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 = 0.4 V·s/rad and an armature resistance of 1 Ω runs from 110 V and draws 5 A. Find its speed n in rpm.

V = 110 VR_a = 1 ΩI_a = 5 Ak = 0.4 V·s/rad

n = rpm

2.Easy

A DC motor with an armature resistance of 0.8 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 = 0.8 Ω

I_start = A

3.Medium

A DC motor with k = 0.3 V·s/rad runs from 24 V. Neglecting all losses at no load, find its no-load speed n₀ in rpm.

V = 24 Vk = 0.3 V·s/rad

n₀ = rpm

4.Medium

A DC motor (k = 2 V·s/rad, R_a = 0.2 Ω) runs from 220 V. At full load it draws 40 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 220 Vk = 2 V·s/radR_a = 0.2 ΩI_FL = 40 A

SR = %

5.Hard

A separately excited DC motor (k = 0.4 V·s/rad at full field, R_a = 0.25 Ω, V = 120 V) drives a constant load torque of 6 N·m. The field flux is reduced to 90 % of its full value. Find the new speed n.

V = 120 Vk = 0.4 V·s/radR_a = 0.25 ΩT = 6 N·mΦ = 90 %

n = rpm

6.Hard

A separately excited DC motor on 110 V (R_a = 0.2 Ω) runs at 1000 rpm with an armature current of 50 A. The load changes and the current becomes 25 A, with the field unchanged. Find the new speed n₂.

V = 110 VR_a = 0.2 Ωn₁ = 1000 rpmI_a1 = 50 AI_a2 = 25 A

n₂ = rpm

Answer key

1. n = 2507 rpm
2. I_start = 137.5 A
3. n₀ = 763.9 rpm
4. SR = 3.774 %
5. n = 3073 rpm
6. n₂ = 1050 rpm

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