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​ = 0.8 ΩIa​ = ?M+−E = 98 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 98 V. Find the armature current Ia.

V = 110 VE = 98 VRa = 0.8 Ω
Ia = A
2.Easy
+−110 VRa​ = 0.25 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 0.25 Ω. Find the back-EMF E.

V = 110 VIa = 25 ARa = 0.25 Ω
E = V
3.Medium
T (N·m)n (rpm)n0​ = ?

A DC motor with k = 2 V·s/rad runs from 240 V. Neglecting all losses at no load, find its no-load speed n0 in rpm.

V = 240 Vk = 2 V·s/rad
n0 = rpm
4.Medium
+−VRa​M+−En = 750 rpmT = ?

A DC motor delivers 2.2 kW of mechanical power at 750 rpm. Find the shaft torque T.

P = 2.2 kWn = 750 rpm
T = N·m
5.Hard
+−240 VD = ?Ra​ = 1.2 ΩM+−En = 800 rpmT = 8 N·m

A DC chopper feeds a motor (k = 1 V·s/rad, Ra = 1.2 Ω) from a 240 V DC bus. The average armature voltage is D·240 V. Find the duty cycle D that runs the motor at 800 rpm with a load of 8 N·m.

Vdc = 240 Vk = 1 V·s/radRa = 1.2 ΩT = 8 N·mn = 800 rpm
D = %
6.Hard
+−48 VRa​ = 0.4 ΩM+−En = ?T = 3 N·m

A separately excited DC motor (k = 0.75 V·s/rad at full field, Ra = 0.4 Ω, V = 48 V) drives a constant load torque of 3 N·m. The field flux is reduced to 75 % of its full value. Find the new speed n.

V = 48 Vk = 0.75 V·s/radRa = 0.4 ΩT = 3 N·mΦ = 75 %
n = rpm

Answer key

  1. 1. Ia = 15 A
  2. 2. E = 103.8 V
  3. 3. n0 = 1146 rpm
  4. 4. T = 28.01 N·m
  5. 5. D = 38.91 %
  6. 6. n = 778.7 rpm

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