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 0.2 Ω is switched directly onto a 48 V supply. Its normal full-load current is 20 A. Find the armature current I_start at the moment of starting.

V = 48 VR_a = 0.2 Ω

I_start = A

2.Easy

A DC motor with k = 0.6 V·s/rad and an armature resistance of 2 Ω runs from 220 V and draws 15 A. Find its speed n in rpm.

V = 220 VR_a = 2 ΩI_a = 15 Ak = 0.6 V·s/rad

n = rpm

3.Medium

A DC motor (k = 0.8 V·s/rad, R_a = 0.25 Ω) runs from 240 V. At full load it draws 30 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 240 Vk = 0.8 V·s/radR_a = 0.25 ΩI_FL = 30 A

SR = %

4.Medium

A DC motor (k = 0.15 N·m/A, R_a = 0.2 Ω) runs from 48 V and drives a load torque of 1.5 N·m. Find its speed n in rpm.

V = 48 Vk = 0.15 N·m/AR_a = 0.2 ΩT = 1.5 N·m

n = rpm

5.Hard

A separately excited DC motor (k = 1.2 V·s/rad at full field, R_a = 1 Ω, V = 240 V) drives a constant load torque of 18 N·m. The field flux is reduced to 80 % of its full value. Find the new speed n.

V = 240 Vk = 1.2 V·s/radR_a = 1 ΩT = 18 N·mΦ = 80 %

n = rpm

6.Hard

A small DC motor has an armature resistance of 2.5 Ω and runs from 110 V. Neglecting rotational losses, find the largest mechanical power P_max it can develop at any speed.

V = 110 VR_a = 2.5 Ω

P_max = W

Answer key

1. I_start = 240 A
2. n = 3024 rpm
3. SR = 3.226 %
4. n = 2928 rpm
5. n = 2201 rpm
6. P_max = 1210 W

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