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 Ω is switched directly onto a 110 V supply. Its normal full-load current is 12 A. Find the armature current I_start at the moment of starting.

V = 110 VR_a = 1 Ω

I_start = A

2.Easy

A DC motor on a 110 V supply has an armature resistance of 0.5 Ω. At its present speed the back-EMF is 102.5 V. Find the armature current I_a.

V = 110 VE = 102.5 VR_a = 0.5 Ω

I_a = A

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 5 W. Find its efficiency η.

V = 48 VI_a = 4 AR_a = 0.8 ΩP_rot = 5 W

η = %

4.Medium

A DC motor draws 5 A from a 120 V supply. Its armature resistance is 2 Ω and its rotational (friction, windage and iron) losses are 44 W. Find its efficiency η.

V = 120 VI_a = 5 AR_a = 2 ΩP_rot = 44 W

η = %

5.Hard

A DC motor with k = 1.5 V·s/rad and R_a = 0.5 Ω must drive a load of 37.5 N·m at 300 rpm. Find the armature voltage V it needs.

k = 1.5 V·s/radR_a = 0.5 ΩT = 37.5 N·mn = 300 rpm

V = V

6.Hard

A 48 V DC motor has an armature resistance of 1.2 Ω. Find the series starting resistance R_st that limits the starting current to 30 A.

V = 48 VR_a = 1.2 ΩI_max = 30 A

R_st = Ω

Answer key

1. I_start = 110 A
2. I_a = 15 A
3. η = 90.73 %
4. η = 84.33 %
5. V = 59.62 V
6. R_st = 0.4 Ω

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