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​ = 0.4 ΩIa​ = 25 AM+−E = ?ωT

A DC motor runs from a 110 V supply and draws an armature current of 25 A. Its armature resistance is 0.4 Ω. Find the back-EMF E.

V = 110 VIa = 25 ARa = 0.4 Ω
E = V
2.Easy
+−110 VRa​ = 0.8 ΩIa​ = ?M+−E = 100.4 VωT

A DC motor on a 110 V supply has an armature resistance of 0.8 Ω. At its present speed the back-EMF is 100.4 V. Find the armature current Ia.

V = 110 VE = 100.4 VRa = 0.8 Ω
Ia = A
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 0.15 V·s/rad runs from 48 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 48 Vk = 0.15 V·s/rad
n0 = rpm
4.Medium
+−VRa​M+−En = 960 rpmT = ?

A DC motor delivers 4 kW of mechanical power at 960 rpm. Find the shaft torque T.

P = 4 kWn = 960 rpm
T = N·m
5.Hard
T (N·m)n (rpm)9 N·m1200 rpm

A DC motor with k = 0.6 V·s/rad and Ra = 0.5 Ω must drive a load of 9 N·m at 1200 rpm. Find the armature voltage V it needs.

k = 0.6 V·s/radRa = 0.5 ΩT = 9 N·mn = 1200 rpm
V = V
6.Hard
+−220 VD = ?Ra​ = 0.5 ΩM+−En = 1000 rpmT = 24 N·m

A DC chopper feeds a motor (k = 0.8 V·s/rad, Ra = 0.5 Ω) from a 220 V DC bus. The average armature voltage is D·220 V. Find the duty cycle D that runs the motor at 1000 rpm with a load of 24 N·m.

Vdc = 220 Vk = 0.8 V·s/radRa = 0.5 ΩT = 24 N·mn = 1000 rpm
D = %

Answer key

  1. 1. E = 100 V
  2. 2. Ia = 12 A
  3. 3. n0 = 3056 rpm
  4. 4. T = 39.79 N·m
  5. 5. V = 82.9 V
  6. 6. D = 44.9 %

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