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​ = 1 ΩIa​ = 8 AM+−En = ?T

A DC motor with k = 0.6 V·s/rad and an armature resistance of 1 Ω runs from 220 V and draws 8 A. Find its speed n in rpm.

V = 220 VRa = 1 ΩIa = 8 Ak = 0.6 V·s/rad
n = rpm
2.Easy
+−24 VRa​ = 0.8 ΩIa​ = 4 AM+−En = ?T

A DC motor with k = 0.25 V·s/rad and an armature resistance of 0.8 Ω runs from 24 V and draws 4 A. Find its speed n in rpm.

V = 24 VRa = 0.8 ΩIa = 4 Ak = 0.25 V·s/rad
n = rpm
3.Medium
+−VRa​M+−En = 2000 rpmT = ?

A DC motor delivers 5.5 kW of mechanical power at 2000 rpm. Find the shaft torque T.

P = 5.5 kWn = 2000 rpm
T = N·m
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 2 N·m/A and Ra = 2 Ω is connected to 220 V. Find its stall torque Tstall, the torque at zero speed.

V = 220 Vk = 2 N·m/ARa = 2 Ω
Tstall = N·m
5.Hard
+−240 VRa​ = 0.8 ΩIa​: 20 → 10 AM+−En = ?T

A separately excited DC motor on 240 V (Ra = 0.8 Ω) runs at 1000 rpm with an armature current of 20 A. The load changes and the current becomes 10 A, with the field unchanged. Find the new speed n2.

V = 240 VRa = 0.8 Ωn1 = 1000 rpmIa1 = 20 AIa2 = 10 A
n2 = rpm
6.Hard
+−240 VRa​ = 0.5 ΩM+−En = ?T = 30 N·m

A separately excited DC motor (k = 0.75 V·s/rad at full field, Ra = 0.5 Ω, V = 240 V) drives a constant load torque of 30 N·m. The field flux is reduced to 85 % of its full value. Find the new speed n.

V = 240 Vk = 0.75 V·s/radRa = 0.5 ΩT = 30 N·mΦ = 85 %
n = rpm

Answer key

  1. 1. n = 3374 rpm
  2. 2. n = 794.5 rpm
  3. 3. T = 26.26 N·m
  4. 4. Tstall = 220 N·m
  5. 5. n2 = 1036 rpm
  6. 6. n = 3243 rpm

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