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 k = 0.2 V·s/rad and an armature resistance of 0.2 Ω runs from 48 V and draws 30 A. Find its speed n in rpm.

V = 48 VR_a = 0.2 ΩI_a = 30 Ak = 0.2 V·s/rad

n = rpm

2.Easy

A DC motor with an armature resistance of 0.4 Ω is switched directly onto a 220 V supply. Its normal full-load current is 20 A. Find the armature current I_start at the moment of starting.

V = 220 VR_a = 0.4 Ω

I_start = A

3.Medium

A DC motor delivers 1.5 kW of mechanical power at 3000 rpm. Find the shaft torque T.

P = 1.5 kWn = 3000 rpm

T = N·m

4.Medium

A DC motor (k = 0.3 V·s/rad, R_a = 0.4 Ω) runs from 48 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 = 48 Vk = 0.3 V·s/radR_a = 0.4 ΩI_FL = 15 A

SR = %

5.Hard

A separately excited DC motor (k = 2 V·s/rad at full field, R_a = 2 Ω, V = 240 V) drives a constant load torque of 10 N·m. The field flux is reduced to 70 % of its full value. Find the new speed n.

V = 240 Vk = 2 V·s/radR_a = 2 ΩT = 10 N·mΦ = 70 %

n = rpm

6.Hard

A small DC motor has an armature resistance of 1.2 Ω and runs from 120 V. Neglecting rotational losses, find the largest mechanical power P_max it can develop at any speed.

V = 120 VR_a = 1.2 Ω

P_max = W

Answer key

1. n = 2005 rpm
2. I_start = 550 A
3. T = 4.775 N·m
4. SR = 14.29 %
5. n = 1540 rpm
6. P_max = 3000 W

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