PAYBACK variable speeds drives represent a totally different approach to variable speed control of AC motors and their respective loads. We have assembled this FAQ sheet to address many of the questions that arise regarding this revolutionary technology.

1. What are the basic principles of operation for PAYBACK variable speed drives?
2. What are the primary benefits of PAYBACK variable speed drives?
3. How did magnetic-coupled variable speed drives evolve?
4. How do PAYBACK variable speed drives save energy if the motor runs continuously?
5. Do PAYBACK variable speed drives cause additional motor heating?
6. Do PAYBACK variable speed drives create any harmonics or other power quality issues?
7. How far can the controller be located from the motor? Are there any limitations?
8. Are PAYBACK variable speed drives subject to nuisance dropouts?
9. Can lightning damage PAYBACK variable speed drives?
10. Can PAYBACK variable speed drives retrofit existing motors?
11. Is there a bypass option?
12. Is there any motor insulation failure or electrical pitting of motor bearings?
13. How do PAYBACK variable speed drives influence power factor?
14. Are PAYBACK variable speed drives easy to maintain?
15. How does the PAYBACK variable speed drive perform with constant torque loads?
16. What's new on the horizon?

What are the basic principles of operation for PAYBACK variable speed drives?

The magnetic coupled concept differs vastly from Variable Frequency Drives in that there is no electrical power interruption to the motor. With the motor running continuously, precise speed control is accomplished by varying the magnetic coupling between the motor's output shaft and the load. Most configurations are comprised of two primary elements:

One element (input portion) is affixed to the motors shaft so as to run continuously. The other element (output portion) will have a connection to the driven load. These two elements are separated by an air gap, and have no other mechanical connection other than supportive bearings. By applying current to the coil of the electromagnet rotor, a polarized magnetic field is produced, creating eddy currents on the surface of the drum, magnetically coupling both components and causing the output portion to turn in the same direction as the motor. The speed of the output is dependent on the strength of the magnetic field which is proportionately controlled by the amount of current applied to the electromagnet.

What are the primary benefits of PAYBACK variable speed drives?

• Simple and economical to install
• Substantial energy savings with variable torque loads
• Does not require expensive or highly skilled personnel to maintain or service
• Low cost of ownership, fast payback
• Allows motor to operate with pure uninterrupted ac power
• No Power quality issues, ZERO HARMONICS
• No special filters, reactors or expensive three phase isolation transformers
• No expensive bypass circuitry
• No nuisance dropouts
• Highest level of immunity to the effects of lightning
• Does not cause additional motor heating, even at continuous low speed requirements
• Does not cause across the line shorts, power surges, or voltage spikes
• Does not cause motor frequency noise
• Does not harm motor winding insulation
• Does not cause electrically induced pitting of motor bearings
• Does not require expensive inverter duty motors
• Controller can be located any distance from motor/drive without causing voltage ring up or motor damage
• True retrofit capability, works with existing motors and existing wiring circuitry
• Same compact low wattage, low voltage, low cost controller for all horsepower drive sizes
  

How did magnetic-coupled variable speed drives evolve?

Foot Mounted Style

In the 1940's and 50's, magnetic coupled devices known as eddy current clutches were effectively used with AC motors and were quickly becoming a popular method of varying the speed of many industrial loads. [Fig 1] Although bulky and inefficient, these workhorses were quite reliable and were used in applications such as punch presses, conveyors, winders, and other machine tool situations. These were oversized foot mounted units that initially were designed as a separately housed clutch assembly with an input shaft and an output shaft to be coupled in line between the motor and the load. Also offered were motor and clutch combination (one piece) packaged units. In those days, the primary focus was in functionality, performance and maintainability, as energy efficiency was not as important a factor as it is today.

Shaft-Mounted Styles

In the 1960's some of the first commercially available motor shaft-mounted magnetically coupled drives were offered to the industry. This new design was originally intended for fractional and small integral horsepower applications, and was novel in that the drive was totally supported by the motor shaft. [Fig 2] The product did however have some drawbacks, namely oversized slip rings and problems with brush alignment. It was the users responsibility to align the brush holder with the slip rings. The intent was to mount the brush holder to the existing motor bolts. Unfortunately, motors supplied by different manufacturers varied significantly and alignment became difficult. Additionally, the slip rings were provided on the drive at the motors shaft entry side, fabricated on a circuit board type material with copper rings facing the motor. Since the diameter of the outer ring was larger than the inner ring, the outer brush would wear out faster. In addition to the uneven and rapid brush wear, the integrity of the circuit board and copper rings were effected by heat, causing separation of the rings from the base material. This design was abandoned soon after initial production.

In the 1980's the problem with brush alignment had been somewhat resolved by a new shaft-mounted design that incorporated a bracket supported by an additional bearing on the drive which maintained reasonable alignment between the brush holder and the slip rings. [Fig 3] This design enjoyed some success in the machine tool industry where reduced run-time hours was common. This basic design was still flawed however, as the slip rings were still located on the motor shaft entry side, causing them to be oversized and progressively larger to accommodate the higher horsepower motor shafts. This created a major headache in the HVAC Air Handler marketplace, as it was soon discovered that 24 hour duty meant brush changes in some cases as often as once every two to three months on the larger drives. Although an improvement over previous efforts, the location of the bearings being cantilevered to the pulley grooves, caused premature drive bearing failure in many instances. The additional bearing used to accommodate the brush holder had an unacceptably high failure rate as well. This high maintenance drive has become virtually obsolete in the air handler industry and has been routinely replaced by the more efficient and reliable new brushless designs.

Another design consideration places the pulley grooves out on the outboard side of the drive, a distance away from the motor face. [Fig 4] This, by far is the poorest of all approaches because it directly jeopardizes the motor bearings' life expectancy. Since the pulley grooves are not located over the nema shaft extension, applying full rated belt tension exceeds the overhung load rating of the motor in many cases. This out-board pulley design has many documented failures in the field, again compounded by drive bearing failure due to cantilevering effect, and motor bearing damage as well.

The most reliable and field proven design distinguishes itself in many ways from the previous concepts. [Fig 5] The rotor/coil assembly rotates constantly with the motor shaft. A one piece drum/pulley portion is the output driving member. The pulley grooves are located in-board, closer to the motor face than any other design. The drives' bearings are located directly under the pulley grooves so that maximum belt tension can be applied continuously on all models without harming the drive bearings and yet remains well under the overhung load capacities of the motor. Because the drum is copper lined, the brushless drive runs cooler, and is more efficient than other magnetic-coupled drives. The drive coil requires only one third to one fourth the wattage of other models. It has the fewest parts, weighs less and has the best operating performance of all available designs. The unblemished track record exceeding five years.

How do PAYBACK variable speed drives save energy if the motor runs continuously?

Because of the nature of the descending torque load itself, the magnetic-coupled drive takes advantage of the energy savings with variable speed.

[Graph] The magnetic-coupled drive will take advantage of the affinity laws on variable torque loads such as centrifugal fans and pumps and does so without altering the voltage or interrupting power to the motor. Even with the motor running continuously, the kW required by the motor changes according to the actual load, i.e.: -loading and unloading of the motor by varying the speed of the load via magnetic coupling between the motor and the load. What you always pay for is the amount of kW used and even with the motor running continuously, the difference in the amount of kW required of any 3 phase motor from no load to full load is very significant.

As example: 7-1/2 hp motor/3phase/60hz (Typical Blower Application)
Measured @ Full Load, max fan speed (typical) = 5.60 kW
Measured @ Min Load, min fan speed = approximately 0.40 to 0.50 kW

GREATER THAN 10 TO 1 kW DIFFERENCE THROUGHOUT ENTIRE SPEED RANGE, THANKS TO THE AFFINITY LAWS. Even greater advantages from full load to no load are realized as the motor horsepower size increases. As a result, the power curves are very similar to VFD's on variable torque loads and the kW savings are significant as well. As a general rule, magnetic-coupled drives are more efficient at the top end of the curve and VFD's are more efficient further down the curve.

Do PAYBACK variable speed drives cause additional motor heating?

The magnetic coupling is electrically isolated from the motor and in effect operates as an infinitely variable, frictionless clutch, allowing the motor to operate as originally designed, at full speed continuously, and with pure uninterrupted ac power. Regardless of the drives' operating speed, the motor never sees any additional heating contributed by the drive. In fact, any additional heat is dissipated by the drive itself, via slip, and not the motor. Since these new drives are efficiently sized to handle the full rated horsepower of these types of loads, the minimal amount of drive heating is effectively dissipated by the drives' own integral fan. Since the motor runs continuously and the drive is simply controllably coupling and uncoupling the load, the effect of the motor loading is no different in operation than if you had incorporated an infinite number of pulley sizes to provide variable speed to the load.

Do PAYBACK variable speed drives create any harmonics or other power quality issues?

Since the magnetic-coupled drive does not interrupt the power source to the motor, there are no current harmonics produced nor is there any resultant voltage distortion. There is never any need for filters, reactors, or full rated, three phase isolation transformers

How far can the controller be located from the motor? Are there any limitations?

The distance from the controller to the magnetic drive and motor have worked successfully at distances up to 2000 feet. The only requirement is that the two wires that provide power to the drive coil be sized large enough to allow for any voltage drop (say 14 gauge, typically). No filters or any other devices are required. There is no concern about ever causing any damage to the motor or drive.

Are PAYBACK variable speed drives subject to nuisance dropouts?

By virtue of the inherently simple design, the drive is always active as long as the motor starter is energized. Transient over voltages, voltage sags, and harmonic distortion from other sources generally do not effect magnetic-coupled drives unless the duration of power interruption is long enough to actually drop out the motor starter circuit.

Can lightning damage PAYBACK variable speed drives?

Since magnetic-coupled drives are isolated from the power source, they provide the highest level of immunity to the effects of lightning.

Can PAYBACK variable speed drives retrofit existing motors?

The shaft-mounted, magnetic-coupled drive allows for true variable speed retrofit of any existing motor. There is never any need for inverter duty motors. All original motor wiring circuitry can always remain undisturbed.

Is there a bypass option?

A simple mechanical full speed lock-up feature is standard on all drives. Full speed electrical bypass can be accomplished with only one diode.

Is there any Motor Insulation Failure or Electrical Pitting of Motor Bearings?

Since the magnetic-coupled drive does not electrically connect to the motor wires, there is never any possibility of causing any electrically induced harm to the motor windings or the motor bearings.

How do PAYBACK variable speed drives influence power factor?

Some utility companies in certain locations may charge a penalty if the facility's total measured power factor is below an acceptable pre-determined level. In hundreds and hundreds of installations, in many varied facilities such as colleges, schools, hospitals, government buildings, and shopping malls, power factor has never been an issue with the new shaft-mounted variable speed drives. However, in the rare event that power factor may require attention, low cost power factor correction capacitors can be easily installed at each motor location as required.

Are PAYBACK variable speed drives easy to maintain?

This simple technology does not require highly skilled personnel to maintain. The same compact low cost controller is used on all drive sizes 1 through 150 horsepower. Unlike high maintenance brushes and slip rings, the rotary brushless plug-in coupling cartridge can be swapped out in a matter of seconds.

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How does the PAYBACK variable speed drive perform with constant torque loads?

In constant torque applications, the drive must be properly sized to allow for additional drive heat dissipation at the lower speed ranges, however there are no added demands on the motor other than the conventional no load to full load conditions, the same as in fixed speed operation considerations. These drives perform excellently in many other applications, such as conveyors, feeders, straighteners, winders, mixers, machine tools, punch presses, etc., as variable voltage, constant current, or closed loop methods.

What's new on the horizon?

Continued development on the next generation of self powered variable speed drives are a priority. These new magnetic coupled drives have their own source of power, an integral generator, automatically active when the motor is running. No external power source is required. Installation cost is reduced further because there is no need for the added cost of an electrician. Simply mount the drive to the motor shaft, connect the belts, and then provide a standard 4/20 ma signal to the drive. Loop powered at ANY HORSEPOWER. Imagine that.