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 220 V supply and draws an armature current of 40 A. Its armature resistance is 0.25 Ω. Find the back-EMF E.

V = 220 VI_a = 40 AR_a = 0.25 Ω

E = V

2.Easy

A DC motor turns at 750 rpm, and its back-EMF at that speed is 228 V. Find the motor constant k in V·s/rad.

E = 228 Vn = 750 rpm

k = V·s/rad

3.Medium

A DC motor with k = 0.25 N·m/A and R_a = 0.5 Ω is connected to 48 V. Find its stall torque T_stall, the torque at zero speed.

V = 48 Vk = 0.25 N·m/AR_a = 0.5 Ω

T_stall = N·m

4.Medium

A DC motor (k = 0.5 V·s/rad, R_a = 1.2 Ω) runs from 120 V. At full load it draws 6 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 120 Vk = 0.5 V·s/radR_a = 1.2 ΩI_FL = 6 A

SR = %

5.Hard

A DC motor (k = 0.6 V·s/rad, R_a = 0.2 Ω) on a fixed 120 V supply drives a load of 12 N·m. Find the series resistance R_s that slows it down to 750 rpm.

V = 120 Vk = 0.6 V·s/radR_a = 0.2 ΩT = 12 N·mn = 750 rpm

R_s = Ω

6.Hard

A DC motor (k = 0.5 V·s/rad, R_a = 0.4 Ω) on a fixed 120 V supply drives a load of 12.5 N·m. Find the series resistance R_s that slows it down to 400 rpm.

V = 120 Vk = 0.5 V·s/radR_a = 0.4 ΩT = 12.5 N·mn = 400 rpm

R_s = Ω

Answer key

1. E = 210 V
2. k = 2.903 V·s/rad
3. T_stall = 24 N·m
4. SR = 6.383 %
5. R_s = 3.444 Ω
6. R_s = 3.562 Ω

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