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 110 V supply and draws an armature current of 5 A. Its armature resistance is 0.8 Ω. Find the back-EMF E.

V = 110 VI_a = 5 AR_a = 0.8 Ω

E = V

2.Easy

A permanent-magnet DC motor has the torque constant k = 0.75 N·m/A. Find the torque T it develops at an armature current of 8 A.

k = 0.75 N·m/AI_a = 8 A

T = N·m

3.Medium

A DC motor (k = 0.5 V·s/rad, R_a = 0.4 Ω) runs from 48 V. At full load it draws 15 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 48 Vk = 0.5 V·s/radR_a = 0.4 ΩI_FL = 15 A

SR = %

4.Medium

A DC motor with k = 0.12 N·m/A and R_a = 1.2 Ω is connected to 48 V. Find its stall torque T_stall, the torque at zero speed.

V = 48 Vk = 0.12 N·m/AR_a = 1.2 Ω

T_stall = N·m

5.Hard

A small DC motor has an armature resistance of 2.5 Ω and runs from 24 V. Neglecting rotational losses, find the largest mechanical power P_max it can develop at any speed.

V = 24 VR_a = 2.5 Ω

P_max = W

6.Hard

A separately excited DC motor on 110 V (R_a = 0.2 Ω) runs at 1000 rpm with an armature current of 30 A. The load changes and the current becomes 15 A, with the field unchanged. Find the new speed n₂.

V = 110 VR_a = 0.2 Ωn₁ = 1000 rpmI_a1 = 30 AI_a2 = 15 A

n₂ = rpm

Answer key

1. E = 106 V
2. T = 6 N·m
3. SR = 14.29 %
4. T_stall = 4.8 N·m
5. P_max = 57.6 W
6. n₂ = 1029 rpm

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