Why Are Drives Easier to Apply Than Soft Starters?

Why Are Drives Easier to Apply Than Soft Starters?

Consider torque requirements, speed control and power distribution when deciding between a soft starter or an AC drive for an application.

By Richard Anderson, product manager, Rockwell Automation

Recent market research shows that the worldwide low-voltage AC drives market continues to remain approximately 20 times larger than the worldwide low-voltage soft starter market. This is a very interesting phenomenon given that there can be substantial cost, size and energy savings when choosing to use a soft starter instead of an AC drive for some applications.

So why are drives applied in situations in which soft starters are sufficient to do the job?

The answer is fairly basic: AC drives will work in almost all applications, but soft starters will not. Some people interpret this to mean that AC drives are easier to specify, install and commission than soft starters. This isn't true. There are many more things to consider when applying an AC drive than when applying a soft starter. A better way to look at this is that AC drives work in a wider variety of applications than soft starters.

All of us probably have heard the old saying “Location, location, location!” when buying real estate. Similarly, when deciding whether to choose a soft starter or an AC drive, the factors that are important are “Application, application, application!”

Torque Requirements

When deciding whether a soft starter or an AC drive should be used, ask the following common application questions:

  • What are the load torque requirements?
  • Is speed control needed during the run mode? Is full-speed reverse operation needed?
  • Are there any power distribution considerations or restrictions?

The load torque requirements of an application are important when choosing a soft starter or an AC Drive. Does the application need high starting torque? Full torque at zero speed? Holding Torque? Soft starters are by definition reduced voltage starters. This means that full torque is not available at zero speed or very low speeds.

Because of this, these load types are typically not recommended for soft starters: extruders, positive displacement pumps, inclined conveyors (overhauling load), lifts, and elevators (unless hydraulic). Meanwhile, an AC drive can produce rated torque from 0 to rated speed and can typically handle all load types.

Speed Control

The ability to control speed is another key application consideration and is dictated by the physical construction of the soft starter and AC drive. A soft starter has silicon-controlled rectifiers (SCRs) that connect the AC input line to the motor. Physical properties allow changing the point in the AC input waveform where the SCR is switched on, but not where it turns off; SCRs turn off on their own.

This allows ramping of motor voltage from 0 to full line voltage (voltage control). This means that line frequency is always applied to the motor and that once the motor is up to speed it will run at the equivalent 60Hz (or 50Hz) rpm value. Full speed direction is determined by line power connection ordering.

In contrast, AC drives have many more active electronic components — typically consisting of a diode front end, DC bus and insulated-gate bipolar transistors (IGBTs). IGBTs can be switched on and off at any point, for as long and as often as you want. Different voltages as well as different frequencies (voltage and frequency control) can be applied to the motor, which results in continuous speed control in either direction.

Power Distribution

AC drives can operate on most practical power supply systems, including those that are closely matched to the motor load. Typically when using a soft starter, the supply source should have the capacity to allow starting currents that are 150% to 450% of the motor load full load amperes. Some AC drives can also operate 3-phase motor loads using single-phase input power. Soft starters can only operate on 3-phase input power.

When deciding whether to choose a soft starter or an AC drive for a particular installation, application characteristics are the primary concern. Torque requirements, speed control and power distribution should be carefully considered.

For soft starter applications, some general rules of thumb include:

  • Reduces mechanical wear and damage to system.
  • Limiting current is prime reason for starting method.
  • Lower starting torque applications.
  • Lightly or moderately loaded applications.
  • Full-speed operation.

For AC drive applications, general guidelines include:

  • Speed control during the run mode.
  • High starting torque.
  • Position control.
  • Holding rotor at zero speed.

No matter which motor control method you select, consider installation, commissioning, and additional filters or circuit protection when specifying a soft starter or AC drive.

Learn more about Rockwell Automation® low-voltage soft starters and low-voltage AC drives.


The Journal From Rockwell Automation and Our PartnerNetwork™ is published by Putman Media, Inc.

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The JOURNAL from Rockwell Automation and Our PartnerNetwork™ is a bimonthly magazine, published by Putman Media, Inc., designed to educate engineers about leading-edge industrial automation methods, trends and technologies.