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 0.4 Ω is switched directly onto a 48 V supply. Its normal full-load current is 10 A. Find the armature current I_start at the moment of starting.

V = 48 VR_a = 0.4 Ω

I_start = A

2.Easy

A DC motor runs from a 240 V supply and draws an armature current of 15 A. Its armature resistance is 1.5 Ω. Find the back-EMF E.

V = 240 VI_a = 15 AR_a = 1.5 Ω

E = V

3.Medium

A DC motor draws 25 A from a 48 V supply. Its armature resistance is 0.25 Ω and its rotational (friction, windage and iron) losses are 52 W. Find its efficiency η.

V = 48 VI_a = 25 AR_a = 0.25 ΩP_rot = 52 W

η = %

4.Medium

A DC motor (k = 0.15 N·m/A, R_a = 0.5 Ω) runs from 24 V and drives a load torque of 0.9 N·m. Find its speed n in rpm.

V = 24 Vk = 0.15 N·m/AR_a = 0.5 ΩT = 0.9 N·m

n = rpm

5.Hard

A separately excited DC motor on 220 V (R_a = 0.8 Ω) runs at 1800 rpm with an armature current of 25 A. The load changes and the current becomes 10 A, with the field unchanged. Find the new speed n₂.

V = 220 VR_a = 0.8 Ωn₁ = 1800 rpmI_a1 = 25 AI_a2 = 10 A

n₂ = rpm

6.Hard

A DC chopper feeds a motor (k = 0.8 V·s/rad, R_a = 1 Ω) from a 240 V DC bus. The average armature voltage is D·240 V. Find the duty cycle D that runs the motor at 750 rpm with a load of 6.4 N·m.

V_dc = 240 Vk = 0.8 V·s/radR_a = 1 ΩT = 6.4 N·mn = 750 rpm

D = %

Answer key

1. I_start = 120 A
2. E = 217.5 V
3. η = 82.65 %
4. n = 1337 rpm
5. n₂ = 1908 rpm
6. D = 29.51 %

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