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Power up with asynchronous motor starting system basics

Starting a motor is a big job for any power source. There must be enough power to energize the windings and activate the shaft. Applying direct power to the motor isn't the best strategy, however. There should be a starting system that applies power in a slow-and-controlled manner. Learn more about asynchronous motor starters so that you can make an educated decision about your system today.


For motors that operate on less than 10KW, the direct-on-line starter is a viable option. It uses the combination of a contactor and circuit breaker. Apply power to the motor, and it initially energizes the contactor. Through the connected circuit breaker, energy slowly moves into the motor.

The de-energizing process works in the same manner. The contactor acts as a front door to the incoming power so that the motor isn't overwhelmed with current. It must be used on smaller motors, however, because large models can overheat with this method.

Rotor Resistance

One of the most basic ways to limit power into a system is by using resistance. Resistor components come in all sorts of values and sizes. When they're used in a starting system, they draw some voltage away from the circuit.

The energy that does make it into the rotor winding is calm and relatively low. It's enough power to get the system running. Most rotor-resistance starters require the removal of the parts after a successful power sequence. The motor can take on the full, incoming voltage at this point.

Frequency Inverter

Electronically controlling incoming power can be expensive, but it works in certain applications. A frequency inverter uses inverter circuits in order to control the incoming current. No current surges will harm your motor, which can ruin windings in a short period of time. Other electronic components are involved so that the power-up sequence can be as seamless as possible.

This system is deceiving because it doesn't look like a separate entity. It becomes a single unit with the motor. The resulting system has few problems and lasts for a long time.


When you have a lightweight load, a star-delta starter is a good addition to the motor assembly. Windings in the shape of a star are first in line for the incoming power. The current drops in the star, which allows the system to start up without any overloads.

The power enters the delta configuration, which increases the current. Increased torque isn't far behind. The motor can run without fail now.

Switches in either manual or automatic modes allow the transition to happen with each startup sequence.


If you have a large motor, you need a specialized way to power it up. This system uses two switches and an auto-transformer. Move the switch to "start," and about 60 percent of the full voltage will flow through it. Your motor has a chance to warm up and activate to its full potential.

Change the switch over to "run," and the full voltage feeds the motor. It continues to run until you shut it off. With controlled power moving through the motor, it can theoretically run for hours on end.

Soft Start

An electronically controlled starter is the soft start. It uses a set of thyristors as a filter to the incoming power. Once the power clears the thyristors, the energy flow is steady and controlled. There are no surges to worry about.

If you have a system with expensive or specialized parts, adding a soft start to their power-up sequence is a clever choice. You'll ultimately save money on the installation. The controlled power moving through the internal parts will only preserve the mechanical operations.

Connect with Kurz when you have any questions about motors or starting systems. It's our goal to support your needs out in the field. Motors can be mysterious on several levels. We aim to help you understand and thrive with today's mechanical wonders.