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
+−220 VRa​M+−E = 205 Vn = 3000 rpmT

A DC motor turns at 3000 rpm, and its back-EMF at that speed is 205 V. Find the motor constant k in V·s/rad.

E = 205 Vn = 3000 rpm
k = V·s/rad
2.Easy
+−220 VRa​ = 0.8 ΩIa​ = 20 AM+−En = ?T

A DC motor with k = 2 V·s/rad and an armature resistance of 0.8 Ω runs from 220 V and draws 20 A. Find its speed n in rpm.

V = 220 VRa = 0.8 ΩIa = 20 Ak = 2 V·s/rad
n = rpm
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.08 N·m/A and Ra = 0.8 Ω is connected to 24 V. Find its stall torque Tstall, the torque at zero speed.

V = 24 Vk = 0.08 N·m/ARa = 0.8 Ω
Tstall = N·m
4.Medium
T (N·m)n (rpm)0.24 N·mn0​n = ?

A DC motor (k = 0.12 N·m/A, Ra = 0.8 Ω) runs from 24 V and drives a load torque of 0.24 N·m. Find its speed n in rpm.

V = 24 Vk = 0.12 N·m/ARa = 0.8 ΩT = 0.24 N·m
n = rpm
5.Hard
+−48 VRs​ = ?Ra​ = 1.5 ΩM+−En = 750 rpmT = 0.5 N·m

A DC motor (k = 0.25 V·s/rad, Ra = 1.5 Ω) on a fixed 48 V supply drives a load of 0.5 N·m. Find the series resistance Rs that slows it down to 750 rpm.

V = 48 Vk = 0.25 V·s/radRa = 1.5 ΩT = 0.5 N·mn = 750 rpm
Rs = Ω
6.Hard
T (N·m)n (rpm)16 N·m1500 rpm

A DC motor with k = 0.4 V·s/rad and Ra = 0.2 Ω must drive a load of 16 N·m at 1500 rpm. Find the armature voltage V it needs.

k = 0.4 V·s/radRa = 0.2 ΩT = 16 N·mn = 1500 rpm
V = V

Answer key

  1. 1. k = 0.6525 V·s/rad
  2. 2. n = 974 rpm
  3. 3. Tstall = 2.4 N·m
  4. 4. n = 1783 rpm
  5. 5. Rs = 12.68 Ω
  6. 6. V = 70.83 V

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