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 runs from a 110 V supply and draws an armature current of 30 A. Its armature resistance is 0.4 Ω. Find the back-EMF E.

V = 110 VI_a = 30 AR_a = 0.4 Ω

E = V

2.Easy

A DC motor runs from a 240 V supply and draws an armature current of 40 A. Its armature resistance is 0.4 Ω. Find the back-EMF E.

V = 240 VI_a = 40 AR_a = 0.4 Ω

E = V

3.Medium

A DC motor with k = 0.12 N·m/A and R_a = 0.8 Ω is connected to 24 V. Find its stall torque T_stall, the torque at zero speed.

V = 24 Vk = 0.12 N·m/AR_a = 0.8 Ω

T_stall = N·m

4.Medium

A DC motor (k = 1.2 V·s/rad, R_a = 2 Ω) runs from 110 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 = 110 Vk = 1.2 V·s/radR_a = 2 ΩI_FL = 6 A

SR = %

5.Hard

A separately excited DC motor (k = 0.12 V·s/rad at full field, R_a = 0.5 Ω, V = 48 V) drives a constant load torque of 0.48 N·m. The field flux is reduced to 80 % of its full value. Find the new speed n.

V = 48 Vk = 0.12 V·s/radR_a = 0.5 ΩT = 0.48 N·mΦ = 80 %

n = rpm

6.Hard

A 120 V DC motor has an armature resistance of 0.4 Ω. Find the series starting resistance R_st that limits the starting current to 20 A.

V = 120 VR_a = 0.4 ΩI_max = 20 A

R_st = Ω

Answer key

1. E = 98 V
2. E = 224 V
3. T_stall = 3.6 N·m
4. SR = 12.24 %
5. n = 4526 rpm
6. R_st = 5.6 Ω

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