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
+−48 VRa​ = 0.4 ΩIa​ = ?M+−E = 45.6 VωT

A DC motor on a 48 V supply has an armature resistance of 0.4 Ω. At its present speed the back-EMF is 45.6 V. Find the armature current Ia.

V = 48 VE = 45.6 VRa = 0.4 Ω
Ia = A
2.Easy
+−24 VRa​ = 0.4 ΩIa​ = 6 AM+−EωT

A DC motor runs from 24 V with an armature current of 6 A. The armature resistance is 0.4 Ω. Find the developed (electromagnetic) power Pdev.

V = 24 VIa = 6 ARa = 0.4 Ω
Pdev = W
3.Medium
+−VRa​M+−En = 1800 rpmT = ?

A DC motor delivers 5.5 kW of mechanical power at 1800 rpm. Find the shaft torque T.

P = 5.5 kWn = 1800 rpm
T = N·m
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.8 N·m/A and Ra = 0.2 Ω is connected to 240 V. Find its stall torque Tstall, the torque at zero speed.

V = 240 Vk = 0.8 N·m/ARa = 0.2 Ω
Tstall = N·m
5.Hard
+−48 VRs​ = ?Ra​ = 2 ΩM+−En = 900 rpmT = 0.4 N·m

A DC motor (k = 0.2 V·s/rad, Ra = 2 Ω) on a fixed 48 V supply drives a load of 0.4 N·m. Find the series resistance Rs that slows it down to 900 rpm.

V = 48 Vk = 0.2 V·s/radRa = 2 ΩT = 0.4 N·mn = 900 rpm
Rs = Ω
6.Hard
+−220 VRa​ = 1.5 ΩM+−En = ?T = 10 N·m

A separately excited DC motor (k = 1 V·s/rad at full field, Ra = 1.5 Ω, V = 220 V) drives a constant load torque of 10 N·m. The field flux is reduced to 85 % of its full value. Find the new speed n.

V = 220 Vk = 1 V·s/radRa = 1.5 ΩT = 10 N·mΦ = 85 %
n = rpm

Answer key

  1. 1. Ia = 6 A
  2. 2. Pdev = 129.6 W
  3. 3. T = 29.18 N·m
  4. 4. Tstall = 960 N·m
  5. 5. Rs = 12.58 Ω
  6. 6. n = 2273 rpm

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