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​ = 0.25 ΩIa​ = 20 AM+−En = ?T

A DC motor with k = 0.25 V·s/rad and an armature resistance of 0.25 Ω runs from 48 V and draws 20 A. Find its speed n in rpm.

V = 48 VRa = 0.25 ΩIa = 20 Ak = 0.25 V·s/rad
n = rpm
2.Easy
+−110 VRa​Ia​ = 12 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.3 N·m/A. Find the torque T it develops at an armature current of 12 A.

k = 0.3 N·m/AIa = 12 A
T = N·m
3.Medium
+−VRa​M+−En = 1450 rpmT = ?

A DC motor delivers 3 kW of mechanical power at 1450 rpm. Find the shaft torque T.

P = 3 kWn = 1450 rpm
T = N·m
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 1 N·m/A and Ra = 0.5 Ω is connected to 220 V. Find its stall torque Tstall, the torque at zero speed.

V = 220 Vk = 1 N·m/ARa = 0.5 Ω
Tstall = N·m
5.Hard
+−220 VRa​ = 1 ΩM+−En = ?T = 24 N·m

A separately excited DC motor (k = 2 V·s/rad at full field, Ra = 1 Ω, V = 220 V) drives a constant load torque of 24 N·m. The field flux is reduced to 85 % of its full value. Find the new speed n.

V = 220 Vk = 2 V·s/radRa = 1 ΩT = 24 N·mΦ = 85 %
n = rpm
6.Hard
T (N·m)n (rpm)4.8 N·m600 rpm

A DC motor with k = 1.2 V·s/rad and Ra = 1.2 Ω must drive a load of 4.8 N·m at 600 rpm. Find the armature voltage V it needs.

k = 1.2 V·s/radRa = 1.2 ΩT = 4.8 N·mn = 600 rpm
V = V

Answer key

  1. 1. n = 1642 rpm
  2. 2. T = 3.6 N·m
  3. 3. T = 19.76 N·m
  4. 4. Tstall = 440 N·m
  5. 5. n = 1156 rpm
  6. 6. V = 80.2 V

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