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 an armature resistance of 0.4 Ω is switched directly onto a 110 V supply. Its normal full-load current is 12 A. Find the armature current I_start at the moment of starting.

V = 110 VR_a = 0.4 Ω

I_start = A

2.Easy

A DC motor runs from 220 V with an armature current of 20 A. The armature resistance is 0.4 Ω. Find the developed (electromagnetic) power P_dev.

V = 220 VI_a = 20 AR_a = 0.4 Ω

P_dev = W

3.Medium

A DC motor (k = 0.4 N·m/A, R_a = 2 Ω) runs from 120 V and drives a load torque of 1.6 N·m. Find its speed n in rpm.

V = 120 Vk = 0.4 N·m/AR_a = 2 ΩT = 1.6 N·m

n = rpm

4.Medium

A DC motor draws 15 A from a 48 V supply. Its armature resistance is 0.25 Ω and its rotational (friction, windage and iron) losses are 53 W. Find its efficiency η.

V = 48 VI_a = 15 AR_a = 0.25 ΩP_rot = 53 W

η = %

5.Hard

A separately excited DC motor (k = 1.5 V·s/rad at full field, R_a = 1 Ω, V = 220 V) drives a constant load torque of 30 N·m. The field flux is reduced to 85 % of its full value. Find the new speed n.

V = 220 Vk = 1.5 V·s/radR_a = 1 ΩT = 30 N·mΦ = 85 %

n = rpm

6.Hard

A separately excited DC motor (k = 0.3 V·s/rad at full field, R_a = 2 Ω, V = 120 V) drives a constant load torque of 1.2 N·m. The field flux is reduced to 70 % of its full value. Find the new speed n.

V = 120 Vk = 0.3 V·s/radR_a = 2 ΩT = 1.2 N·mΦ = 70 %

n = rpm

Answer key

1. I_start = 275 A
2. P_dev = 4240 W
3. n = 2674 rpm
4. η = 84.83 %
5. n = 1471 rpm
6. n = 4937 rpm

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