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

A DC motor with k = 0.5 V·s/rad and an armature resistance of 0.4 Ω runs from 120 V and draws 30 A. Find its speed n in rpm.

V = 120 VR_a = 0.4 ΩI_a = 30 Ak = 0.5 V·s/rad

n = rpm

2.Easy

A DC motor with k = 0.75 V·s/rad and an armature resistance of 1.5 Ω runs from 120 V and draws 6 A. Find its speed n in rpm.

V = 120 VR_a = 1.5 ΩI_a = 6 Ak = 0.75 V·s/rad

n = rpm

3.Medium

A DC motor (k = 1 V·s/rad, R_a = 0.25 Ω) runs from 240 V. At full load it draws 40 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 240 Vk = 1 V·s/radR_a = 0.25 ΩI_FL = 40 A

SR = %

4.Medium

A DC motor draws 15 A from a 220 V supply. Its armature resistance is 2 Ω and its rotational (friction, windage and iron) losses are 171 W. Find its efficiency η.

V = 220 VI_a = 15 AR_a = 2 ΩP_rot = 171 W

η = %

5.Hard

A separately excited DC motor on 120 V (R_a = 0.5 Ω) runs at 1440 rpm with an armature current of 15 A. The load changes and the current becomes 9 A, with the field unchanged. Find the new speed n₂.

V = 120 VR_a = 0.5 Ωn₁ = 1440 rpmI_a1 = 15 AI_a2 = 9 A

n₂ = rpm

6.Hard

A DC chopper feeds a motor (k = 0.75 V·s/rad, R_a = 0.2 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 500 rpm with a load of 9 N·m.

V_dc = 110 Vk = 0.75 V·s/radR_a = 0.2 ΩT = 9 N·mn = 500 rpm

D = %

Answer key

1. n = 2063 rpm
2. n = 1413 rpm
3. SR = 4.348 %
4. η = 81.18 %
5. n₂ = 1478 rpm
6. D = 37.88 %

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