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 runs from a 120 V supply and draws an armature current of 20 A. Its armature resistance is 0.8 Ω. Find the back-EMF E.

V = 120 VI_a = 20 AR_a = 0.8 Ω

E = V

2.Easy

A DC motor with an armature resistance of 0.8 Ω is switched directly onto a 220 V supply. Its normal full-load current is 10 A. Find the armature current I_start at the moment of starting.

V = 220 VR_a = 0.8 Ω

I_start = A

3.Medium

A DC motor with k = 1 N·m/A and R_a = 1.2 Ω is connected to 240 V. Find its stall torque T_stall, the torque at zero speed.

V = 240 Vk = 1 N·m/AR_a = 1.2 Ω

T_stall = N·m

4.Medium

A DC motor draws 40 A from a 110 V supply. Its armature resistance is 0.25 Ω and its rotational (friction, windage and iron) losses are 320 W. Find its efficiency η.

V = 110 VI_a = 40 AR_a = 0.25 ΩP_rot = 320 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 45 N·m at 600 rpm. Find the armature voltage V it needs.

k = 1.5 V·s/radR_a = 0.5 ΩT = 45 N·mn = 600 rpm

V = V

6.Hard

A separately excited DC motor (k = 1 V·s/rad at full field, R_a = 0.25 Ω, V = 220 V) drives a constant load torque of 40 N·m. The field flux is reduced to 80 % of its full value. Find the new speed n.

V = 220 Vk = 1 V·s/radR_a = 0.25 ΩT = 40 N·mΦ = 80 %

n = rpm

Answer key

1. E = 104 V
2. I_start = 275 A
3. T_stall = 200 N·m
4. η = 83.64 %
5. V = 109.2 V
6. n = 2477 rpm

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