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 8 A. The armature resistance is 0.2 Ω. Find the developed (electromagnetic) power P_dev.

V = 48 VI_a = 8 AR_a = 0.2 Ω

P_dev = W

2.Easy

A DC motor runs from 240 V with an armature current of 25 A. The armature resistance is 1.2 Ω. Find the developed (electromagnetic) power P_dev.

V = 240 VI_a = 25 AR_a = 1.2 Ω

P_dev = W

3.Medium

A DC motor draws 8 A from a 120 V supply. Its armature resistance is 2 Ω and its rotational (friction, windage and iron) losses are 33 W. Find its efficiency η.

V = 120 VI_a = 8 AR_a = 2 ΩP_rot = 33 W

η = %

4.Medium

A DC motor (k = 0.3 V·s/rad, R_a = 0.25 Ω) runs from 120 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 = 120 Vk = 0.3 V·s/radR_a = 0.25 ΩI_FL = 30 A

SR = %

5.Hard

A separately excited DC motor (k = 1 V·s/rad at full field, R_a = 0.8 Ω, V = 110 V) drives a constant load torque of 8 N·m. The field flux is reduced to 75 % of its full value. Find the new speed n.

V = 110 Vk = 1 V·s/radR_a = 0.8 ΩT = 8 N·mΦ = 75 %

n = rpm

6.Hard

A 48 V DC motor has an armature resistance of 1 Ω. Find the series starting resistance R_st that limits the starting current to 30 A.

V = 48 VR_a = 1 ΩI_max = 30 A

R_st = Ω

Answer key

1. P_dev = 371.2 W
2. P_dev = 5250 W
3. η = 83.23 %
4. SR = 6.667 %
5. n = 1292 rpm
6. R_st = 0.6 Ω

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