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 110 V with an armature current of 12 A. The armature resistance is 0.4 Ω. Find the developed (electromagnetic) power P_dev.

V = 110 VI_a = 12 AR_a = 0.4 Ω

P_dev = W

2.Easy

A DC motor runs from 220 V with an armature current of 8 A. The armature resistance is 1.2 Ω. Find the developed (electromagnetic) power P_dev.

V = 220 VI_a = 8 AR_a = 1.2 Ω

P_dev = W

3.Medium

A DC motor with k = 0.12 V·s/rad runs from 48 V. Neglecting all losses at no load, find its no-load speed n₀ in rpm.

V = 48 Vk = 0.12 V·s/rad

n₀ = rpm

4.Medium

A DC motor (k = 1.2 V·s/rad, R_a = 1.5 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 = 1.2 V·s/radR_a = 1.5 ΩI_FL = 15 A

SR = %

5.Hard

A separately excited DC motor (k = 0.75 V·s/rad at full field, R_a = 1.5 Ω, V = 110 V) drives a constant load torque of 6 N·m. The field flux is reduced to 80 % of its full value. Find the new speed n.

V = 110 Vk = 0.75 V·s/radR_a = 1.5 ΩT = 6 N·mΦ = 80 %

n = rpm

6.Hard

A small DC motor has an armature resistance of 4 Ω and runs from 24 V. Neglecting rotational losses, find the largest mechanical power P_max it can develop at any speed.

V = 24 VR_a = 4 Ω

P_max = W

Answer key

1. P_dev = 1262 W
2. P_dev = 1683 W
3. n₀ = 3820 rpm
4. SR = 10.34 %
5. n = 1512 rpm
6. P_max = 36 W

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