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 1.5 Ω is switched directly onto a 220 V supply. Its normal full-load current is 8 A. Find the armature current I_start at the moment of starting.

V = 220 VR_a = 1.5 Ω

I_start = A

2.Easy

A DC motor runs from a 220 V supply and draws an armature current of 6 A. Its armature resistance is 2 Ω. Find the back-EMF E.

V = 220 VI_a = 6 AR_a = 2 Ω

E = V

3.Medium

A DC motor with k = 0.5 V·s/rad runs from 48 V. Neglecting all losses at no load, find its no-load speed n₀ in rpm.

V = 48 Vk = 0.5 V·s/rad

n₀ = rpm

4.Medium

A DC motor draws 5 A from a 48 V supply. Its armature resistance is 0.4 Ω and its rotational (friction, windage and iron) losses are 14 W. Find its efficiency η.

V = 48 VI_a = 5 AR_a = 0.4 ΩP_rot = 14 W

η = %

5.Hard

A DC motor with k = 1 V·s/rad and R_a = 1.5 Ω must drive a load of 4 N·m at 400 rpm. Find the armature voltage V it needs.

k = 1 V·s/radR_a = 1.5 ΩT = 4 N·mn = 400 rpm

V = V

6.Hard

A DC chopper feeds a motor (k = 0.8 V·s/rad, R_a = 0.8 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 300 rpm with a load of 20 N·m.

V_dc = 240 Vk = 0.8 V·s/radR_a = 0.8 ΩT = 20 N·mn = 300 rpm

D = %

Answer key

1. I_start = 146.7 A
2. E = 208 V
3. n₀ = 916.7 rpm
4. η = 90 %
5. V = 47.89 V
6. D = 18.81 %

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