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 an armature resistance of 0.5 Ω is switched directly onto a 240 V supply. Its normal full-load current is 15 A. Find the armature current I_start at the moment of starting.

V = 240 VR_a = 0.5 Ω

I_start = A

2.Easy

A DC motor with an armature resistance of 2 Ω is switched directly onto a 240 V supply. Its normal full-load current is 6 A. Find the armature current I_start at the moment of starting.

V = 240 VR_a = 2 Ω

I_start = A

3.Medium

A DC motor draws 6 A from a 120 V supply. Its armature resistance is 1.2 Ω and its rotational (friction, windage and iron) losses are 41 W. Find its efficiency η.

V = 120 VI_a = 6 AR_a = 1.2 ΩP_rot = 41 W

η = %

4.Medium

A DC motor (k = 1 V·s/rad, R_a = 0.25 Ω) runs from 240 V. At full load it draws 30 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 = 30 A

SR = %

5.Hard

A DC motor (k = 0.6 V·s/rad, R_a = 0.8 Ω) on a fixed 110 V supply drives a load of 7.2 N·m. Find the series resistance R_s that slows it down to 600 rpm.

V = 110 Vk = 0.6 V·s/radR_a = 0.8 ΩT = 7.2 N·mn = 600 rpm

R_s = Ω

6.Hard

A DC motor with k = 0.1 V·s/rad and R_a = 0.5 Ω must drive a load of 0.2 N·m at 1500 rpm. Find the armature voltage V it needs.

k = 0.1 V·s/radR_a = 0.5 ΩT = 0.2 N·mn = 1500 rpm

V = V

Answer key

1. I_start = 480 A
2. I_start = 120 A
3. η = 88.31 %
4. SR = 3.226 %
5. R_s = 5.225 Ω
6. V = 16.71 V

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