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 = 0.15 V·s/rad and an armature resistance of 0.2 Ω runs from 48 V and draws 20 A. Find its speed n in rpm.

V = 48 VR_a = 0.2 ΩI_a = 20 Ak = 0.15 V·s/rad

n = rpm

2.Easy

A DC motor with an armature resistance of 0.8 Ω is switched directly onto a 48 V supply. Its normal full-load current is 8 A. Find the armature current I_start at the moment of starting.

V = 48 VR_a = 0.8 Ω

I_start = A

3.Medium

A DC motor draws 2 A from a 48 V supply. Its armature resistance is 0.8 Ω and its rotational (friction, windage and iron) losses are 7 W. Find its efficiency η.

V = 48 VI_a = 2 AR_a = 0.8 ΩP_rot = 7 W

η = %

4.Medium

A DC motor with k = 0.4 V·s/rad runs from 120 V. Neglecting all losses at no load, find its no-load speed n₀ in rpm.

V = 120 Vk = 0.4 V·s/rad

n₀ = rpm

5.Hard

A separately excited DC motor (k = 1 V·s/rad at full field, R_a = 0.8 Ω, V = 120 V) drives a constant load torque of 5 N·m. The field flux is reduced to 70 % of its full value. Find the new speed n.

V = 120 Vk = 1 V·s/radR_a = 0.8 ΩT = 5 N·mΦ = 70 %

n = rpm

6.Hard

A DC motor with k = 1.5 V·s/rad and R_a = 1.5 Ω must drive a load of 22.5 N·m at 900 rpm. Find the armature voltage V it needs.

k = 1.5 V·s/radR_a = 1.5 ΩT = 22.5 N·mn = 900 rpm

V = V

Answer key

1. n = 2801 rpm
2. I_start = 60 A
3. η = 89.38 %
4. n₀ = 2865 rpm
5. n = 1559 rpm
6. V = 163.9 V

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