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 1450 rpm, and its back-EMF at that speed is 115 V. Find the motor constant k in V·s/rad.

E = 115 Vn = 1450 rpm

k = V·s/rad

2.Easy

A DC motor with k = 0.3 V·s/rad and an armature resistance of 2 Ω runs from 120 V and draws 2 A. Find its speed n in rpm.

V = 120 VR_a = 2 ΩI_a = 2 Ak = 0.3 V·s/rad

n = rpm

3.Medium

A DC motor with k = 0.8 V·s/rad runs from 240 V. Neglecting all losses at no load, find its no-load speed n₀ in rpm.

V = 240 Vk = 0.8 V·s/rad

n₀ = rpm

4.Medium

A DC motor draws 8 A from a 120 V supply. Its armature resistance is 1 Ω and its rotational (friction, windage and iron) losses are 54 W. Find its efficiency η.

V = 120 VI_a = 8 AR_a = 1 ΩP_rot = 54 W

η = %

5.Hard

A separately excited DC motor (k = 0.4 V·s/rad at full field, R_a = 2 Ω, V = 110 V) drives a constant load torque of 1.6 N·m. The field flux is reduced to 80 % of its full value. Find the new speed n.

V = 110 Vk = 0.4 V·s/radR_a = 2 ΩT = 1.6 N·mΦ = 80 %

n = rpm

6.Hard

A separately excited DC motor (k = 1 V·s/rad at full field, R_a = 0.5 Ω, V = 220 V) drives a constant load torque of 40 N·m. The field flux is reduced to 70 % of its full value. Find the new speed n.

V = 220 Vk = 1 V·s/radR_a = 0.5 ΩT = 40 N·mΦ = 70 %

n = rpm

Answer key

1. k = 0.7574 V·s/rad
2. n = 3692 rpm
3. n₀ = 2865 rpm
4. η = 87.71 %
5. n = 2984 rpm
6. n = 2611 rpm

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