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 0.4 Ω is switched directly onto a 240 V supply. Its normal full-load current is 30 A. Find the armature current I_start at the moment of starting.

V = 240 VR_a = 0.4 Ω

I_start = A

2.Easy

A permanent-magnet DC motor has the torque constant k = 0.05 N·m/A. Find the torque T it develops at an armature current of 8 A.

k = 0.05 N·m/AI_a = 8 A

T = N·m

3.Medium

A DC motor delivers 3 kW of mechanical power at 1450 rpm. Find the shaft torque T.

P = 3 kWn = 1450 rpm

T = N·m

4.Medium

A DC motor with k = 0.12 N·m/A and R_a = 0.2 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 = 0.2 Ω

T_stall = N·m

5.Hard

A separately excited DC motor (k = 0.6 V·s/rad at full field, R_a = 1 Ω, V = 120 V) drives a constant load torque of 4.8 N·m. The field flux is reduced to 90 % of its full value. Find the new speed n.

V = 120 Vk = 0.6 V·s/radR_a = 1 ΩT = 4.8 N·mΦ = 90 %

n = rpm

6.Hard

A DC chopper feeds a motor (k = 1 V·s/rad, R_a = 1 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 500 rpm with a load of 5 N·m.

V_dc = 220 Vk = 1 V·s/radR_a = 1 ΩT = 5 N·mn = 500 rpm

D = %

Answer key

1. I_start = 600 A
2. T = 0.4 N·m
3. T = 19.76 N·m
4. T_stall = 14.4 N·m
5. n = 1965 rpm
6. D = 26.07 %

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