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 48 V with an armature current of 5 A. The armature resistance is 1 Ω. Find the developed (electromagnetic) power P_dev.

V = 48 VI_a = 5 AR_a = 1 Ω

P_dev = W

2.Easy

A DC motor on a 110 V supply has an armature resistance of 0.8 Ω. At its present speed the back-EMF is 106 V. Find the armature current I_a.

V = 110 VE = 106 VR_a = 0.8 Ω

I_a = A

3.Medium

A DC motor draws 12 A from a 48 V supply. Its armature resistance is 0.4 Ω and its rotational (friction, windage and iron) losses are 31 W. Find its efficiency η.

V = 48 VI_a = 12 AR_a = 0.4 ΩP_rot = 31 W

η = %

4.Medium

A DC motor with k = 1 N·m/A and R_a = 0.4 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 = 0.4 Ω

T_stall = N·m

5.Hard

A 48 V DC motor has an armature resistance of 0.5 Ω. Find the series starting resistance R_st that limits the starting current to 60 A.

V = 48 VR_a = 0.5 ΩI_max = 60 A

R_st = Ω

6.Hard

A DC chopper feeds a motor (k = 0.15 V·s/rad, R_a = 0.4 Ω) from a 48 V DC bus. The average armature voltage is D·48 V. Find the duty cycle D that runs the motor at 900 rpm with a load of 1.8 N·m.

V_dc = 48 Vk = 0.15 V·s/radR_a = 0.4 ΩT = 1.8 N·mn = 900 rpm

D = %

Answer key

1. P_dev = 215 W
2. I_a = 5 A
3. η = 84.62 %
4. T_stall = 600 N·m
5. R_st = 0.3 Ω
6. D = 39.45 %

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