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 runs from 48 V with an armature current of 10 A. The armature resistance is 0.2 Ω. Find the developed (electromagnetic) power P_dev.

V = 48 VI_a = 10 AR_a = 0.2 Ω

P_dev = W

2.Easy

A DC motor with k = 0.4 V·s/rad and an armature resistance of 2 Ω runs from 110 V and draws 5 A. Find its speed n in rpm.

V = 110 VR_a = 2 ΩI_a = 5 Ak = 0.4 V·s/rad

n = rpm

3.Medium

A DC motor (k = 1 V·s/rad, R_a = 1 Ω) runs from 120 V. At full load it draws 10 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 120 Vk = 1 V·s/radR_a = 1 ΩI_FL = 10 A

SR = %

4.Medium

A DC motor draws 2 A from a 24 V supply. Its armature resistance is 0.5 Ω and its rotational (friction, windage and iron) losses are 3 W. Find its efficiency η.

V = 24 VI_a = 2 AR_a = 0.5 ΩP_rot = 3 W

η = %

5.Hard

A small DC motor has an armature resistance of 3 Ω and runs from 110 V. Neglecting rotational losses, find the largest mechanical power P_max it can develop at any speed.

V = 110 VR_a = 3 Ω

P_max = W

6.Hard

A separately excited DC motor (k = 0.4 V·s/rad at full field, R_a = 1.5 Ω, V = 110 V) drives a constant load torque of 2.4 N·m. The field flux is reduced to 70 % of its full value. Find the new speed n.

V = 110 Vk = 0.4 V·s/radR_a = 1.5 ΩT = 2.4 N·mΦ = 70 %

n = rpm

Answer key

1. P_dev = 460 W
2. n = 2387 rpm
3. SR = 9.091 %
4. η = 89.58 %
5. P_max = 1008 W
6. n = 3313 rpm

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