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Purvis Industries

VFD Stopping Techniques

Posted on Tuesday Jan 29, 2019 at 06:12PM in Electrical

We have a history of receiving inquiries on how to add braking to VFD applications. The terminology can often be challenging and confusing. There is always a possibility that the application is not a good fit. Below are three different types of  braking techniques often used with a VFD. 

What is Regenerative Braking?

Regeneration Braking occurs when a coasting or decelerating electric motor is driven by the load causing the motor to generate electricity. This is very similar to a generator. This can be best described using is a four-quadrant graph with Speed and Torque are plotted on X-Y Axis.

The motor is in “Regenerative Braking (REGEN)” when the following two combinations are met.

Combination 1:

Speed is Positive (+) and Torque is Negative (-)

Combination 2:

Speed is Negative (-) and Torque is Positive (+)

What is Regenerative Braking used for?

Regenerative Braking is used for applications that require continuous braking or have high inertia loads. This is the most expensive solution, but some of the cost can often be recovered by feeding the generated electricity back into a power grid.  For example, this is similar to a conveyor that is running downhill where the weight on the belt causes the belt to move without a power driven motor.


What is Flux Braking?

Flux braking is a controlled method used to increase motor loss (slip). Motor loss, also known as slip, is the total difference in speed between the rotor speed and the rotating electrical field of the stator. When braking is required, the flux (current) in the motor is increased, which in turn increases the motor’s capability to brake. By raising the level of magnetization (current) in the motor, the load can be quickly decelerated. By increasing the flux in the motor, the energy of the mechanical system changes to thermal energy in the motor.


What is Dynamic Braking?

Dynamic Braking is where a braking chopper is used with a braking resistor. As the demand for braking increases, a switch turns on and bleeds the excess energy off to a resistor. Basically, the braking chopper serves as an electrical switch that connects the VFDs DC bus voltage and to a resistor, where the braking energy is converted into heat. During the deceleration period, the motor switches to a generator operation and supplies energy back through the inverter.



  • Regenerative Braking – Breaking energy is converted to electricity and returned to the grid
  • Usually the most expensive solution
  • Often requires a Regenerative Drive
  • Can be used without limit


  • Flux Braking – Uses current to create magnetic flux to stop a motor
  • Good for lower horsepower(HP)
  • Over fluxing reduces slip, which reduces the braking capability
  • Usually the most cost efficient solution
  • Can only be used intermittently due to the heat generated in the motor


  • Dynamic Braking – Requires a brake chopper and brake resistor
  • Power is dissipated by heat
  • Brake Resistor must be sized by load and duty cycle
  • Can only be used intermittently from  heat generated from the resistor

Let's Start Something

Posted on Friday Jan 04, 2019 at 02:54PM in Electrical

Consider this; every motor needs a way to be started and stopped, and in some applications, reversed. The 3 most common will be discussed below, including the pros and cons of each method.

Direct On Line (DOL), also referred to as across the line starting, is the most common with the lowest initial cost, but with the most drawbacks. This is accomplished with a motor starter, which consists of a contactor and a motor overload. This method delivers high torque at startup, but puts the most stress on the system, mechanically and electrically. When using this type of starting, the starter immediately delivers full voltage to the motor, resulting in a temporary inrush current resulting in current draws up to 6-8 times the Full Load Amp (FLA) rating of the motor. An example of this is a 30HP motor with a nominal FLA of 40 Amps; starting DOL, the motor could see 320 Amps until the load gets up to speed. It stresses the motor windings, reducing motor life and can cause belts to squeal, reducing their life, and cause severe stress to couplings, gearboxes and whatever else is connected to the motor shaft, reducing their life as well. For reversing applications, the correct starter must be utilized. It consists of 2 contactors (to switch 2 of the 3 leads) to achieve reverse, and an overload to protect the motor. The overloads react relatively slowly versus the next 2 options. Note: The motor always goes to full speed.

 Soft starters are another method of starting a motor. It ramps up the voltage being delivered to the motor, resulting in a “soft” start.  It reduces the inrush current and can prevent belt slippage and is easier on couplings and the other components connected to the motor. There is a reduction in the startup torque with this method. It is easier on the motor windings because the inrush is typically half of the DOL method above. Most soft starters can be reversed with a switch closure to an input on the device. This is slightly higher in cost vs DOL. Again, the motor always goes to full speed but there is more control of the start and stop.

Variable Frequency Drives, also referred to as VFDs, can deliver full torque at start up and there is no inrush current with this method. Ramp up (acceleration) and ramp down (deceleration) times can be set for smooth starts and stops. Adjustable speed is another benefit of this type of motor control. It is by far the easiest on your motor and system, if the motor is suitable for use on a VFD. Minimum and maximum speeds need to be considered to ensure the correct motor is being used. This method comes at a higher cost than the previous 2 types, but in the right application, it can reduce energy consumption and there are incentives available from the utility companies to offset the initial cost, but the energy savings with also help pay for the VFD. To learn more about the incentives available, you can go to www.DSIREUSA.ORG . All that is required is the Zip Code of the customer location for a list of Policies and Incentives.

Feel free to contact your Purvis Industries sales representative to get connected with one of our Triad RAE for assistance in locating the best solution for you!


Posted on Tuesday Dec 04, 2018 at 03:38PM in Belting & Conveyors

Article By: Dave Corson, Business Development Manager, IMSCO - Mining Division

In today’s increasing demands put on belt conveyors to transport material from point A to point B, the proper selection of the components used on your belt conveyors becomes even more critical. This includes, but is not limited to higher tonnage demands, wider belts to meet these demands, and the ever increasing belt speeds (FPM) that must be achieved for these increasing tonnages.

Over my 30+ years of experience working around these conveyors in the mining industry, I have had multiple discussions with mine managers, engineers, maintenance superintendants, etc. regarding the “myths” and “truths” around selecting conveyor idlers for their projects. There are a number of “quality” manufacturers in the USA, some of which manufacture BOTH, tapered roller and ball bearing idlers.

Let’s look at some of the “myths” surrounding the ball bearing -vs- tapered roller bearing  controversy.

  • Ball Bearing idlers have lower rotational resistance.
    • This is FALSE. The rotational resistance of the conveyor idler roll, depends on the grease and seal arrangement of the idler. NOT the bearing.

  • Tapered Roller bearings MUST BE REGREASED.
    • This is FALSE. Although regreasing any bearing is always recommended to extend the life of the bearing and equipment, “Sealed for Life” tapered roller bearing idlers have been manufactured in the USA for over 40 years, and have proven themselves in the most demanding mining applications.

  • Tapered Roller bearings MUST BE PRELOADED.
    • This is FALSE. Quality manufacturers of TRB idlers, assemble their roll heads with strictly controlled, proprietary end play tolerances. NOT PRELOADED!

  • Tapered Roller bearing idlers allow only 3 minutes of angular misalignment.
    • This is FALSE. Quality manufacturers of TRB conveyor idlers, use “Modified Geometry” tapered roller bearings, and are tested to withstand up to 16 minutes of angular deflection.

We offer our customers many options when “selecting” the BEST “solution” for maximizing the life and efficiencies of the components on their belt conveyors. Choosing the right component for their application, is key to providing long term solutions for them.

In upcoming discussions, we will look at the different bearings and calculated L10 life associated with the different bearings used today in conveyor idlers.





Posted on Tuesday Dec 04, 2018 at 04:13AM in Electrical

Article By: Don Lovella, Triad Automation Business Development Manager

There are many reasons our customers use variable frequency drives, but probably the most popular reason is to control the speed of their electric motor. While other customers might want to use a VDF to start and stop their machine in a specific location or time. There are other customers who might want to use a VFD to minimize the impact on the mechanical system. VFD's accomplish all these tasks by controlling the speed, position or acceleration of an electric motor. While this all sounds great, there is still more a VFD can do for you and that is save energy! Saving energy is probably the most overlooked benefit of using a VFD.

For example,  a typical variable torque application such as a fan or a centrical pumps energy consumption can be reduced approximately 50% by slowing down the motor just 25%!

Here are some interesting facts about electric motors.

  • Electric Motors consume approximately 28% of the world's total electrical energy production.
  • The cost of an electric motor is approximately 1% of the total cost of ownership of an an electric motor while the other 99% is the cost of the electricity to operate that motor.
  • Approximately half of all electric motors installed today could reduce energy consumption with the addition of a VFD.

This means there are literally hundreds of thousands, if not million, application within our industry that could benefit by the addition of a VFD! I'm sure by now you're thinking to yourself this all sounds great, but the I'm not sure we can afford the expense of adding VFD's. What if I were to tell you that electrical utility providers offer rebates to their customers to install VFD's and save energy? This sounds crazy, but on average the rebate from an electrical utility provider for a VFD package install is $125 a horse power. I know you're thinking I don't have the time to search through my utility providers website to find the rebates. I have an answer for that as well. There is a website '' that we have used for many years to assist our customers in locating rebates provided by their utility provider. All you have to do is enter your zip code in the search field and ti will take them to a page that shows rebates provided by their utility providers. 


Posted on Friday Oct 05, 2018 at 07:54PM in Fluid Power

By: Ron Polvado

To start, there are three basic types of accumulators:

1) Bladder

2) Piston

3) Diaphragm

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Posted on Thursday Jul 26, 2018 at 02:39PM in Electrical

          Many of our customers have the need for electric motor braking whether they know it or not. Most of them are unaware of the potential time and energy savings they could receive from electric motor braking.

          Once it has been determined braking is required there are several things that need to be known. The first thing that must be known is the weight and speed of the rotating load, next is how fast that load needs to be stopped and lastly how frequently this load needs to be stopped. Once these factors are known it can be determined if the VFD drive can stop the load on its own or if additional items are required. In the event the VFD drive needs additional items to stop the load you might need to add a braking resistor and or the brake chopper option to the VFD drives order code. On some occasions you will need to add an external brake module to a VFD drive that doesn’t have the option for a brake chopper. These items are used together to dissipate the excess energy that is created when stopping a rotating load.

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Posted on Thursday Jun 14, 2018 at 01:19PM in Electrical

Article Submitted By: Robert Witte

ABB PSTX Soft Starters

        When ABB first released their new PSTX Soft Starter, they were only available up to 370 amps (300 Hp @ 480v) but they have now released the full product line up to 1250 amps (1000 Hp  @ 480v). Previously, all ABB Soft Starters were a part of the ABB Low Voltage Products Group, along with products like Circuit Breakers, Across the Line Starters, Disconnect Switches, Pilot Devices, etc. Recently the Soft Starter product line has been re-aligned into the ABB Low Voltage Drives Group. 


The PSTX Soft Starters are the most feature rich Soft Starters on the market today. The detachable keypad is modeled after the current ABB Drives keypad, such as the ACS880, ACS580 and soon to be released ACS480. Navigating through the keypad to program the Soft Starter parameters has the same flow as the current ABB drives, making programming very easy, even without using the manual for assistance. This user-friendly keypad is standard on all PSTX Softs Starters.

  The PSTX Soft Starters offer a Slow Speed Jog Function, in both Forward and Reverse Directions. This feature allows for greater flexibility when operating equipment such as Conveyor Belts and Cranes. This feature provides positioning capabilities, allowing the operator to take greater control of their process.


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Posted on Monday Jun 04, 2018 at 04:25PM in Mechanical

Article by: Dan Whitehouse

Bearing failure due to lubrication is a very common occurrence. About 50% of bearing failure is related to lubrication. Below is a section of an article from that details eight failure mechanisms.  (Full article can be read here.)

When in doubt, it does not hurt to ask when lubrication comes into play.

1. Unsuitable Lubricant - First, you must choose the correct lubrication for the application. Fundamental properties, such as the viscosity, additive package and consistency (for grease), should be carefully selected based on the bearing type, speed factor and operating conditions. If these factors are not thoroughly considered and an unsuitable lubricant is applied, the lubricant may become overly stressed or be insufficient for the machine's lubrication needs. In either situation, the bearing will likely undergo premature wear and failure.

2. Lack of Lubricant - For greased bearing applications, the correct regreasing volume and frequency must be established to ensure the bearing load zones are lubricated properly. Too much time between regreasing intervals or applying too little grease will cause excessive boundary and bearing wear.

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Posted on Monday Jun 04, 2018 at 04:22PM in Fluid Power

By: Steve McKown

The life blood of the hydraulic system is the fluid, and it is usually the last thing customers think about.  Contamination in the fluid causes wear and damage to the moving components inside a system and can cause it to fail prematurely.  Here are some key points on filtration.

New hydraulic fluid: New fluid should be filtered before it is used.  Contamination gets into the fluid during the processing and packaging phases.  Filter carts are available to clean this fluid before use.  

Cleaning the tank: If the filters in the system clog they will bypass and return unfiltered fluid to the tank.  Particles settle out of the fluid over time leaving a layer of gunk at the bottom of the tank.  Most tanks have a panel that can be removed to clean this out.  Keeping the tank clean will increase the life of the filters. 

Suction filters in the tank: In the past most tanks were equipped with a screen suction filter on the suction tube.  The tank manufacturers are going away from this because if the screen clogs it can pull a vacuum on the pump causing a catastrophic failure.  They now rely on the high and low-pressure filters to catch debris in the system.  This makes the high-pressure filtration even more important.

Desiccant Filler Breathers:  Moisture acts like an abrasive in the hydraulic systems and can cause severe damage.  Using a desiccant breather cap removes the water vapor from the air. 

Return Filter:  this is the filter you will see most often on the power units.  This cleans the fluid as it returns from the system and removes the particulates introduced into the fluid by the seals, hoses, and moving parts in the system before returning to the tank.  If you have a worn rod seal on a cylinder it will pull dirt into the cylinder contaminating the fluid.  If you have older hoses they deteriorate and dirty the fluid.  

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Posted on Friday May 11, 2018 at 07:41AM in Belting & Conveyors

By: Ken Harville

Looking for a better posi-drive belt option? Do you have complaints about excessive stretching, wavy belt edges, and sprocket disengagement? Do you need to replace plastic modular with fabric belting? Well then here is the answer, Gates Mectrol's Posiclean PC-10 and PC-20 Kevlar reinforced, posi-drive belting.

PC-10 and PC-20 are 1” and 2” pitch respectively, and their Kevlar reinforcement eliminates belt stretch, wavy belt edges and sprocket disengagement. Further, these 2 belts allow customers the ability to drive these belts without the aid of UHMW “shoes”. Shoes increase belt wrap at the drive sprockets, but they also cause wear and tear on the belt’s surface and edges.

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Posted on Friday May 11, 2018 at 07:29AM in Mechanical

By: Blake Timmons

With the increased use of Variable Frequency Drives (VFDs) in industrial and commercial electric motors, there becomes a source of current flow through the bearing. Note that inverter-induced bearing currents and premature bearing failures occur in a relatively small percentage of installations and applications. Nevertheless, it's best to understand the topic when you run across the problem.

The damage to the outer or inner race of a motor bearing will look something like the pictures below.



The damage to the outer or inner race of a motor bearing will look something like the pictures below. In these photos. Notice that the “fluting” is seen as symmetrical damage which is a common sign of a bearing current issue. Also notice that damage can also occur that is not symmetrical, shown by individual random spots on metal surfaces. With motors using with an inverter, you need to be aware of the high-frequency current paths from the motor back to the inverter and to ground. This will help in understanding potential bearing current problems and remedies. High frequency motor bearing currents can occur in any motor driven by a drive using Pulse Width Modulation (PWM).

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