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 110 V supply and draws an armature current of 30 A. Its armature resistance is 0.4 Ω. Find the back-EMF E.

V = 110 VI_a = 30 AR_a = 0.4 Ω

E = V

2.Easy

A DC motor with k = 0.8 V·s/rad and an armature resistance of 0.5 Ω runs from 120 V and draws 25 A. Find its speed n in rpm.

V = 120 VR_a = 0.5 ΩI_a = 25 Ak = 0.8 V·s/rad

n = rpm

3.Medium

A DC motor (k = 0.6 V·s/rad, R_a = 1.2 Ω) runs from 120 V. At full load it draws 10 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 120 Vk = 0.6 V·s/radR_a = 1.2 ΩI_FL = 10 A

SR = %

4.Medium

A DC motor with k = 0.75 N·m/A and R_a = 0.2 Ω is connected to 48 V. Find its stall torque T_stall, the torque at zero speed.

V = 48 Vk = 0.75 N·m/AR_a = 0.2 Ω

T_stall = N·m

5.Hard

A DC motor with k = 1.5 V·s/rad and R_a = 2 Ω must drive a load of 3 N·m at 400 rpm. Find the armature voltage V it needs.

k = 1.5 V·s/radR_a = 2 ΩT = 3 N·mn = 400 rpm

V = V

6.Hard

A DC chopper feeds a motor (k = 2 V·s/rad, R_a = 1.2 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 300 rpm with a load of 20 N·m.

V_dc = 220 Vk = 2 V·s/radR_a = 1.2 ΩT = 20 N·mn = 300 rpm

D = %

Answer key

1. E = 98 V
2. n = 1283 rpm
3. SR = 11.11 %
4. T_stall = 180 N·m
5. V = 66.83 V
6. D = 34.01 %

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