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​ = 1.2 ΩIa​ = 8 AM+−E = ?ωT

A DC motor runs from a 120 V supply and draws an armature current of 8 A. Its armature resistance is 1.2 Ω. Find the back-EMF E.

V = 120 VIa = 8 ARa = 1.2 Ω
E = V
2.Easy
+−120 VRa​M+−E = 108 Vn = 600 rpmT

A DC motor turns at 600 rpm, and its back-EMF at that speed is 108 V. Find the motor constant k in V·s/rad.

E = 108 Vn = 600 rpm
k = V·s/rad
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.6 N·m/A and Ra = 0.5 Ω is connected to 48 V. Find its stall torque Tstall, the torque at zero speed.

V = 48 Vk = 0.6 N·m/ARa = 0.5 Ω
Tstall = N·m
4.Medium
T (N·m)n (rpm)0.2 N·mn0​n = ?

A DC motor (k = 0.1 N·m/A, Ra = 1.5 Ω) runs from 24 V and drives a load torque of 0.2 N·m. Find its speed n in rpm.

V = 24 Vk = 0.1 N·m/ARa = 1.5 ΩT = 0.2 N·m
n = rpm
5.Hard
+−110 VD = ?Ra​ = 0.4 ΩM+−En = 500 rpmT = 9 N·m

A DC chopper feeds a motor (k = 0.6 V·s/rad, Ra = 0.4 Ω) from a 110 V DC bus. The average armature voltage is D·110 V. Find the duty cycle D that runs the motor at 500 rpm with a load of 9 N·m.

Vdc = 110 Vk = 0.6 V·s/radRa = 0.4 ΩT = 9 N·mn = 500 rpm
D = %
6.Hard
T (N·m)n (rpm)3.75 N·m600 rpm

A DC motor with k = 0.75 V·s/rad and Ra = 0.8 Ω must drive a load of 3.75 N·m at 600 rpm. Find the armature voltage V it needs.

k = 0.75 V·s/radRa = 0.8 ΩT = 3.75 N·mn = 600 rpm
V = V

Answer key

  1. 1. E = 110.4 V
  2. 2. k = 1.719 V·s/rad
  3. 3. Tstall = 57.6 N·m
  4. 4. n = 2005 rpm
  5. 5. D = 34.01 %
  6. 6. V = 51.12 V

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