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 240 V supply and draws an armature current of 40 A. Its armature resistance is 0.25 Ω. Find the back-EMF E.

V = 240 VI_a = 40 AR_a = 0.25 Ω

E = V

2.Easy

A DC motor turns at 900 rpm, and its back-EMF at that speed is 23 V. Find the motor constant k in V·s/rad.

E = 23 Vn = 900 rpm

k = V·s/rad

3.Medium

A DC motor draws 4 A from a 120 V supply. Its armature resistance is 2 Ω and its rotational (friction, windage and iron) losses are 27 W. Find its efficiency η.

V = 120 VI_a = 4 AR_a = 2 ΩP_rot = 27 W

η = %

4.Medium

A DC motor draws 10 A from a 110 V supply. Its armature resistance is 1.5 Ω and its rotational (friction, windage and iron) losses are 29 W. Find its efficiency η.

V = 110 VI_a = 10 AR_a = 1.5 ΩP_rot = 29 W

η = %

5.Hard

A DC motor with k = 1.2 V·s/rad and R_a = 0.2 Ω must drive a load of 48 N·m at 500 rpm. Find the armature voltage V it needs.

k = 1.2 V·s/radR_a = 0.2 ΩT = 48 N·mn = 500 rpm

V = V

6.Hard

A separately excited DC motor on 120 V (R_a = 0.25 Ω) runs at 1440 rpm with an armature current of 50 A. The load changes and the current becomes 75 A, with the field unchanged. Find the new speed n₂.

V = 120 VR_a = 0.25 Ωn₁ = 1440 rpmI_a1 = 50 AI_a2 = 75 A

n₂ = rpm

Answer key

1. E = 230 V
2. k = 0.244 V·s/rad
3. η = 87.71 %
4. η = 83.73 %
5. V = 70.83 V
6. n₂ = 1356 rpm

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