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 = 227.5 VωT

A DC motor on a 240 V supply has an armature resistance of 0.25 Ω. At its present speed the back-EMF is 227.5 V. Find the armature current Ia.

V = 240 VE = 227.5 VRa = 0.25 Ω
Ia = A
2.Easy
+−120 VRa​ = 0.8 ΩIa​ = 15 AM+−EωT

A DC motor runs from 120 V with an armature current of 15 A. The armature resistance is 0.8 Ω. Find the developed (electromagnetic) power Pdev.

V = 120 VIa = 15 ARa = 0.8 Ω
Pdev = W
3.Medium
T (N·m)n (rpm)1.5 N·mn0​n = ?

A DC motor (k = 0.3 N·m/A, Ra = 2 Ω) runs from 110 V and drives a load torque of 1.5 N·m. Find its speed n in rpm.

V = 110 Vk = 0.3 N·m/ARa = 2 ΩT = 1.5 N·m
n = rpm
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.8 N·m/A and Ra = 1.2 Ω is connected to 240 V. Find its stall torque Tstall, the torque at zero speed.

V = 240 Vk = 0.8 N·m/ARa = 1.2 Ω
Tstall = N·m
5.Hard
+−220 VRs​ = ?Ra​ = 1.2 ΩM+−En = 400 rpmT = 30 N·m

A DC motor (k = 1.5 V·s/rad, Ra = 1.2 Ω) on a fixed 220 V supply drives a load of 30 N·m. Find the series resistance Rs that slows it down to 400 rpm.

V = 220 Vk = 1.5 V·s/radRa = 1.2 ΩT = 30 N·mn = 400 rpm
Rs = Ω
6.Hard
+−48 VRs​ = ?Ra​ = 2 ΩM+−En = 400 rpmT = 0.3 N·m

A DC motor (k = 0.15 V·s/rad, Ra = 2 Ω) on a fixed 48 V supply drives a load of 0.3 N·m. Find the series resistance Rs that slows it down to 400 rpm.

V = 48 Vk = 0.15 V·s/radRa = 2 ΩT = 0.3 N·mn = 400 rpm
Rs = Ω

Answer key

  1. 1. Ia = 50 A
  2. 2. Pdev = 1620 W
  3. 3. n = 3183 rpm
  4. 4. Tstall = 160 N·m
  5. 5. Rs = 6.658 Ω
  6. 6. Rs = 18.86 Ω

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