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​M+−E = 45.5 Vn = 900 rpmT

A DC motor turns at 900 rpm, and its back-EMF at that speed is 45.5 V. Find the motor constant k in V·s/rad.

E = 45.5 Vn = 900 rpm
k = V·s/rad
2.Easy
+−240 VRa​ = 0.8 ΩIa​ = 15 AM+−EωT

A DC motor runs from 240 V with an armature current of 15 A. The armature resistance is 0.8 Ω. Find the developed (electromagnetic) power Pdev.

V = 240 VIa = 15 ARa = 0.8 Ω
Pdev = W
3.Medium
+−VRa​M+−En = 1000 rpmT = ?

A DC motor delivers 11 kW of mechanical power at 1000 rpm. Find the shaft torque T.

P = 11 kWn = 1000 rpm
T = N·m
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.3 N·m/A and Ra = 0.25 Ω is connected to 48 V. Find its stall torque Tstall, the torque at zero speed.

V = 48 Vk = 0.3 N·m/ARa = 0.25 Ω
Tstall = N·m
5.Hard
T (N·m)n (rpm)30 N·m750 rpm

A DC motor with k = 2 V·s/rad and Ra = 1.5 Ω must drive a load of 30 N·m at 750 rpm. Find the armature voltage V it needs.

k = 2 V·s/radRa = 1.5 ΩT = 30 N·mn = 750 rpm
V = V
6.Hard
+−48 VRa​ = 2 ΩM+−En = ?T = 0.24 N·m

A separately excited DC motor (k = 0.12 V·s/rad at full field, Ra = 2 Ω, V = 48 V) drives a constant load torque of 0.24 N·m. The field flux is reduced to 85 % of its full value. Find the new speed n.

V = 48 Vk = 0.12 V·s/radRa = 2 ΩT = 0.24 N·mΦ = 85 %
n = rpm

Answer key

  1. 1. k = 0.4828 V·s/rad
  2. 2. Pdev = 3420 W
  3. 3. T = 105 N·m
  4. 4. Tstall = 57.6 N·m
  5. 5. V = 179.6 V
  6. 6. n = 4053 rpm

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