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 = 2 V·s/rad and an armature resistance of 0.5 Ω runs from 240 V and draws 50 A. Find its speed n in rpm.

V = 240 VR_a = 0.5 ΩI_a = 50 Ak = 2 V·s/rad

n = rpm

2.Easy

A permanent-magnet DC motor has the torque constant k = 0.15 N·m/A. Find the torque T it develops at an armature current of 8 A.

k = 0.15 N·m/AI_a = 8 A

T = N·m

3.Medium

A DC motor (k = 0.6 V·s/rad, R_a = 1 Ω) runs from 48 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 = 48 Vk = 0.6 V·s/radR_a = 1 ΩI_FL = 6 A

SR = %

4.Medium

A DC motor with k = 0.3 N·m/A and R_a = 0.4 Ω is connected to 48 V. Find its stall torque T_stall, the torque at zero speed.

V = 48 Vk = 0.3 N·m/AR_a = 0.4 Ω

T_stall = N·m

5.Hard

A DC motor (k = 1.5 V·s/rad, R_a = 1.2 Ω) on a fixed 120 V supply drives a load of 6 N·m. Find the series resistance R_s that slows it down to 600 rpm.

V = 120 Vk = 1.5 V·s/radR_a = 1.2 ΩT = 6 N·mn = 600 rpm

R_s = Ω

6.Hard

A separately excited DC motor (k = 1.5 V·s/rad at full field, R_a = 0.25 Ω, V = 240 V) drives a constant load torque of 60 N·m. The field flux is reduced to 80 % of its full value. Find the new speed n.

V = 240 Vk = 1.5 V·s/radR_a = 0.25 ΩT = 60 N·mΦ = 80 %

n = rpm

Answer key

1. n = 1027 rpm
2. T = 1.2 N·m
3. SR = 14.29 %
4. T_stall = 36 N·m
5. R_s = 5.238 Ω
6. n = 1810 rpm

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