When buying an asynchronous motor, one of the quantities most facilities overlook, yet which causes the most trouble in the field, is the starting current. At the first instant of motion the motor draws a current far above its rated current; this current both stresses the grid and directly affects the choice of fuse, breaker and cable. On nameplates this value is often given as LRA (Locked Rotor Amps) or as the starting current to rated current ratio (Ia/In). In this guide we cover what LRA is, why it is so high and how to reduce it, comparing the methods of direct-on-line (DOL), star-delta, soft starter and VFD. Our aim is to help you correctly select your motor with a starting method that will neither stress your grid nor be unsuited to your application.

Asynchronous motor starting current LRA chart and switchgear panel

What Is LRA (Locked Rotor Amps)?

LRA is the current the motor draws the instant it is connected to the grid while at standstill (with the rotor not yet turning). Because the motor is stopped at that first moment, the rotor is considered locked, and the current drawn in this condition is typically 6–8 times the rated current (In). LRA is therefore the moment of highest current draw. The Ia/In ratio on the nameplate gives the ratio of the motor starting current to the rated current and is a basic reference for fuse, thermal and cable selection. To read the nameplate values correctly, our article on reading the IE3 motor nameplate is a good start.

The starting current is not just an instant; it stays high throughout the time it takes the motor to reach rated speed. If the load inertia is large, this time lengthens and the high current lasts longer, meaning thermal stress in both the motor and the grid. For the start-up time and inertia relationship, our article on impact load motor, flywheel and inertia is valuable.

Why Is the Starting Current So High?

When an asynchronous motor is at standstill, there is a large speed difference (high slip) between the rotor and the stator. In this condition the current induced in the rotor circuit is very high, and accordingly the current the stator draws from the grid also rises. As the motor speeds up, the slip decreases, the induced current falls and the current slowly drops to the rated value. So the high starting current is a natural consequence of the asynchronous motor working principle. Our article on slip and actual speed explains the slip-speed relationship in detail.

The practical effects of high starting current are significant: it can cause voltage drop on the grid (flickering lights, other devices being affected), fuse/breaker tripping, and serious problems especially in facilities fed by a generator. For the starting current problem in generator-fed facilities, our articles on motor selection on a generator-run site and generator kVA - motor kW matching show the way directly.

Effect on the Grid and Switchgear

The high starting current directly determines the choice of protection and switchgear. The fuse and thermal relay must be selected so they do not trip on the starting current but do protect on overload; the cable cross-section is also sized to carry the starting current. For protection device selection, our articles on thermal, relay and fuse selection and purchasing protection devices are core resources.

Starting methods comparison DOL star-delta soft starter VFD

Methods to Reduce Starting Current: DOL vs Star-Delta vs Soft Starter vs VFD

There are several ways to reduce the starting current, and each has its own advantage, disadvantage and suitable application.

Direct-On-Line (DOL)

The simplest and most economical method; the motor is connected to the grid at full voltage. The starting current is the highest (Ia/In ~6–8) but the starting torque is also full. It is preferred in small-power motors and where the grid is strong. In high powers it is generally unsuitable because it stresses the grid.

Star-Delta Starting

The motor is connected in star at start and in delta at speed; this reduces the starting current to roughly one third. However the starting torque also drops to roughly one third, so it is suitable only in applications where the load demands low torque at start (a pump or fan starting unloaded or lightly loaded). For star-delta details, see star-delta or soft starter and, for the wiring diagram, star-delta wiring diagram. For terminal bridging, the guide on terminal connection (230/400V) is also important.

Soft Starter

A soft starter limits the starting current in a controlled way by raising the voltage gradually and smooths the start. It is more flexible and adjustable than star-delta; it reduces mechanical stress and the grid shock. It is common in loads such as conveyors, crushers and pumps. With a crusher example, our article on starting a crusher motor offers a practical comparison.

VFD (Frequency Inverter)

A VFD raises the frequency gradually from zero, both reducing the starting current below the rated current and providing speed adjustment and energy saving. The lowest starting current and the highest control are achieved with a VFD; however its initial investment is higher than the other methods. It is preferred wherever speed adjustment and saving are needed. For VFD selection, our articles on VFD with an asynchronous motor and, for saving, VFD energy savings in pumps and fans show the way.

Choosing the Right Starting Method by Application

Which method is suitable depends on the motor power, the load torque characteristic, the grid strength and the need for speed adjustment. In small-power, simple applications DOL is enough. In medium-power motors starting unloaded, star-delta is an economical solution. In applications starting under load or needing reduced mechanical shock, a soft starter stands out. If speed adjustment, constant pressure or energy saving is needed, a VFD is the most correct choice. To understand the torque characteristic, our articles on torque classes (Design N/H) and, for constant/variable torque, constant torque or variable torque are core references.

Behaviour of the Starting Current by Load Type

How long the starting current stays high depends largely on the load type. A motor starting unloaded or lightly loaded (for example an unloaded fan or a pump with a closed valve) accelerates quickly; the high current is short-lived. By contrast, a motor starting under load (a loaded conveyor, a crusher, a high-inertia mill) accelerates slowly and the high current lasts longer. As this time lengthens, the thermal load on the motor and cable increases; therefore not only the magnitude of the starting current but also its duration matters.

For this reason, when selecting a starting method, both the peak value of the current and the start-up time must be considered. In applications under load and with high inertia, the low torque of star-delta can lengthen the start-up and, paradoxically, increase the thermal load; in this case a soft starter or VFD is the more correct choice. To evaluate the load torque characteristic and start-up behaviour together, our article on motor load ratio and right sizing is useful. In variable-torque loads such as pumps and fans, the starting current is generally more manageable; in constant-torque loads such as conveyors and crushers, the starting plan becomes critical.

Frequent Start-Stop and Motor Life

How many times a day a motor starts and stops is an important parameter for its life. Each start brings a high current and the associated heating; in applications with frequent start-stop, this heat build-up stresses the motor. Therefore in motors that energise very frequently, both the duty type (intermittent duty types instead of S1) and the starting method must be selected carefully. Soft starters and VFDs reduce the stress that frequent start-stop puts on the motor by providing a soft start. For duty type selection, our guide on duty type (S1-S6) selection shows the way.

The thermal stress caused by high starting current can wear the winding insulation over the long term; therefore in frequently starting applications the insulation class and temperature monitoring become important. For temperature monitoring, our articles on protection with PT100 and PTC thermistor and, for early failure causes, electric motor lifespan and early failure causes are valuable. When the right motor, the right starting method and the right protection come together, the high starting current ceases to be a risk and turns into a manageable parameter.

All your asynchronous motor and three-phase motor needs can be planned together with the right starting method. You can review our efficient electric motors product group and the HEM Motor home page. In applications needing low speed with a reducer, our worm gear reducers group can also be considered. In high powers a 690V connection provides a low-current advantage; our article on 690V asynchronous motor selection covers this topic.

Frequently Asked Questions

What is the difference between LRA and rated current?

The rated current (In) is the current the motor draws while running normally at rated power. LRA (locked rotor amps) is the current the motor draws as it starts from standstill, typically 6–8 times the rated current. LRA is the moment of highest current draw and directly determines the choice of fuse, breaker and cable.

Does star-delta reduce the starting current in every motor?

Star-delta starting reduces the starting current to roughly one third; however it also reduces the starting torque by the same proportion. Therefore it works only in applications starting unloaded or lightly loaded. In motors starting under load, star-delta may be insufficient; in that case a soft starter or VFD is more suitable.

Does the motor get damaged when the starting current is high?

A short-term high starting current is normal and causes no problem in a correctly sized motor. However frequent start-stop, long start-up times or insufficient protection can cause thermal stress in the winding. This risk is managed with the right starting method and the right protection devices.

Get a Quote

Contact us to correctly select your asynchronous motor with a starting method that will neither stress your grid nor be unsuited to your application. Share your power, load type and grid conditions; let us clarify the suitable motor and starting solution together. Phone: +90 (532) 345 49 86 — or request a fast quote via our contact page.

Purchasing and Selection Checklist

  • Is the LRA / Ia-In ratio on the nameplate noted?
  • Is the load torque characteristic (starting unloaded or under load) identified?
  • Are the grid strength and voltage-drop risk evaluated?
  • If there is generator feeding, is the starting current matched?
  • Is the starting method (DOL / star-delta / soft starter / VFD) suited to the application?
  • Are the fuse, thermal and cable cross-section selected for the starting current?
  • If speed adjustment or energy saving is needed, is a VFD considered?
  • Is the terminal connection (star/delta) bridged correctly?