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 0.5 Ω is switched directly onto a 110 V supply. Its normal full-load current is 8 A. Find the armature current I_start at the moment of starting.

V = 110 VR_a = 0.5 Ω

I_start = A

2.Easy

A DC motor runs from a 220 V supply and draws an armature current of 20 A. Its armature resistance is 0.4 Ω. Find the back-EMF E.

V = 220 VI_a = 20 AR_a = 0.4 Ω

E = V

3.Medium

A DC motor draws 30 A from a 220 V supply. Its armature resistance is 1 Ω and its rotational (friction, windage and iron) losses are 285 W. Find its efficiency η.

V = 220 VI_a = 30 AR_a = 1 ΩP_rot = 285 W

η = %

4.Medium

A DC motor (k = 1.2 N·m/A, R_a = 0.8 Ω) runs from 220 V and drives a load torque of 14.4 N·m. Find its speed n in rpm.

V = 220 Vk = 1.2 N·m/AR_a = 0.8 ΩT = 14.4 N·m

n = rpm

5.Hard

A small DC motor has an armature resistance of 2 Ω and runs from 24 V. Neglecting rotational losses, find the largest mechanical power P_max it can develop at any speed.

V = 24 VR_a = 2 Ω

P_max = W

6.Hard

A DC motor (k = 0.8 V·s/rad, R_a = 0.5 Ω) on a fixed 110 V supply drives a load of 6.4 N·m. Find the series resistance R_s that slows it down to 1000 rpm.

V = 110 Vk = 0.8 V·s/radR_a = 0.5 ΩT = 6.4 N·mn = 1000 rpm

R_s = Ω

Answer key

1. I_start = 220 A
2. E = 212 V
3. η = 82.05 %
4. n = 1674 rpm
5. P_max = 72 W
6. R_s = 2.778 Ω

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