Motor Start Up Current Calculator

Motor Start Up Current Formula:

\[ I = \frac{kVA}{V} \times \frac{1000}{\sqrt{3}} \text{ (for 3-phase)} \]

Start Up Current (I):

Unit Converter ▲

Unit Converter ▼

From:	To:

1. What is Motor Start Up Current?

Motor start up current, also known as inrush current, is the instantaneous high current drawn by a motor when it is first turned on. This current is typically much higher than the normal operating current and can be several times the full-load current rating.

2. How Does the Calculator Work?

The calculator uses the 3-phase motor start up current formula:

\[ I = \frac{kVA}{V} \times \frac{1000}{\sqrt{3}} \]

Where:

\( I \) — Start up current (Amperes)
\( kVA \) — Apparent power rating (kiloVolt-Amperes)
\( V \) — Voltage (Volts)
\( \sqrt{3} \) — Constant for 3-phase systems (approximately 1.732)

Explanation: This formula calculates the initial current surge when a 3-phase motor starts, which is crucial for proper circuit protection and equipment sizing.

3. Importance of Start Up Current Calculation

Details: Accurate start up current calculation is essential for selecting appropriate circuit breakers, fuses, and other protective devices. It helps prevent nuisance tripping while ensuring adequate protection for the motor and electrical system.

4. Using the Calculator

Tips: Enter the motor's kVA rating and operating voltage. Ensure values are positive and in correct units (kVA for apparent power, V for voltage).

5. Frequently Asked Questions (FAQ)

Q1: Why is start up current higher than running current?
A: Start up current is higher due to the initial resistance to rotation and the need to establish magnetic fields in the motor before it reaches operating speed.

Q2: How long does start up current last?
A: Typically, start up current lasts only a few cycles (0.1-0.5 seconds) until the motor reaches approximately 75% of its rated speed.

Q3: Can start up current damage electrical systems?
A: If not properly accounted for, high start up current can cause voltage dips, nuisance tripping of protective devices, and potential damage to motor windings.

Q4: Are there ways to reduce start up current?
A: Yes, methods include soft starters, variable frequency drives (VFDs), star-delta starters, and reduced voltage starting techniques.

Q5: Does this formula work for single-phase motors?
A: No, this formula is specifically for 3-phase motors. Single-phase motors have different start up current characteristics and calculation methods.