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 on a 220 V supply has an armature resistance of 1.5 Ω. At its present speed the back-EMF is 205 V. Find the armature current I_a.

V = 220 VE = 205 VR_a = 1.5 Ω

I_a = A

2.Easy

A DC motor runs from a 220 V supply and draws an armature current of 50 A. Its armature resistance is 0.2 Ω. Find the back-EMF E.

V = 220 VI_a = 50 AR_a = 0.2 Ω

E = V

3.Medium

A DC motor draws 4 A from a 48 V supply. Its armature resistance is 0.8 Ω and its rotational (friction, windage and iron) losses are 7 W. Find its efficiency η.

V = 48 VI_a = 4 AR_a = 0.8 ΩP_rot = 7 W

η = %

4.Medium

A DC motor draws 10 A from a 48 V supply. Its armature resistance is 0.4 Ω and its rotational (friction, windage and iron) losses are 26 W. Find its efficiency η.

V = 48 VI_a = 10 AR_a = 0.4 ΩP_rot = 26 W

η = %

5.Hard

A small DC motor has an armature resistance of 0.8 Ω and runs from 110 V. Neglecting rotational losses, find the largest mechanical power P_max it can develop at any speed.

V = 110 VR_a = 0.8 Ω

P_max = W

6.Hard

A separately excited DC motor on 120 V (R_a = 0.25 Ω) runs at 900 rpm with an armature current of 50 A. The load changes and the current becomes 12.5 A, with the field unchanged. Find the new speed n₂.

V = 120 VR_a = 0.25 Ωn₁ = 900 rpmI_a1 = 50 AI_a2 = 12.5 A

n₂ = rpm

Answer key

1. I_a = 10 A
2. E = 210 V
3. η = 89.69 %
4. η = 86.25 %
5. P_max = 3781 W
6. n₂ = 978.5 rpm

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