

Answer key

  1. 1. k = 1.981 V·s/rad
  2. 2. n = 1324 rpm
  3. 3. Tstall = 137.5 N·m
  4. 4. Tstall = 55 N·m
  5. 5. n2 = 1269 rpm
  6. 6. V = 45.51 V

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