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 240 V with an armature current of 20 A. The armature resistance is 1 Ω. Find the developed (electromagnetic) power P_dev.

V = 240 VI_a = 20 AR_a = 1 Ω

P_dev = W

2.Easy

A DC motor turns at 1500 rpm, and its back-EMF at that speed is 108 V. Find the motor constant k in V·s/rad.

E = 108 Vn = 1500 rpm

k = V·s/rad

3.Medium

A DC motor (k = 1 V·s/rad, R_a = 0.5 Ω) runs from 110 V. At full load it draws 25 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 110 Vk = 1 V·s/radR_a = 0.5 ΩI_FL = 25 A

SR = %

4.Medium

A DC motor with k = 1 N·m/A and R_a = 0.2 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 = 0.2 Ω

T_stall = N·m

5.Hard

A separately excited DC motor on 220 V (R_a = 1 Ω) runs at 1440 rpm with an armature current of 25 A. The load changes and the current becomes 12.5 A, with the field unchanged. Find the new speed n₂.

V = 220 VR_a = 1 Ωn₁ = 1440 rpmI_a1 = 25 AI_a2 = 12.5 A

n₂ = rpm

6.Hard

A 120 V DC motor has an armature resistance of 0.2 Ω. Find the series starting resistance R_st that limits the starting current to 100 A.

V = 120 VR_a = 0.2 ΩI_max = 100 A

R_st = Ω

Answer key

1. P_dev = 4400 W
2. k = 0.6875 V·s/rad
3. SR = 12.82 %
4. T_stall = 1200 N·m
5. n₂ = 1532 rpm
6. R_st = 1 Ω

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