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 2 Ω is switched directly onto a 220 V supply. Its normal full-load current is 4 A. Find the armature current I_start at the moment of starting.

V = 220 VR_a = 2 Ω

I_start = A

2.Easy

A DC motor runs from 240 V with an armature current of 20 A. The armature resistance is 1 Ω. Find the developed (electromagnetic) power P_dev.

V = 240 VI_a = 20 AR_a = 1 Ω

P_dev = W

3.Medium

A DC motor (k = 2 N·m/A, R_a = 0.4 Ω) runs from 240 V and drives a load torque of 50 N·m. Find its speed n in rpm.

V = 240 Vk = 2 N·m/AR_a = 0.4 ΩT = 50 N·m

n = rpm

4.Medium

A DC motor draws 50 A from a 120 V supply. Its armature resistance is 0.25 Ω and its rotational (friction, windage and iron) losses are 269 W. Find its efficiency η.

V = 120 VI_a = 50 AR_a = 0.25 ΩP_rot = 269 W

η = %

5.Hard

A DC motor with k = 0.4 V·s/rad and R_a = 1.5 Ω must drive a load of 2 N·m at 1000 rpm. Find the armature voltage V it needs.

k = 0.4 V·s/radR_a = 1.5 ΩT = 2 N·mn = 1000 rpm

V = V

6.Hard

A separately excited DC motor (k = 0.8 V·s/rad at full field, R_a = 2 Ω, V = 120 V) drives a constant load torque of 4.8 N·m. The field flux is reduced to 85 % of its full value. Find the new speed n.

V = 120 Vk = 0.8 V·s/radR_a = 2 ΩT = 4.8 N·mΦ = 85 %

n = rpm

Answer key

1. I_start = 110 A
2. P_dev = 4400 W
3. n = 1098 rpm
4. η = 85.1 %
5. V = 49.39 V
6. n = 1487 rpm

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