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 2 Ω is switched directly onto a 240 V supply. Its normal full-load current is 12 A. Find the armature current I_start at the moment of starting.

V = 240 VR_a = 2 Ω

I_start = A

2.Easy

A DC motor turns at 1500 rpm, and its back-EMF at that speed is 216 V. Find the motor constant k in V·s/rad.

E = 216 Vn = 1500 rpm

k = V·s/rad

3.Medium

A DC motor (k = 1.5 V·s/rad, R_a = 0.5 Ω) runs from 240 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 = 240 Vk = 1.5 V·s/radR_a = 0.5 ΩI_FL = 40 A

SR = %

4.Medium

A DC motor (k = 0.3 N·m/A, R_a = 0.2 Ω) runs from 24 V and drives a load torque of 4.5 N·m. Find its speed n in rpm.

V = 24 Vk = 0.3 N·m/AR_a = 0.2 ΩT = 4.5 N·m

n = rpm

5.Hard

A DC chopper feeds a motor (k = 0.8 V·s/rad, R_a = 1.5 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 800 rpm with a load of 8 N·m.

V_dc = 220 Vk = 0.8 V·s/radR_a = 1.5 ΩT = 8 N·mn = 800 rpm

D = %

6.Hard

A DC motor (k = 0.15 V·s/rad, R_a = 0.2 Ω) on a fixed 24 V supply drives a load of 0.9 N·m. Find the series resistance R_s that slows it down to 500 rpm.

V = 24 Vk = 0.15 V·s/radR_a = 0.2 ΩT = 0.9 N·mn = 500 rpm

R_s = Ω

Answer key

1. I_start = 120 A
2. k = 1.375 V·s/rad
3. SR = 9.091 %
4. n = 668.5 rpm
5. D = 37.28 %
6. R_s = 2.491 Ω

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