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 an armature resistance of 1.2 Ω is switched directly onto a 120 V supply. Its normal full-load current is 8 A. Find the armature current I_start at the moment of starting.

V = 120 VR_a = 1.2 Ω

I_start = A

2.Easy

A DC motor runs from 240 V with an armature current of 15 A. The armature resistance is 1.2 Ω. Find the developed (electromagnetic) power P_dev.

V = 240 VI_a = 15 AR_a = 1.2 Ω

P_dev = W

3.Medium

A DC motor (k = 1.2 N·m/A, R_a = 1 Ω) runs from 220 V and drives a load torque of 18 N·m. Find its speed n in rpm.

V = 220 Vk = 1.2 N·m/AR_a = 1 ΩT = 18 N·m

n = rpm

4.Medium

A DC motor (k = 0.5 V·s/rad, R_a = 0.8 Ω) runs from 48 V. At full load it draws 8 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 48 Vk = 0.5 V·s/radR_a = 0.8 ΩI_FL = 8 A

SR = %

5.Hard

A DC motor with k = 0.75 V·s/rad and R_a = 0.25 Ω must drive a load of 4.5 N·m at 300 rpm. Find the armature voltage V it needs.

k = 0.75 V·s/radR_a = 0.25 ΩT = 4.5 N·mn = 300 rpm

V = V

6.Hard

A separately excited DC motor (k = 0.4 V·s/rad at full field, R_a = 0.4 Ω, V = 110 V) drives a constant load torque of 10 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 = 0.4 ΩT = 10 N·mΦ = 80 %

n = rpm

Answer key

1. I_start = 100 A
2. P_dev = 3330 W
3. n = 1631 rpm
4. SR = 15.38 %
5. V = 25.06 V
6. n = 2910 rpm

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