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 1.5 Ω is switched directly onto a 110 V supply. Its normal full-load current is 4 A. Find the armature current I_start at the moment of starting.

V = 110 VR_a = 1.5 Ω

I_start = A

2.Easy

A DC motor with an armature resistance of 1 Ω is switched directly onto a 220 V supply. Its normal full-load current is 12 A. Find the armature current I_start at the moment of starting.

V = 220 VR_a = 1 Ω

I_start = A

3.Medium

A DC motor (k = 0.6 V·s/rad, R_a = 0.2 Ω) runs from 220 V. At full load it draws 40 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 220 Vk = 0.6 V·s/radR_a = 0.2 ΩI_FL = 40 A

SR = %

4.Medium

A DC motor (k = 2 V·s/rad, R_a = 0.5 Ω) runs from 240 V. At full load it draws 20 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 240 Vk = 2 V·s/radR_a = 0.5 ΩI_FL = 20 A

SR = %

5.Hard

A small DC motor has an armature resistance of 0.5 Ω and runs from 48 V. Neglecting rotational losses, find the largest mechanical power P_max it can develop at any speed.

V = 48 VR_a = 0.5 Ω

P_max = W

6.Hard

A separately excited DC motor on 120 V (R_a = 1 Ω) runs at 1500 rpm with an armature current of 12 A. The load changes and the current becomes 6 A, with the field unchanged. Find the new speed n₂.

V = 120 VR_a = 1 Ωn₁ = 1500 rpmI_a1 = 12 AI_a2 = 6 A

n₂ = rpm

Answer key

1. I_start = 73.33 A
2. I_start = 220 A
3. SR = 3.774 %
4. SR = 4.348 %
5. P_max = 1152 W
6. n₂ = 1583 rpm

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