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 turns at 1200 rpm, and its back-EMF at that speed is 208 V. Find the motor constant k in V·s/rad.

E = 208 Vn = 1200 rpm

k = V·s/rad

2.Easy

A DC motor with k = 0.4 V·s/rad and an armature resistance of 0.8 Ω runs from 48 V and draws 2 A. Find its speed n in rpm.

V = 48 VR_a = 0.8 ΩI_a = 2 Ak = 0.4 V·s/rad

n = rpm

3.Medium

A DC motor draws 50 A from a 120 V supply. Its armature resistance is 0.2 Ω and its rotational (friction, windage and iron) losses are 220 W. Find its efficiency η.

V = 120 VI_a = 50 AR_a = 0.2 ΩP_rot = 220 W

η = %

4.Medium

A DC motor (k = 0.12 N·m/A, R_a = 0.4 Ω) runs from 48 V and drives a load torque of 1.8 N·m. Find its speed n in rpm.

V = 48 Vk = 0.12 N·m/AR_a = 0.4 ΩT = 1.8 N·m

n = rpm

5.Hard

A DC motor (k = 0.4 V·s/rad, R_a = 0.8 Ω) on a fixed 48 V supply drives a load of 0.8 N·m. Find the series resistance R_s that slows it down to 750 rpm.

V = 48 Vk = 0.4 V·s/radR_a = 0.8 ΩT = 0.8 N·mn = 750 rpm

R_s = Ω

6.Hard

A DC chopper feeds a motor (k = 1.2 V·s/rad, R_a = 1 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 14.4 N·m.

V_dc = 240 Vk = 1.2 V·s/radR_a = 1 ΩT = 14.4 N·mn = 300 rpm

D = %

Answer key

1. k = 1.655 V·s/rad
2. n = 1108 rpm
3. η = 88 %
4. n = 3342 rpm
5. R_s = 7.492 Ω
6. D = 20.71 %

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