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 = 0.15 V·s/rad and an armature resistance of 0.4 Ω runs from 48 V and draws 15 A. Find its speed n in rpm.

V = 48 VR_a = 0.4 ΩI_a = 15 Ak = 0.15 V·s/rad

n = rpm

2.Easy

A permanent-magnet DC motor has the torque constant k = 2 N·m/A. Find the torque T it develops at an armature current of 6 A.

k = 2 N·m/AI_a = 6 A

T = N·m

3.Medium

A DC motor delivers 0.5 kW of mechanical power at 1500 rpm. Find the shaft torque T.

P = 0.5 kWn = 1500 rpm

T = N·m

4.Medium

A DC motor draws 15 A from a 48 V supply. Its armature resistance is 0.2 Ω and its rotational (friction, windage and iron) losses are 41 W. Find its efficiency η.

V = 48 VI_a = 15 AR_a = 0.2 ΩP_rot = 41 W

η = %

5.Hard

A DC chopper feeds a motor (k = 0.12 V·s/rad, R_a = 0.25 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 1500 rpm with a load of 0.96 N·m.

V_dc = 220 Vk = 0.12 V·s/radR_a = 0.25 ΩT = 0.96 N·mn = 1500 rpm

D = %

6.Hard

A separately excited DC motor (k = 0.3 V·s/rad at full field, R_a = 2 Ω, V = 110 V) drives a constant load torque of 1.8 N·m. The field flux is reduced to 75 % of its full value. Find the new speed n.

V = 110 Vk = 0.3 V·s/radR_a = 2 ΩT = 1.8 N·mΦ = 75 %

n = rpm

Answer key

1. n = 2674 rpm
2. T = 12 N·m
3. T = 3.183 N·m
4. η = 88.06 %
5. D = 9.477 %
6. n = 3989 rpm

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