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 a 120 V supply and draws an armature current of 25 A. Its armature resistance is 0.5 Ω. Find the back-EMF E.

V = 120 VI_a = 25 AR_a = 0.5 Ω

E = V

2.Easy

A DC motor with an armature resistance of 0.4 Ω is switched directly onto a 220 V supply. Its normal full-load current is 40 A. Find the armature current I_start at the moment of starting.

V = 220 VR_a = 0.4 Ω

I_start = A

3.Medium

A DC motor (k = 1 N·m/A, R_a = 0.4 Ω) runs from 220 V and drives a load torque of 20 N·m. Find its speed n in rpm.

V = 220 Vk = 1 N·m/AR_a = 0.4 ΩT = 20 N·m

n = rpm

4.Medium

A DC motor (k = 0.3 V·s/rad, R_a = 1.5 Ω) runs from 110 V. At full load it draws 10 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 110 Vk = 0.3 V·s/radR_a = 1.5 ΩI_FL = 10 A

SR = %

5.Hard

A DC motor (k = 0.6 V·s/rad, R_a = 0.8 Ω) on a fixed 220 V supply drives a load of 7.2 N·m. Find the series resistance R_s that slows it down to 1500 rpm.

V = 220 Vk = 0.6 V·s/radR_a = 0.8 ΩT = 7.2 N·mn = 1500 rpm

R_s = Ω

6.Hard

A 240 V DC motor has an armature resistance of 0.8 Ω. Find the series starting resistance R_st that limits the starting current to 60 A.

V = 240 VR_a = 0.8 ΩI_max = 60 A

R_st = Ω

Answer key

1. E = 107.5 V
2. I_start = 550 A
3. n = 2024 rpm
4. SR = 15.79 %
5. R_s = 9.679 Ω
6. R_st = 3.2 Ω

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