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
+−120 VRa​ = 0.8 ΩIa​ = ?M+−E = 112 VωT

A DC motor on a 120 V supply has an armature resistance of 0.8 Ω. At its present speed the back-EMF is 112 V. Find the armature current Ia.

V = 120 VE = 112 VRa = 0.8 Ω
Ia = A
2.Easy
+−120 VRa​ = 2 ΩIa​ = 4 AM+−En = ?T

A DC motor with k = 0.8 V·s/rad and an armature resistance of 2 Ω runs from 120 V and draws 4 A. Find its speed n in rpm.

V = 120 VRa = 2 ΩIa = 4 Ak = 0.8 V·s/rad
n = rpm
3.Medium
T (N·m)n (rpm)0.6 N·mn0​n = ?

A DC motor (k = 0.3 N·m/A, Ra = 1.5 Ω) runs from 24 V and drives a load torque of 0.6 N·m. Find its speed n in rpm.

V = 24 Vk = 0.3 N·m/ARa = 1.5 ΩT = 0.6 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 = 1.2 Ω is connected to 110 V. Find its stall torque Tstall, the torque at zero speed.

V = 110 Vk = 0.5 N·m/ARa = 1.2 Ω
Tstall = N·m
5.Hard
+−220 VRa​ = 0.8 ΩM+−En = ?T = 15 N·m

A separately excited DC motor (k = 0.75 V·s/rad at full field, Ra = 0.8 Ω, V = 220 V) drives a constant load torque of 15 N·m. The field flux is reduced to 85 % of its full value. Find the new speed n.

V = 220 Vk = 0.75 V·s/radRa = 0.8 ΩT = 15 N·mΦ = 85 %
n = rpm
6.Hard
T (N·m)n (rpm)0.6 N·m400 rpm

A DC motor with k = 0.1 V·s/rad and Ra = 0.4 Ω must drive a load of 0.6 N·m at 400 rpm. Find the armature voltage V it needs.

k = 0.1 V·s/radRa = 0.4 ΩT = 0.6 N·mn = 400 rpm
V = V

Answer key

  1. 1. Ia = 10 A
  2. 2. n = 1337 rpm
  3. 3. n = 668.5 rpm
  4. 4. Tstall = 45.83 N·m
  5. 5. n = 3013 rpm
  6. 6. V = 6.589 V

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