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​ = 1 ΩIa​ = ?M+−E = 43 VωT

A DC motor on a 48 V supply has an armature resistance of 1 Ω. At its present speed the back-EMF is 43 V. Find the armature current Ia.

V = 48 VE = 43 VRa = 1 Ω
Ia = A
2.Easy
+−120 VRa​M+−E = 104 Vn = 1200 rpmT

A DC motor turns at 1200 rpm, and its back-EMF at that speed is 104 V. Find the motor constant k in V·s/rad.

E = 104 Vn = 1200 rpm
k = V·s/rad
3.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 1 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 N·m/ARa = 0.8 Ω
Tstall = N·m
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 1.5 V·s/rad runs from 220 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 220 Vk = 1.5 V·s/rad
n0 = rpm
5.Hard
+−240 VRa​ = 0.4 ΩIa​: 25 → 50 AM+−En = ?T

A separately excited DC motor on 240 V (Ra = 0.4 Ω) runs at 1500 rpm with an armature current of 25 A. The load changes and the current becomes 50 A, with the field unchanged. Find the new speed n2.

V = 240 VRa = 0.4 Ωn1 = 1500 rpmIa1 = 25 AIa2 = 50 A
n2 = rpm
6.Hard
+−240 VRst​ = ?Ra​ = 0.4 Ω60 AM+−E = 0n = 0T

A 240 V DC motor has an armature resistance of 0.4 Ω. Find the series starting resistance Rst that limits the starting current to 60 A.

V = 240 VRa = 0.4 ΩImax = 60 A
Rst = Ω

Answer key

  1. 1. Ia = 5 A
  2. 2. k = 0.8276 V·s/rad
  3. 3. Tstall = 300 N·m
  4. 4. n0 = 1401 rpm
  5. 5. n2 = 1435 rpm
  6. 6. Rst = 3.6 Ω

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