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​Ia​ = 6 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 1.2 N·m/A. Find the torque T it develops at an armature current of 6 A.

k = 1.2 N·m/AIa = 6 A
T = N·m
2.Easy
+−120 VRa​M+−E = 108 Vn = 1500 rpmT

A DC motor turns at 1500 rpm, and its back-EMF at that speed is 108 V. Find the motor constant k in V·s/rad.

E = 108 Vn = 1500 rpm
k = V·s/rad
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 0.12 N·m/A and Ra = 0.2 Ω is connected to 24 V. Find its stall torque Tstall, the torque at zero speed.

V = 24 Vk = 0.12 N·m/ARa = 0.2 Ω
Tstall = N·m
4.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
5.Hard
+−220 VRst​ = ?Ra​ = 0.25 Ω10 AM+−E = 0n = 0T

A 220 V DC motor has an armature resistance of 0.25 Ω. Find the series starting resistance Rst that limits the starting current to 10 A.

V = 220 VRa = 0.25 ΩImax = 10 A
Rst = Ω
6.Hard
+−240 VRa​ = 0.5 ΩIa​: 15 → 22.5 AM+−En = ?T

A separately excited DC motor on 240 V (Ra = 0.5 Ω) runs at 1440 rpm with an armature current of 15 A. The load changes and the current becomes 22.5 A, with the field unchanged. Find the new speed n2.

V = 240 VRa = 0.5 Ωn1 = 1440 rpmIa1 = 15 AIa2 = 22.5 A
n2 = rpm

Answer key

  1. 1. T = 7.2 N·m
  2. 2. k = 0.6875 V·s/rad
  3. 3. Tstall = 14.4 N·m
  4. 4. T = 19.76 N·m
  5. 5. Rst = 21.75 Ω
  6. 6. n2 = 1417 rpm

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