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
+−240 VRa​ = 1.5 ΩIa​ = 20 AM+−En = ?T

A DC motor with k = 1 V·s/rad and an armature resistance of 1.5 Ω runs from 240 V and draws 20 A. Find its speed n in rpm.

V = 240 VRa = 1.5 ΩIa = 20 Ak = 1 V·s/rad
n = rpm
2.Easy
+−220 VRa​Ia​ = 15 AM+−EωT = ?

A permanent-magnet DC motor has the torque constant k = 0.75 N·m/A. Find the torque T it develops at an armature current of 15 A.

k = 0.75 N·m/AIa = 15 A
T = N·m
3.Medium
T (N·m)n (rpm)0.16 N·mn0​nFL​

A DC motor (k = 0.08 V·s/rad, Ra = 1.5 Ω) runs from 24 V. At full load it draws 2 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − nFL)/nFL in percent.

V = 24 Vk = 0.08 V·s/radRa = 1.5 ΩIFL = 2 A
SR = %
4.Medium
+−VRa​M+−En = 1200 rpmT = ?

A DC motor delivers 4 kW of mechanical power at 1200 rpm. Find the shaft torque T.

P = 4 kWn = 1200 rpm
T = N·m
5.Hard
+−220 VRa​ = 0.4 ΩIa​: 40 → 20 AM+−En = ?T

A separately excited DC motor on 220 V (Ra = 0.4 Ω) runs at 900 rpm with an armature current of 40 A. The load changes and the current becomes 20 A, with the field unchanged. Find the new speed n2.

V = 220 VRa = 0.4 Ωn1 = 900 rpmIa1 = 40 AIa2 = 20 A
n2 = rpm
6.Hard
+−220 VD = ?Ra​ = 2 ΩM+−En = 500 rpmT = 1.5 N·m

A DC chopper feeds a motor (k = 0.75 V·s/rad, Ra = 2 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 500 rpm with a load of 1.5 N·m.

Vdc = 220 Vk = 0.75 V·s/radRa = 2 ΩT = 1.5 N·mn = 500 rpm
D = %

Answer key

  1. 1. n = 2005 rpm
  2. 2. T = 11.25 N·m
  3. 3. SR = 14.29 %
  4. 4. T = 31.83 N·m
  5. 5. n2 = 935.3 rpm
  6. 6. D = 19.67 %

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