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 = 1.2 V·s/rad and an armature resistance of 0.25 Ω runs from 110 V and draws 30 A. Find its speed n in rpm.

V = 110 VR_a = 0.25 ΩI_a = 30 Ak = 1.2 V·s/rad

n = rpm

2.Easy

A DC motor runs from a 220 V supply and draws an armature current of 25 A. Its armature resistance is 1 Ω. Find the back-EMF E.

V = 220 VI_a = 25 AR_a = 1 Ω

E = V

3.Medium

A DC motor (k = 1 V·s/rad, R_a = 2 Ω) runs from 240 V. At full load it draws 6 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 240 Vk = 1 V·s/radR_a = 2 ΩI_FL = 6 A

SR = %

4.Medium

A DC motor (k = 0.4 V·s/rad, R_a = 1 Ω) runs from 48 V. At full load it draws 2 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 48 Vk = 0.4 V·s/radR_a = 1 ΩI_FL = 2 A

SR = %

5.Hard

A separately excited DC motor (k = 1.2 V·s/rad at full field, R_a = 0.2 Ω, V = 220 V) drives a constant load torque of 48 N·m. The field flux is reduced to 70 % of its full value. Find the new speed n.

V = 220 Vk = 1.2 V·s/radR_a = 0.2 ΩT = 48 N·mΦ = 70 %

n = rpm

6.Hard

A DC motor with k = 0.3 V·s/rad and R_a = 0.2 Ω must drive a load of 2.4 N·m at 600 rpm. Find the armature voltage V it needs.

k = 0.3 V·s/radR_a = 0.2 ΩT = 2.4 N·mn = 600 rpm

V = V

Answer key

1. n = 815.7 rpm
2. E = 195 V
3. SR = 5.263 %
4. SR = 4.348 %
5. n = 2371 rpm
6. V = 20.45 V

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