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 turns at 1200 rpm, and its back-EMF at that speed is 114 V. Find the motor constant k in V·s/rad.

E = 114 Vn = 1200 rpm

k = V·s/rad

2.Easy

A DC motor runs from 48 V with an armature current of 8 A. The armature resistance is 0.25 Ω. Find the developed (electromagnetic) power P_dev.

V = 48 VI_a = 8 AR_a = 0.25 Ω

P_dev = W

3.Medium

A DC motor (k = 1.2 N·m/A, R_a = 0.2 Ω) runs from 240 V and drives a load torque of 48 N·m. Find its speed n in rpm.

V = 240 Vk = 1.2 N·m/AR_a = 0.2 ΩT = 48 N·m

n = rpm

4.Medium

A DC motor draws 30 A from a 220 V supply. Its armature resistance is 1 Ω and its rotational (friction, windage and iron) losses are 456 W. Find its efficiency η.

V = 220 VI_a = 30 AR_a = 1 ΩP_rot = 456 W

η = %

5.Hard

A DC motor (k = 1 V·s/rad, R_a = 0.25 Ω) on a fixed 120 V supply drives a load of 25 N·m. Find the series resistance R_s that slows it down to 300 rpm.

V = 120 Vk = 1 V·s/radR_a = 0.25 ΩT = 25 N·mn = 300 rpm

R_s = Ω

6.Hard

A separately excited DC motor (k = 0.3 V·s/rad at full field, R_a = 0.2 Ω, V = 24 V) drives a constant load torque of 1.5 N·m. The field flux is reduced to 80 % of its full value. Find the new speed n.

V = 24 Vk = 0.3 V·s/radR_a = 0.2 ΩT = 1.5 N·mΦ = 80 %

n = rpm

Answer key

1. k = 0.9072 V·s/rad
2. P_dev = 368 W
3. n = 1846 rpm
4. η = 79.45 %
5. R_s = 3.293 Ω
6. n = 905.2 rpm

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