Start Your Motors

Which Starter Fits the Application?: Expanded Machine Capabilities Might Warrant a Switch From Full-Voltage Starters to VFDs or Soft Starters

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We build some pretty basic machines that have only limited need for variable speed control of 240/480 Vac motors from 5 to 20 hp. That's going to change as we expand the machines' capabilities and design them to integrate into systems. We're arguing about whether to switch from full-voltage starters to either VFDs or soft starters. We think we could save energy costs with VFDs because we sometimes run at faster speeds than we need to, but that's not a big deal at the moment. Soft starts would clearly be easier on the system at startup. Cost can't be ignored.

—from October '09 Control Design

ANSWERS

Power Vs. Speed for Fans and Centrifugal Pumps

This machine builder has touched on a subject near to my heart and the basic mission of Vacon. That is the basic value statement of drives and why we say that every ac motor deserves a Vacon drive.

It is quite true that ac drives can save a tremendous amount of energy. That's particularly true when you reduce the speed on fans and centrifugal pumps. There, the relationship between speed change and power consumption is the most opportune for saving money. However, in all applications, the motor's energy consumption is at least related to rotational speed in a linear fashion.

The efficiency of every industrial process can be positively influenced by the variable speed capability of drives, some more so than others. Savings in material waste and the optimum use of an operator's skill set is the primary advantage here.
An electric motor starts very abruptly when powered from full voltage, often causing excessive wear and shortened life for mechanical components. This also is true when mechanical braking is employed. The ramp controls of the Vacon ac drive provide a subtle start and stop and extends the life of these mechanical components, saving maintenance time and money.

Whether you reverse the motor in your application or not, you still have needs to switch the motor on and off. All other motor-starting mechanisms, other than Vacon drives, have moving parts that wear and fail. Drives result in additional savings, particularly in applications where there's lots of starting and reversing, since this is where mechanical means wear out so quickly.

Finally, I don't know which market this builder appeals to, but the phase conversion capabilities of Vacon drives can extend the use of the machine to other markets. Vacon drives can allow three phase motors to be used effectively from single phase power. This can open residential and agricultural markets, where three phase power is not always available.

Tim Park, product marketing manager,
Vacon, www.vacon.com

Full Vs. Reduced Voltage

When starting and stopping a motor, both soft starters and variable-frequency drives (VFDs) reduce the wear and tear on the motor and transmission, reduce torque shock on the load and also reduce the high inrush current draw associated with full-voltage starters. This, in turn, allows for less maintenance cost and downtime. However, if your applications allow for speed regulation, a VFD must be used, and it also may reduce energy costs.

With that said, there are a few things to consider when applying soft starters and VFDs.

An inverter-duty motor is required for VFDs, while any motor will do for a soft starter. This allows the soft starter to be a direct replacement for full-voltage starters and any reduced voltage starters.

Soft starters are less complicated to program and connect, compared to VFDs.

The soft starter almost always will be less expensive than the VFD.

The VFD converts ac to dc and then dc back to ac to control the frequency that controls the speed of the motor. This can lead to harmonic distortion (interference) in the power line.

The soft starter uses thyristors to control the voltage used to start and stop the motor, resulting in no harmonic distortion.
Unlike VFDs, soft starters can be bypassed while running to reduce heat.

The bottom line: If speed regulation is not required, a soft starter would be the best method of reduced voltage starting.

Lorenzo Di Maso, business development area manager, low voltage soft starters,
ABB, www.abb.us/lowvoltage

Weigh the Advantages

Soft start controllers are able to smoothly ramp up the motor and therefore able to reduce mechanical and electro-dynamic stresses in the system.

From both a mechanical and electrical standpoint, there are a variety of advantages to employing soft starts, rather than across-the-line starters. Overall, soft starting reduces downtime and lowers costs. Further, soft starting is able to:

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