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 = 1.5 V·s/rad and an armature resistance of 1.5 Ω runs from 110 V and draws 10 A. Find its speed n in rpm.

V = 110 VR_a = 1.5 ΩI_a = 10 Ak = 1.5 V·s/rad

n = rpm

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 with k = 0.75 V·s/rad runs from 240 V. Neglecting all losses at no load, find its no-load speed n₀ in rpm.

V = 240 Vk = 0.75 V·s/rad

n₀ = rpm

4.Medium

A DC motor (k = 0.6 V·s/rad, R_a = 0.5 Ω) runs from 240 V. At full load it draws 15 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 240 Vk = 0.6 V·s/radR_a = 0.5 ΩI_FL = 15 A

SR = %

5.Hard

A separately excited DC motor on 120 V (R_a = 0.5 Ω) runs at 900 rpm with an armature current of 30 A. The load changes and the current becomes 15 A, with the field unchanged. Find the new speed n₂.

V = 120 VR_a = 0.5 Ωn₁ = 900 rpmI_a1 = 30 AI_a2 = 15 A

n₂ = rpm

6.Hard

A small DC motor has an armature resistance of 1.5 Ω and runs from 48 V. Neglecting rotational losses, find the largest mechanical power P_max it can develop at any speed.

V = 48 VR_a = 1.5 Ω

P_max = W

Answer key

1. n = 604.8 rpm
2. P_dev = 5250 W
3. n₀ = 3056 rpm
4. SR = 3.226 %
5. n₂ = 964.3 rpm
6. P_max = 384 W

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