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 k = 1.5 V·s/rad and an armature resistance of 0.25 Ω runs from 240 V and draws 40 A. Find its speed n in rpm.

V = 240 VR_a = 0.25 ΩI_a = 40 Ak = 1.5 V·s/rad

n = rpm

2.Easy

A DC motor runs from 220 V with an armature current of 6 A. The armature resistance is 1.5 Ω. Find the developed (electromagnetic) power P_dev.

V = 220 VI_a = 6 AR_a = 1.5 Ω

P_dev = W

3.Medium

A DC motor draws 20 A from a 110 V supply. Its armature resistance is 0.2 Ω and its rotational (friction, windage and iron) losses are 64 W. Find its efficiency η.

V = 110 VI_a = 20 AR_a = 0.2 ΩP_rot = 64 W

η = %

4.Medium

A DC motor with k = 0.8 N·m/A and R_a = 0.4 Ω is connected to 110 V. Find its stall torque T_stall, the torque at zero speed.

V = 110 Vk = 0.8 N·m/AR_a = 0.4 Ω

T_stall = N·m

5.Hard

A separately excited DC motor on 240 V (R_a = 1 Ω) runs at 900 rpm with an armature current of 12 A. The load changes and the current becomes 4.8 A, with the field unchanged. Find the new speed n₂.

V = 240 VR_a = 1 Ωn₁ = 900 rpmI_a1 = 12 AI_a2 = 4.8 A

n₂ = rpm

6.Hard

A DC chopper feeds a motor (k = 0.75 V·s/rad, R_a = 2 Ω) from a 220 V DC bus. The average armature voltage is D·220 V. Find the duty cycle D that runs the motor at 1200 rpm with a load of 3 N·m.

V_dc = 220 Vk = 0.75 V·s/radR_a = 2 ΩT = 3 N·mn = 1200 rpm

D = %

Answer key

1. n = 1464 rpm
2. P_dev = 1266 W
3. η = 93.45 %
4. T_stall = 220 N·m
5. n₂ = 928.4 rpm
6. D = 46.48 %

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