5 Hp, 230 V shunt motor takes 18 Amperes when operating at full load. The shunt field resistance is 115 ohms and the armature resistance is 0.25 ohm. Calculate the starter resistance if the starting torque is limited to 1.5 times the full load torque (take a 3-V brush drop)

Question

5 Hp, 230 V shunt motor takes 18 Amperes when operating at full load. The shunt field resistance is 115 ohms and the armature resistance is 0.25 ohm. Calculate the starter resistance if the starting torque is limited to 1.5 times the full load torque (take a 3-V brush drop)

Answer 1

To calculate the starter resistance, we first need to determine the full load torque of the motor.

The formula for calculating torque is:

Torque = (Armature Current * Armature Resistance) / (Shunt Field Current * Shunt Field Resistance)

Given:
- Horsepower (Hp) = 5
- Voltage (V) = 230
- Full load current (I) = 18 A
- Shunt field resistance (Rf) = 115 ohms
- Armature resistance (Ra) = 0.25 ohm

To calculate the full load torque, we need to convert the horsepower to watts:

Power (W) = Horsepower * 746

Now, we can calculate the full load torque:

Full Load Torque (T) = (Power * 1000) / (2 * pi * Speed)

Assuming the speed is 1200 RPM for a typical 5 Hp motor, the value of Speed is 1200.

Next, we can calculate the armature current and the shunt field current:

Armature Current (Ia) = Full Load Current - Shunt Field Current
Shunt Field Current (If) = Full Load Current - Armature Current

Substituting the given values:

Ia = 18 A - If
If = 18 A - Ia

Now, let's calculate the full load torque value:

T = (Power * 1000) / (2 * pi * Speed)

Substitute the values and convert the horsepower:

T = (5 * 746 * 1000) / (2 * pi * 1200)

Now, we can calculate the starter torque, given that the starting torque is limited to 1.5 times the full load torque:

Starter Torque = 1.5 * Full Load Torque

Once we have the starter torque, we can calculate the starter current using the formula:

Starter Current = (Armature Current * Starter Resistance) / (Armature Resistance + Starter Resistance)

Considering that there is a 3-V brush drop, we adjust the applied voltage:

Applied Voltage = Voltage - 3

Next, we can calculate the starter resistance:

Starter Resistance = (Applied Voltage - Starter Current * Armature Resistance) / Starter Current

By substituting the values into the formula, we can calculate the starter resistance.

However, please note that without the specific values for power, speed, and torque, the exact calculations cannot be provided. Please provide the necessary information to obtain accurate results.