Question: A 250-V, 4-pole, wave-wound d.c. series motor has 782 conductors on itsarmature. It has armature and series field resistance of 0.75 ohm. The motor takesa current of 40 A. Estimate its speed and gross torque developed if it has a flux per pole of 25 mWb Answer: Step 1 Mechanical Engineering homework question answer, step 1, image 1 Mechanical Engineering homework question answer, step 1, image 2 Step 2 Mechanical Engineering homework question answer, step 2, image 1 Step 3 Mechanical Engineering homework question answer, step 3, image 1

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 Given:

A DC shunt motor with armature resistance of 0.12 Ohm is fed with a 220 V supply. It is started using a variable resistance starter and starting characteristics are provided with a minimum current of 162 A and a maximum current of 270 A.

Step 2

We know that at the instant of starting, the back emf induced in the armature is zero. At this instant, if no starter is used, there will be a maximum current in the armature. This maximum current flowing in the armature circuit is given by

Answer (a): The maximum current that can flow through the armature if there is no starting resistance is present is 1833.33 A .

Step 3

When a starter is connected in series of armature circuit, the starting current will be limited. The armature current will be reduced from 1833.33 A to 270 A and as its speed increases, back emf increases. So, the armature current will also be reduced. Here, the minimum armature current flowing in the circuit with the starter is 162 A.

Answer (b): 162 A .

Step 4

From the starting characteristics, the ratio of upper limit and lower limit of current can be written as

The total armature circuit resistance (including starter resistance) at the instant of starting is

The number of starting resistor section (n) can be calculated as

For an integral choice, n = 5.

Answer (c): 5 .

Step 5

At the instant of starting, the total armature resistance (including starting resistance) calculated is 0.8148 Ohm. The required starter resistance is

Answer (d): 0.69 Ohm .