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 = 1.5 V·s/rad and an armature resistance of 1 Ω runs from 240 V and draws 25 A. Find its speed n in rpm.

V = 240 VR_a = 1 ΩI_a = 25 Ak = 1.5 V·s/rad

n = rpm

2.Easy

A DC motor runs from 48 V with an armature current of 12 A. The armature resistance is 0.2 Ω. Find the developed (electromagnetic) power P_dev.

V = 48 VI_a = 12 AR_a = 0.2 Ω

P_dev = W

3.Medium

A DC motor with k = 1 V·s/rad runs from 240 V. Neglecting all losses at no load, find its no-load speed n₀ in rpm.

V = 240 Vk = 1 V·s/rad

n₀ = rpm

4.Medium

A DC motor (k = 0.6 V·s/rad, R_a = 0.4 Ω) runs from 240 V. At full load it draws 20 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 240 Vk = 0.6 V·s/radR_a = 0.4 ΩI_FL = 20 A

SR = %

5.Hard

A DC chopper feeds a motor (k = 0.12 V·s/rad, R_a = 0.4 Ω) from a 48 V DC bus. The average armature voltage is D·48 V. Find the duty cycle D that runs the motor at 900 rpm with a load of 0.6 N·m.

V_dc = 48 Vk = 0.12 V·s/radR_a = 0.4 ΩT = 0.6 N·mn = 900 rpm

D = %

6.Hard

A 110 V DC motor has an armature resistance of 1 Ω. Find the series starting resistance R_st that limits the starting current to 40 A.

V = 110 VR_a = 1 ΩI_max = 40 A

R_st = Ω

Answer key

1. n = 1369 rpm
2. P_dev = 547.2 W
3. n₀ = 2292 rpm
4. SR = 3.448 %
5. D = 27.73 %
6. R_st = 1.75 Ω

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