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 on a 120 V supply has an armature resistance of 0.2 Ω. At its present speed the back-EMF is 116 V. Find the armature current I_a.

V = 120 VE = 116 VR_a = 0.2 Ω

I_a = A

2.Easy

A DC motor turns at 2400 rpm, and its back-EMF at that speed is 208 V. Find the motor constant k in V·s/rad.

E = 208 Vn = 2400 rpm

k = V·s/rad

3.Medium

A DC motor draws 30 A from a 120 V supply. Its armature resistance is 0.2 Ω and its rotational (friction, windage and iron) losses are 205 W. Find its efficiency η.

V = 120 VI_a = 30 AR_a = 0.2 ΩP_rot = 205 W

η = %

4.Medium

A DC motor draws 20 A from a 220 V supply. Its armature resistance is 0.8 Ω and its rotational (friction, windage and iron) losses are 122 W. Find its efficiency η.

V = 220 VI_a = 20 AR_a = 0.8 ΩP_rot = 122 W

η = %

5.Hard

A separately excited DC motor (k = 0.8 V·s/rad at full field, R_a = 0.2 Ω, V = 240 V) drives a constant load torque of 32 N·m. The field flux is reduced to 80 % of its full value. Find the new speed n.

V = 240 Vk = 0.8 V·s/radR_a = 0.2 ΩT = 32 N·mΦ = 80 %

n = rpm

6.Hard

A separately excited DC motor (k = 0.6 V·s/rad at full field, R_a = 0.8 Ω, V = 48 V) drives a constant load torque of 3 N·m. The field flux is reduced to 70 % of its full value. Find the new speed n.

V = 48 Vk = 0.6 V·s/radR_a = 0.8 ΩT = 3 N·mΦ = 70 %

n = rpm

Answer key

1. I_a = 20 A
2. k = 0.8276 V·s/rad
3. η = 89.31 %
4. η = 89.95 %
5. n = 3432 rpm
6. n = 961.4 rpm

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