Using Soft Starters verses a VFD

When to use a Solid State Starter and when to use a Variable Frequency Drive is a topic that comes up quite often.  Most times it is generated by a need that is expressed by an equipment manufacturer for their equipment or a piece of equipment that too large to start across-the-line due to line restrictions or limitations.  Which method to use really depends on the application and what the expectations are for the driven equipment.

Sometimes it is a mandate by the power company that requires that a motor be started by “reduced voltage” means so as to not stress the utility line.  It can be at a site where the primary source of power is a portable generator.  Other times it may be applicable for a desired or needed reduction in mechanical stress for the driven equipment or a system that is connected to the equipment such as a pipeline connected to a pump.  Even a conveyor can be a concern and is typically a good application for either method.  So what makes the determination as to which method we should use?

Does a VFD make a good soft starter?  The answer to that is yes.  Under most circumstances a VFD does make a great soft starter and can soft stop if needed with complete and smooth regulation on most loads.  But if the load is to be run at 100% speed after initial start-up until such time as it is no longer required, it is generally a bit extravagant to use a drive for this application.

Newer technology has brought the cost of general purpose Solid State Starters down due to the ability of many of today’s soft starters to control a motor with only regulating two of the three phases.  Better phase control and switching technology has made it so this is done without the distortion that was typical on earlier units which were not very well accepted in the past.

One note, notice I said “general purpose” soft starts.  While these starters perform well starting and stopping a centrifugal pump, a fan and some light duty conveyor applications, I still hesitate applying them on applications where starting torque is an absolute necessity.  Loads such as a crusher or any load that requires all the torque a motor can wring out at reduced current still require a three-phase controller style soft starter.  Also on these applications a true “Shunt Duty” starter should be used in place of the units that have “built-in” bypass.

If the load is to be regulated for flow, product delivery, pressure or what have you, a VFD is probably the way to go.  Mainly due to the fact that if you are going to throttle the load in any manner, you might as well be benefiting from the reduction of required kW that the drive will provide and in doing so will save you money.  Depending on the load and application, the energy saving benefits of using a Variable Frequency Controller is still the number one reason to apply a drive.  Many applications such as pumps and fans running at 20% speed reduction can have huge cost savings in comparison to throttling or dampening.  In most application, you can expect save the initial cost of the VFD in less than 6 to 12 months depending on the hours of average run time during a day, week or month.  So you get the benefit of desired work and cost savings using a VFD if applied correctly.

I still get people telling me that they have been told that applying a Solid State Starter will save them on their demand charge.  This is not correct in most cases.  Under most circumstances demand is calculated over a 15 minute window by most utilities.  It is typically not just a “blip on the scope” caused by starting a specific load as most believe.  Let’s just take a look at starting a 100 horsepower motor.

Using today’s information here is what we would expect to see with an across-the-line starter driving a centrifugal pump:

FLA = 122 amps

LRA =720 amps  

Typical Start time = .86 sec

Now using a Soft Starter in current control mode set to 300%

Max current =366

Calculating average inertia or angular acceleration of the pump and motor and based on a common pump curve and by changing the initial head over time of acceleration, we can come up with a hypothetical time of about 4-6 seconds.

Now, which is greater: 366 amps for 4-6 seconds or a max of 720 amps reached during the .86 second time?  The answer is, in the eye of the utility they are equal to each other therefore there is no demand difference.  Calculating in all the factors of starting a motor still require that a certain amount of power be consumed to accomplish a start.  Only by regulating voltage, current and frequency can this be altered and that would require a VFD.

If you have an application that you are looking at for either a drive or a soft starter, call us and we can discuss your application and see what would be best for your need.  We may not have all the answers to your questions, but we know where to look and whom to call.

Scott