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 turns at 750 rpm, and its back-EMF at that speed is 114 V. Find the motor constant k in V·s/rad.

E = 114 Vn = 750 rpm

k = V·s/rad

2.Easy

A DC motor with k = 0.6 V·s/rad and an armature resistance of 0.5 Ω runs from 220 V and draws 50 A. Find its speed n in rpm.

V = 220 VR_a = 0.5 ΩI_a = 50 Ak = 0.6 V·s/rad

n = rpm

3.Medium

A DC motor (k = 0.75 V·s/rad, R_a = 0.5 Ω) runs from 110 V. At full load it draws 15 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 110 Vk = 0.75 V·s/radR_a = 0.5 ΩI_FL = 15 A

SR = %

4.Medium

A DC motor delivers 15 kW of mechanical power at 750 rpm. Find the shaft torque T.

P = 15 kWn = 750 rpm

T = N·m

5.Hard

A DC motor (k = 0.8 V·s/rad, R_a = 0.4 Ω) on a fixed 120 V supply drives a load of 8 N·m. Find the series resistance R_s that slows it down to 500 rpm.

V = 120 Vk = 0.8 V·s/radR_a = 0.4 ΩT = 8 N·mn = 500 rpm

R_s = Ω

6.Hard

A DC chopper feeds a motor (k = 1.5 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 15 N·m.

V_dc = 220 Vk = 1.5 V·s/radR_a = 1.2 ΩT = 15 N·mn = 300 rpm

D = %

Answer key

1. k = 1.451 V·s/rad
2. n = 3104 rpm
3. SR = 7.317 %
4. T = 191 N·m
5. R_s = 7.411 Ω
6. D = 26.87 %

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