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 on a 48 V supply has an armature resistance of 0.4 Ω. At its present speed the back-EMF is 43.2 V. Find the armature current I_a.

V = 48 VE = 43.2 VR_a = 0.4 Ω

I_a = A

2.Easy

A permanent-magnet DC motor has the torque constant k = 1 N·m/A. Find the torque T it develops at an armature current of 15 A.

k = 1 N·m/AI_a = 15 A

T = N·m

3.Medium

A DC motor draws 5 A from a 110 V supply. Its armature resistance is 2 Ω and its rotational (friction, windage and iron) losses are 40 W. Find its efficiency η.

V = 110 VI_a = 5 AR_a = 2 ΩP_rot = 40 W

η = %

4.Medium

A DC motor delivers 1 kW of mechanical power at 2900 rpm. Find the shaft torque T.

P = 1 kWn = 2900 rpm

T = N·m

5.Hard

A separately excited DC motor (k = 0.2 V·s/rad at full field, R_a = 1.5 Ω, V = 48 V) drives a constant load torque of 0.4 N·m. The field flux is reduced to 80 % of its full value. Find the new speed n.

V = 48 Vk = 0.2 V·s/radR_a = 1.5 ΩT = 0.4 N·mΦ = 80 %

n = rpm

6.Hard

A DC chopper feeds a motor (k = 0.75 V·s/rad, R_a = 1 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 1000 rpm with a load of 9 N·m.

V_dc = 220 Vk = 0.75 V·s/radR_a = 1 ΩT = 9 N·mn = 1000 rpm

D = %

Answer key

1. I_a = 12 A
2. T = 15 N·m
3. η = 83.64 %
4. T = 3.293 N·m
5. n = 2641 rpm
6. D = 41.15 %

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