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 = 0.8 V·s/rad and an armature resistance of 1 Ω runs from 110 V and draws 8 A. Find its speed n in rpm.

V = 110 VR_a = 1 ΩI_a = 8 Ak = 0.8 V·s/rad

n = rpm

2.Easy

A DC motor runs from 220 V with an armature current of 12 A. The armature resistance is 1.5 Ω. Find the developed (electromagnetic) power P_dev.

V = 220 VI_a = 12 AR_a = 1.5 Ω

P_dev = W

3.Medium

A DC motor (k = 0.75 N·m/A, R_a = 0.4 Ω) runs from 120 V and drives a load torque of 22.5 N·m. Find its speed n in rpm.

V = 120 Vk = 0.75 N·m/AR_a = 0.4 ΩT = 22.5 N·m

n = rpm

4.Medium

A DC motor draws 30 A from a 220 V supply. Its armature resistance is 0.25 Ω and its rotational (friction, windage and iron) losses are 383 W. Find its efficiency η.

V = 220 VI_a = 30 AR_a = 0.25 ΩP_rot = 383 W

η = %

5.Hard

A separately excited DC motor (k = 1.2 V·s/rad at full field, R_a = 1.2 Ω, V = 110 V) drives a constant load torque of 7.2 N·m. The field flux is reduced to 85 % of its full value. Find the new speed n.

V = 110 Vk = 1.2 V·s/radR_a = 1.2 ΩT = 7.2 N·mΦ = 85 %

n = rpm

6.Hard

A small DC motor has an armature resistance of 1.5 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 = 1.5 Ω

P_max = W

Answer key

1. n = 1218 rpm
2. P_dev = 2424 W
3. n = 1375 rpm
4. η = 90.79 %
5. n = 950.5 rpm
6. P_max = 2017 W

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