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 an armature resistance of 1 Ω is switched directly onto a 120 V supply. Its normal full-load current is 8 A. Find the armature current I_start at the moment of starting.

V = 120 VR_a = 1 Ω

I_start = A

2.Easy

A DC motor turns at 1800 rpm, and its back-EMF at that speed is 216 V. Find the motor constant k in V·s/rad.

E = 216 Vn = 1800 rpm

k = V·s/rad

3.Medium

A DC motor with k = 0.2 N·m/A and R_a = 0.4 Ω is connected to 48 V. Find its stall torque T_stall, the torque at zero speed.

V = 48 Vk = 0.2 N·m/AR_a = 0.4 Ω

T_stall = N·m

4.Medium

A DC motor (k = 0.3 V·s/rad, R_a = 0.2 Ω) runs from 120 V. At full load it draws 30 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 120 Vk = 0.3 V·s/radR_a = 0.2 ΩI_FL = 30 A

SR = %

5.Hard

A separately excited DC motor (k = 0.4 V·s/rad at full field, R_a = 0.4 Ω, V = 110 V) drives a constant load torque of 12 N·m. The field flux is reduced to 70 % of its full value. Find the new speed n.

V = 110 Vk = 0.4 V·s/radR_a = 0.4 ΩT = 12 N·mΦ = 70 %

n = rpm

6.Hard

A DC motor with k = 1 V·s/rad and R_a = 0.4 Ω must drive a load of 40 N·m at 600 rpm. Find the armature voltage V it needs.

k = 1 V·s/radR_a = 0.4 ΩT = 40 N·mn = 600 rpm

V = V

Answer key

1. I_start = 120 A
2. k = 1.146 V·s/rad
3. T_stall = 24 N·m
4. SR = 5.263 %
5. n = 3167 rpm
6. V = 78.83 V

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