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
+−240 VRa​M+−E = 215 Vn = 750 rpmT

A DC motor turns at 750 rpm, and its back-EMF at that speed is 215 V. Find the motor constant k in V·s/rad.

E = 215 Vn = 750 rpm
k = V·s/rad
2.Easy
+−24 VRa​ = 0.5 ΩIa​ = 5 AM+−E = ?ωT

A DC motor runs from a 24 V supply and draws an armature current of 5 A. Its armature resistance is 0.5 Ω. Find the back-EMF E.

V = 24 VIa = 5 ARa = 0.5 Ω
E = V
3.Medium
T (N·m)n (rpm)1.2 N·mn0​n = ?

A DC motor (k = 0.6 N·m/A, Ra = 2 Ω) runs from 48 V and drives a load torque of 1.2 N·m. Find its speed n in rpm.

V = 48 Vk = 0.6 N·m/ARa = 2 ΩT = 1.2 N·m
n = rpm
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.75 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 = 0.75 N·m/ARa = 0.4 Ω
Tstall = N·m
5.Hard
+−48 VRa​ = 2.5 ΩM+−EωT

A small DC motor has an armature resistance of 2.5 Ω and runs from 48 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 48 VRa = 2.5 Ω
Pmax = W
6.Hard
T (N·m)n (rpm)14.4 N·m600 rpm

A DC motor with k = 1.2 V·s/rad and Ra = 0.4 Ω must drive a load of 14.4 N·m at 600 rpm. Find the armature voltage V it needs.

k = 1.2 V·s/radRa = 0.4 ΩT = 14.4 N·mn = 600 rpm
V = V

Answer key

  1. 1. k = 2.737 V·s/rad
  2. 2. E = 21.5 V
  3. 3. n = 700.3 rpm
  4. 4. Tstall = 412.5 N·m
  5. 5. Pmax = 230.4 W
  6. 6. V = 80.2 V

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