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​ = 0.8 ΩIa​ = 5 AM+−E = ?ωT

A DC motor runs from a 48 V supply and draws an armature current of 5 A. Its armature resistance is 0.8 Ω. Find the back-EMF E.

V = 48 VIa = 5 ARa = 0.8 Ω
E = V
2.Easy
+−220 VRa​ = 0.8 ΩIa​ = ?M+−E = 196 VωT

A DC motor on a 220 V supply has an armature resistance of 0.8 Ω. At its present speed the back-EMF is 196 V. Find the armature current Ia.

V = 220 VE = 196 VRa = 0.8 Ω
Ia = A
3.Medium
T (N·m)n (rpm)1.8 N·mn0​nFL​

A DC motor (k = 0.3 V·s/rad, Ra = 2 Ω) runs from 110 V. At full load it draws 6 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 110 Vk = 0.3 V·s/radRa = 2 ΩIFL = 6 A
SR = %
4.Medium
T (N·m)n (rpm)25 N·mn0​n = ?

A DC motor (k = 1 N·m/A, Ra = 0.4 Ω) runs from 220 V and drives a load torque of 25 N·m. Find its speed n in rpm.

V = 220 Vk = 1 N·m/ARa = 0.4 ΩT = 25 N·m
n = rpm
5.Hard
+−110 VD = ?Ra​ = 1.2 ΩM+−En = 750 rpmT = 1 N·m

A DC chopper feeds a motor (k = 0.5 V·s/rad, Ra = 1.2 Ω) from a 110 V DC bus. The average armature voltage is D·110 V. Find the duty cycle D that runs the motor at 750 rpm with a load of 1 N·m.

Vdc = 110 Vk = 0.5 V·s/radRa = 1.2 ΩT = 1 N·mn = 750 rpm
D = %
6.Hard
T (N·m)n (rpm)20 N·m750 rpm

A DC motor with k = 0.8 V·s/rad and Ra = 0.5 Ω must drive a load of 20 N·m at 750 rpm. Find the armature voltage V it needs.

k = 0.8 V·s/radRa = 0.5 ΩT = 20 N·mn = 750 rpm
V = V

Answer key

  1. 1. E = 44 V
  2. 2. Ia = 30 A
  3. 3. SR = 12.24 %
  4. 4. n = 2005 rpm
  5. 5. D = 37.88 %
  6. 6. V = 75.33 V

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