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 1000 rpm, and its back-EMF at that speed is 224 V. Find the motor constant k in V·s/rad.

E = 224 Vn = 1000 rpm

k = V·s/rad

2.Easy

A DC motor on a 110 V supply has an armature resistance of 0.25 Ω. At its present speed the back-EMF is 105 V. Find the armature current I_a.

V = 110 VE = 105 VR_a = 0.25 Ω

I_a = A

3.Medium

A DC motor (k = 0.8 N·m/A, R_a = 1 Ω) runs from 240 V and drives a load torque of 20 N·m. Find its speed n in rpm.

V = 240 Vk = 0.8 N·m/AR_a = 1 ΩT = 20 N·m

n = rpm

4.Medium

A DC motor (k = 0.6 V·s/rad, R_a = 1 Ω) runs from 110 V. At full load it draws 4 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 110 Vk = 0.6 V·s/radR_a = 1 ΩI_FL = 4 A

SR = %

5.Hard

A separately excited DC motor (k = 0.3 V·s/rad at full field, R_a = 1.5 Ω, V = 120 V) drives a constant load torque of 2.4 N·m. The field flux is reduced to 70 % of its full value. Find the new speed n.

V = 120 Vk = 0.3 V·s/radR_a = 1.5 ΩT = 2.4 N·mΦ = 70 %

n = rpm

6.Hard

A DC motor (k = 0.15 V·s/rad, R_a = 0.25 Ω) on a fixed 48 V supply drives a load of 2.25 N·m. Find the series resistance R_s that slows it down to 1500 rpm.

V = 48 Vk = 0.15 V·s/radR_a = 0.25 ΩT = 2.25 N·mn = 1500 rpm

R_s = Ω

Answer key

1. k = 2.139 V·s/rad
2. I_a = 20 A
3. n = 2566 rpm
4. SR = 3.774 %
5. n = 4677 rpm
6. R_s = 1.379 Ω

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