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
+−120 VRa​ = 1.5 ΩIa​ = ?M+−E = 111 VωT

A DC motor on a 120 V supply has an armature resistance of 1.5 Ω. At its present speed the back-EMF is 111 V. Find the armature current Ia.

V = 120 VE = 111 VRa = 1.5 Ω
Ia = A
2.Easy
+−24 VRa​ = 0.2 ΩIa​ = ?M+−E = 23 VωT

A DC motor on a 24 V supply has an armature resistance of 0.2 Ω. At its present speed the back-EMF is 23 V. Find the armature current Ia.

V = 24 VE = 23 VRa = 0.2 Ω
Ia = A
3.Medium
+−VRa​M+−En = 1800 rpmT = ?

A DC motor delivers 11 kW of mechanical power at 1800 rpm. Find the shaft torque T.

P = 11 kWn = 1800 rpm
T = N·m
4.Medium
T (N·m)n (rpm)Tst​ = ?n0​

A DC motor with k = 1 N·m/A and Ra = 1 Ω is connected to 110 V. Find its stall torque Tstall, the torque at zero speed.

V = 110 Vk = 1 N·m/ARa = 1 Ω
Tstall = N·m
5.Hard
+−110 VRa​ = 0.5 ΩM+−EωT

A small DC motor has an armature resistance of 0.5 Ω and runs from 110 V. Neglecting rotational losses, find the largest mechanical power Pmax it can develop at any speed.

V = 110 VRa = 0.5 Ω
Pmax = W
6.Hard
+−240 VD = ?Ra​ = 0.5 ΩM+−En = 500 rpmT = 6.4 N·m

A DC chopper feeds a motor (k = 0.8 V·s/rad, Ra = 0.5 Ω) from a 240 V DC bus. The average armature voltage is D·240 V. Find the duty cycle D that runs the motor at 500 rpm with a load of 6.4 N·m.

Vdc = 240 Vk = 0.8 V·s/radRa = 0.5 ΩT = 6.4 N·mn = 500 rpm
D = %

Answer key

  1. 1. Ia = 6 A
  2. 2. Ia = 5 A
  3. 3. T = 58.36 N·m
  4. 4. Tstall = 110 N·m
  5. 5. Pmax = 6050 W
  6. 6. D = 19.12 %

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