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
+−110 VRa​ = 1 ΩIa​ = ?M+−E = 98 VωT

A DC motor on a 110 V supply has an armature resistance of 1 Ω. At its present speed the back-EMF is 98 V. Find the armature current Ia.

V = 110 VE = 98 VRa = 1 Ω
Ia = A
2.Easy
+−110 VRa​ = 0.4 ΩIa​ = ?M+−E = 104 VωT

A DC motor on a 110 V supply has an armature resistance of 0.4 Ω. At its present speed the back-EMF is 104 V. Find the armature current Ia.

V = 110 VE = 104 VRa = 0.4 Ω
Ia = A
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.5 V·s/rad runs from 48 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 48 Vk = 0.5 V·s/rad
n0 = rpm
4.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.8 V·s/rad runs from 120 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 120 Vk = 0.8 V·s/rad
n0 = rpm
5.Hard
+−110 VRst​ = ?Ra​ = 1 Ω100 AM+−E = 0n = 0T

A 110 V DC motor has an armature resistance of 1 Ω. Find the series starting resistance Rst that limits the starting current to 100 A.

V = 110 VRa = 1 ΩImax = 100 A
Rst = Ω
6.Hard
+−240 VRa​ = 1.2 ΩM+−En = ?T = 12 N·m

A separately excited DC motor (k = 1 V·s/rad at full field, Ra = 1.2 Ω, V = 240 V) drives a constant load torque of 12 N·m. The field flux is reduced to 90 % of its full value. Find the new speed n.

V = 240 Vk = 1 V·s/radRa = 1.2 ΩT = 12 N·mΦ = 90 %
n = rpm

Answer key

  1. 1. Ia = 12 A
  2. 2. Ia = 15 A
  3. 3. n0 = 916.7 rpm
  4. 4. n0 = 1432 rpm
  5. 5. Rst = 0.1 Ω
  6. 6. n = 2377 rpm

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