Locked Rotor Current Calculator

Full Load Current (A):

Service Factor:

Starting Current Multiplier:

Inrush current, known as “locked rotor current,” is a surge of current when a motor starts. This happens for a brief moment after turning on the motor. It’s called “locked rotor” because it’s like the amount needed to turn a motor’s shaft from a standstill. When a motor overloads, nearly reaching a stop, it also draws a lot of current. The protective devices must handle this high but short-lived current spike without sacrificing their job of guarding against other issues, like short circuits.

Key Takeaways

Inrush current, also called locked rotor current, can reach levels 20 times greater than the normal current levels during motor startup.
After the initial inrush of current, the motor’s current subsides to a level of 4-8 times the normal running current.
Motor branch circuits can experience inrush currents for a few seconds at startup, hence needing overcurrent devices that can withstand this.
Inverse-time circuit-breakers and time-delay fuses are designed to endure inrush currents during motor startup, providing protection against short circuits and ignoring the overload for a short period.
The increase in fuse or circuit-breaker ampacity allowed by Table 430.52 helps maintain the circuit during the initial inrush current as it subsides and reaches a normal operating level.

What is Locked Rotor Current?

When an AC motor first turns on, it draws a lot more current than usual. This happens even when the motor is not running, which is why it’s called locked rotor current.

This surge of current is necessary because the motor needs a big push to overcome the static resistance. This push is what gets the motor’s shaft moving.

Inrush Current During Motor Startup

At the start, the current can be as high as 20 times more than usual. But once the motor starts rotating, this extra high current quickly drops. It then stabilizes at about 4 – 8 times what’s needed for normal operation.

After a moment, this starting current goes back to normal. This is when the motor has reached its full speed.

Excessive Current Draw at Motor Energization

When a motor is starting with a Direct-On Line method, it draws this initial locked rotor current. How much current it draws depends on the voltage, how it’s started, and the equipment it’s connected to.

Overcoming Static Shaft Resistance

The time the locked rotor current lasts depends on what the motor is connected to. For example, pumps might draw it for 5 seconds; fans for 10-30 seconds.

This high current, needed to get the motor started, can be more than 10 times the full load current. It mainly happens in the first half-cycle of starting when the motor’s rotor has almost zero speed. This lasts for a short time, 10-30 milliseconds, a bit longer with bigger equipment.

Locked Rotor Current and Motor Components

The induction motor stator and induction motor rotor have different magnetic fields. This difference leads to a big inrush current at startup. As the rotor begins turning, it catches up with the stator’s magnetic field. This reduces the difference between the magnetic fields. Thus, the inrush current also decreases.

Stator and Rotor Winding Interactions

A standard AC induction motor always has some amount of slip. This means the two magnetic fields never fully align. The rotor always lags the stator’s magnetic field at startup to a certain extent. The motor’s “slip” is described as a percentage. The final torque from the shaft is the outcome of the magnetic force, adjusting for this slip.

Magnetic Field Differences at Startup

At startup, an electric motor draws about 5 times its regular current for the first half second. This high initial current surge is called the locked rotor current. It happens because there’s almost no back EMF at first. This allows a current flow like a three-phase short circuit. This inrush current lasts 10-30 milliseconds. It might last a bit longer because of the equipment’s inertia.

Protecting Motor Circuits from Locked Rotor Current

The National Electrical Code asks for extra safety for motor systems. It needs protection for the motor’s feeder and branch circuits. It also needs overload protection to avoid damage to motors. Each phase of the motor has its current flow checked carefully.

NEC Requirements for Motor Protection

Breakers with a trip-curve feature can handle very high current for a short time before tripping. This delay helps keep the motor circuit safe. It can endure high current for a bit, which is common at motor startup, without causing trouble.

Inverse Time Circuit Breakers and Time-Delay Fuses

Devices like breakers and fuses have built-in delays for this reason. They also have a bigger size limit allowed. This combo lets motor circuits handle the huge current surges when motors begin running.

Locked Rotor Current and Motor Sizing

During startup, motors can draw a lot of current. To cope with this, overcurrent devices can be much larger, up to 225%-400%. This is shown in Table 430.52. Fuses have tighter rules on being made larger than this table. But, electricians can make circuit-breakers up to 400% bigger for small loads. For bigger loads, they can go up to 300% larger. This is to deal with the big current spike when a motor starts. Inverse time circuit-breakers and time-delay fuses are key in handling this initial current surge.

Calculating and Estimating Locked Rotor Current

Figuring out the locked rotor current (LRC) of a motor is usually simple. Check the motor’s nameplate first. It often shows the motor nameplate locked rotor amperes (LRA). This tells you the motor’s starting current needs.

Nameplate Locked Rotor Amperes (LRA)

But, sometimes the LRA isn’t directly given on the motor’s nameplate. In these situations, look for the kVA Code Letter, or “Code.” This code helps you estimate the starting kVA and LRA. The NEC® 2023 Table 430.7(B) lists a range of kVA/HP for each Code. This range shows the relationship between starting kVA and HP.

kVA Code Letter and LRA Estimation

For motors with no clear LRA, here’s the equation to estimate it: LRA=1000*(kVA/HP)/Voltage. This computes a close LRA figure based on the motor’s kVA Code and HP size.

Generator Sizing for Motor Loads

Matching a generator size to motor loads is vital. Pick the LRA at the low startup voltage. This is because the generator’s output can drop because of the motor’s initial high current. Use the NEC’s high end of the kVA/HP range to size your generator right. This ensures the generator can meet the motor’s startup demands.

Conclusion

Locked rotor current, or inrush current, is the extra electric current an induction motor uses at startup when the rotor isn’t moving. This current can go up to 20 times the usual running current of the motor. It’s a key factor in motor circuit design and protective device settings.

It’s vital to correctly figure out the locked rotor current. This helps in picking the best circuit breakers, fuses, and generators to safeguard the motor gear. It ensures the motors work well.locked rotor current summary and motor starting current protection play a big role in keeping motor systems safe.

It’s important to remember that LRA (Locked Rotor Amps) is at least 6 times the FLA (Full Load Amps). Also, the length of cords and its effect on LRA and voltage drop is crucial to check. Don’t forget to consider the issues that can come with generators having lower amp rates. Checking all these ensures the motor is protected and works reliably.

FAQ

What is inrush current or locked rotor current?

Inrush current is a surge of electricity that happens right when a motor starts. It can be so high that it’s like the motor is stuck, unable to move. We call this “locked rotor current.” This high current lasts only for a short time.

How much higher is the inrush current compared to normal running current?

At the start, a motor can draw in 20 times more electricity than when it runs normally.

What causes the high inrush current at motor startup?

The main cause is the difference in how the stator and rotor windings’ magnetic fields react. When the motor starts, the rotor tries to catch up with the stator. This difference causes the high initial inrush current. Once the rotation is stable, the inrush current goes down.

How does the National Electrical Code (NEC) address protection against inrush current?

The NEC has rules to keep motors safe from inrush currents. These rules cover the motor’s feeder circuit, its branch-circuit, and overload protection. They ensure that the current is safe in every part of the motor’s electrical system.

How can the inrush current be estimated or calculated?

Look for the motor’s LRA on its nameplate. If you can’t find it, check for the Code Letter indicating the starting kVA and motor HP ratio. Using this Code, you can guess the LRA and starting kVA.

Locked Rotor Current Calculator