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 = 900 rpmT

A DC motor turns at 900 rpm, and its back-EMF at that speed is 23 V. Find the motor constant k in V·s/rad.

E = 23 Vn = 900 rpm
k = V·s/rad
2.Easy
+−220 VRa​ = 0.25 ΩIa​ = ?M+−E = 0n = 0T

A DC motor with an armature resistance of 0.25 Ω is switched directly onto a 220 V supply. Its normal full-load current is 30 A. Find the armature current Istart at the moment of starting.

V = 220 VRa = 0.25 Ω
Istart = A
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.25 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 = 0.25 N·m/ARa = 1.2 Ω
Tstall = 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 48 V. Find its stall torque Tstall, the torque at zero speed.

V = 48 Vk = 0.6 N·m/ARa = 1.2 Ω
Tstall = N·m
5.Hard
+−120 VRa​ = 0.8 ΩM+−En = ?T = 3.6 N·m

A separately excited DC motor (k = 0.6 V·s/rad at full field, Ra = 0.8 Ω, V = 120 V) drives a constant load torque of 3.6 N·m. The field flux is reduced to 90 % of its full value. Find the new speed n.

V = 120 Vk = 0.6 V·s/radRa = 0.8 ΩT = 3.6 N·mΦ = 90 %
n = rpm
6.Hard
+−24 VRa​ = 1 ΩM+−EωT

A small DC motor has an armature resistance of 1 Ω and runs from 24 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 24 VRa = 1 Ω
Pmax = W

Answer key

  1. 1. k = 0.244 V·s/rad
  2. 2. Istart = 880 A
  3. 3. Tstall = 10 N·m
  4. 4. Tstall = 24 N·m
  5. 5. n = 2028 rpm
  6. 6. Pmax = 144 W

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