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 runs from 220 V with an armature current of 25 A. The armature resistance is 1 Ω. Find the developed (electromagnetic) power P_dev.

V = 220 VI_a = 25 AR_a = 1 Ω

P_dev = W

2.Easy

A DC motor with an armature resistance of 1 Ω is switched directly onto a 220 V supply. Its normal full-load current is 25 A. Find the armature current I_start at the moment of starting.

V = 220 VR_a = 1 Ω

I_start = A

3.Medium

A DC motor (k = 1.5 V·s/rad, R_a = 0.5 Ω) runs from 120 V. At full load it draws 25 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 120 Vk = 1.5 V·s/radR_a = 0.5 ΩI_FL = 25 A

SR = %

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 = 0.6 V·s/rad at full field, R_a = 2 Ω, V = 220 V) drives a constant load torque of 3 N·m. The field flux is reduced to 90 % of its full value. Find the new speed n.

V = 220 Vk = 0.6 V·s/radR_a = 2 ΩT = 3 N·mΦ = 90 %

n = rpm

6.Hard

A separately excited DC motor on 48 V (R_a = 0.4 Ω) runs at 1500 rpm with an armature current of 15 A. The load changes and the current becomes 7.5 A, with the field unchanged. Find the new speed n₂.

V = 48 VR_a = 0.4 Ωn₁ = 1500 rpmI_a1 = 15 AI_a2 = 7.5 A

n₂ = rpm

Answer key

1. P_dev = 4875 W
2. I_start = 220 A
3. SR = 11.63 %
4. n₀ = 2865 rpm
5. n = 3694 rpm
6. n₂ = 1607 rpm

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