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 0.25 Ω is switched directly onto a 110 V supply. Its normal full-load current is 50 A. Find the armature current I_start at the moment of starting.

V = 110 VR_a = 0.25 Ω

I_start = A

2.Easy

A DC motor with k = 0.15 V·s/rad and an armature resistance of 0.8 Ω runs from 48 V and draws 8 A. Find its speed n in rpm.

V = 48 VR_a = 0.8 ΩI_a = 8 Ak = 0.15 V·s/rad

n = rpm

3.Medium

A DC motor (k = 0.15 V·s/rad, R_a = 1.2 Ω) runs from 48 V. At full load it draws 2 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 48 Vk = 0.15 V·s/radR_a = 1.2 ΩI_FL = 2 A

SR = %

4.Medium

A DC motor draws 4 A from a 120 V supply. Its armature resistance is 1 Ω and its rotational (friction, windage and iron) losses are 37 W. Find its efficiency η.

V = 120 VI_a = 4 AR_a = 1 ΩP_rot = 37 W

η = %

5.Hard

A DC motor (k = 1.5 V·s/rad, R_a = 1.5 Ω) on a fixed 120 V supply drives a load of 9 N·m. Find the series resistance R_s that slows it down to 300 rpm.

V = 120 Vk = 1.5 V·s/radR_a = 1.5 ΩT = 9 N·mn = 300 rpm

R_s = Ω

6.Hard

A separately excited DC motor (k = 0.75 V·s/rad at full field, R_a = 2 Ω, V = 240 V) drives a constant load torque of 4.5 N·m. The field flux is reduced to 90 % of its full value. Find the new speed n.

V = 240 Vk = 0.75 V·s/radR_a = 2 ΩT = 4.5 N·mΦ = 90 %

n = rpm

Answer key

1. I_start = 440 A
2. n = 2648 rpm
3. SR = 5.263 %
4. η = 88.96 %
5. R_s = 10.65 Ω
6. n = 3207 rpm

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