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 k = 0.75 V·s/rad and an armature resistance of 0.5 Ω runs from 240 V and draws 25 A. Find its speed n in rpm.

V = 240 VR_a = 0.5 ΩI_a = 25 Ak = 0.75 V·s/rad

n = rpm

2.Easy

A DC motor on a 110 V supply has an armature resistance of 0.25 Ω. At its present speed the back-EMF is 100 V. Find the armature current I_a.

V = 110 VE = 100 VR_a = 0.25 Ω

I_a = A

3.Medium

A DC motor draws 2 A from a 24 V supply. Its armature resistance is 1.5 Ω and its rotational (friction, windage and iron) losses are 2 W. Find its efficiency η.

V = 24 VI_a = 2 AR_a = 1.5 ΩP_rot = 2 W

η = %

4.Medium

A DC motor (k = 0.08 N·m/A, R_a = 0.4 Ω) runs from 24 V and drives a load torque of 0.32 N·m. Find its speed n in rpm.

V = 24 Vk = 0.08 N·m/AR_a = 0.4 ΩT = 0.32 N·m

n = rpm

5.Hard

A separately excited DC motor (k = 0.8 V·s/rad at full field, R_a = 0.25 Ω, V = 220 V) drives a constant load torque of 24 N·m. The field flux is reduced to 85 % of its full value. Find the new speed n.

V = 220 Vk = 0.8 V·s/radR_a = 0.25 ΩT = 24 N·mΦ = 85 %

n = rpm

6.Hard

A DC chopper feeds a motor (k = 1 V·s/rad, R_a = 2 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 500 rpm with a load of 6 N·m.

V_dc = 240 Vk = 1 V·s/radR_a = 2 ΩT = 6 N·mn = 500 rpm

D = %

Answer key

1. n = 2897 rpm
2. I_a = 40 A
3. η = 83.33 %
4. n = 2674 rpm
5. n = 2966 rpm
6. D = 26.82 %

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