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 = 44 Vn = 1500 rpmT

A DC motor turns at 1500 rpm, and its back-EMF at that speed is 44 V. Find the motor constant k in V·s/rad.

E = 44 Vn = 1500 rpm
k = V·s/rad
2.Easy
+−220 VRa​ = 1.2 ΩIa​ = 15 AM+−E = ?ωT

A DC motor runs from a 220 V supply and draws an armature current of 15 A. Its armature resistance is 1.2 Ω. Find the back-EMF E.

V = 220 VIa = 15 ARa = 1.2 Ω
E = V
3.Medium
+−VRa​M+−En = 3000 rpmT = ?

A DC motor delivers 11 kW of mechanical power at 3000 rpm. Find the shaft torque T.

P = 11 kWn = 3000 rpm
T = N·m
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 1.5 N·m/A and Ra = 0.8 Ω is connected to 240 V. Find its stall torque Tstall, the torque at zero speed.

V = 240 Vk = 1.5 N·m/ARa = 0.8 Ω
Tstall = N·m
5.Hard
T (N·m)n (rpm)8 N·m600 rpm

A DC motor with k = 2 V·s/rad and Ra = 2 Ω must drive a load of 8 N·m at 600 rpm. Find the armature voltage V it needs.

k = 2 V·s/radRa = 2 ΩT = 8 N·mn = 600 rpm
V = V
6.Hard
+−110 VRa​ = 0.2 ΩIa​: 25 → 37.5 AM+−En = ?T

A separately excited DC motor on 110 V (Ra = 0.2 Ω) runs at 1800 rpm with an armature current of 25 A. The load changes and the current becomes 37.5 A, with the field unchanged. Find the new speed n2.

V = 110 VRa = 0.2 Ωn1 = 1800 rpmIa1 = 25 AIa2 = 37.5 A
n2 = rpm

Answer key

  1. 1. k = 0.2801 V·s/rad
  2. 2. E = 202 V
  3. 3. T = 35.01 N·m
  4. 4. Tstall = 450 N·m
  5. 5. V = 133.7 V
  6. 6. n2 = 1757 rpm

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