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​ = 0.25 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.25 Ω is switched directly onto a 240 V supply. Its normal full-load current is 30 A. Find the armature current Istart at the moment of starting.

V = 240 VRa = 0.25 Ω
Istart = A
2.Easy
+−220 VRa​M+−E = 212.5 Vn = 3000 rpmT

A DC motor turns at 3000 rpm, and its back-EMF at that speed is 212.5 V. Find the motor constant k in V·s/rad.

E = 212.5 Vn = 3000 rpm
k = V·s/rad
3.Medium
T (N·m)n (rpm)6 N·mn0​n = ?

A DC motor (k = 0.75 N·m/A, Ra = 0.2 Ω) runs from 48 V and drives a load torque of 6 N·m. Find its speed n in rpm.

V = 48 Vk = 0.75 N·m/ARa = 0.2 ΩT = 6 N·m
n = rpm
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.5 N·m/A and Ra = 2 Ω is connected to 48 V. Find its stall torque Tstall, the torque at zero speed.

V = 48 Vk = 0.5 N·m/ARa = 2 Ω
Tstall = N·m
5.Hard
T (N·m)n (rpm)0.5 N·m750 rpm

A DC motor with k = 0.25 V·s/rad and Ra = 0.8 Ω must drive a load of 0.5 N·m at 750 rpm. Find the armature voltage V it needs.

k = 0.25 V·s/radRa = 0.8 ΩT = 0.5 N·mn = 750 rpm
V = V
6.Hard
+−48 VRa​ = 4 ΩM+−EωT

A small DC motor has an armature resistance of 4 Ω and runs from 48 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 48 VRa = 4 Ω
Pmax = W

Answer key

  1. 1. Istart = 960 A
  2. 2. k = 0.6764 V·s/rad
  3. 3. n = 590.8 rpm
  4. 4. Tstall = 12 N·m
  5. 5. V = 21.23 V
  6. 6. Pmax = 144 W

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