Smith Electric Motorworks ©  Legal

Re-Defining Good Converter Design:

The High-Resistance Rotor

This discussion would not be complete without a look at rotary design for rotary converters.

1. History of the High Resistance Rotor: Benefits

The popular rotary for the past 40 years uses what the industry calls a High Resistance (HR) rotor. In its original construction, the end, or "short" rings of an ordinary 3-phase motor's rotor were drastically cut or alternately slotted end-for-end. This altered the rotor's magnetic field, and would allow the rotary to then start and attain operating speed with only a few motor-run capacitors connected.

The invention of this device was critical to the development of a workable multi-motor rotary phase converter, since at the time the only other starting means available involved starting the rotary motor mechanically with a "pony motor"--not the most user-friendly method--or using some single-phase motor-starting (electrolytic) capacitors and a single-phase motor starting relay to start the converter motor, in much the same fashion as the static converter back in figure 4.

Now the electolytic motor-starting capacitors of 40 years ago were about as stable as dynamite caps in a hot oven, and the single-phase motor-starting relays left a lot to chance. The capacitors had only paper-tissue insulation between layers of foil. The whole of it was wrapped in a cardboard tube and sealed with wax. In damp weather, the motor-starting capacitors might fall apart, or simply explode without warning. If the unreliable motor-starting relay applied power to the start capacitors overly long, their paper insulation could turn to charcoal from the heat. The carbon would take over as a conductor. Fire would then roar through the whole group like a lantern in a straw pile.

Considering the options of its day, the high resistance rotor was a great gift to the industry. It enabled people who otherwise could not have done so to have economical and reliable 3-phase power. The people who designed the high-resistance rotor deserve the commendation of everyone in the industry.

Open, drip-proof high-resistance rotary manufactured by Baldor Electric is used by at least 5 major converter manufacturers, giving rise to the myth that all phase converters are equal. This rotary is constructed of sheet steel and has die-cast aluminum end-bells.

2. Weaknesses of the device: Performance

Unfortunately, the HR rotor has some problems. Some have been addressed recently by manufacturers, but many are simply the trade-off you get with the design. From a performance standpoint, the major benefits from the HR rotor are attained at low rotor rpm, that is, when the rotary motor is starting. When used in a normal 3-phase motor, a high resistance rotor is a high starting torque, low full-load rpm device--in other words, it is a high-slip rotor. Remember that this is just the opposite of what a high-performance rotary phase converter needs, to produce magnetic fields and voltages which stay ahead of operated load motors.

Because of this inherent design deficiency, the HR converters have difficulty maintaining rotor RPM under load or when loads are initiated, so that 3-phase voltages produced by the rotary converter may be poorly formed. Recognizing this, one manufacturer tried to solve the problem of rotor-slowing by adding flywheel weights to the rotor, but this only aids during load initiation (load motors starting). Under continuous full-load operation the rotor will still slow and synchronize its RPM with the load motors, causing the problems mentioned above. During some early testing, we observed that the HR rotor had a slight advantage in capacitor current being distributed to load motors over a broad load range. This advantage, however, was offset by a precipitous voltage drop when a load was initiated--greater and more prolonged than the voltage loss suffered by a low-resistance rotary of the same NEMA frame size and under comparable load conditions.

3. Heat Problems

By nature, and the high-resistant rotor restricts current flow. When you place a resistor in series with a flow of electrical current, the product is heat, and high resistance rotors waste a considerable amount of energy and this fashion. In fact, the heat produced can be so great that the user generally is warned not to allow one of these converters to idle for long periods unloaded. The full current from the rotary's capacitor bank flows into the rotor at idle, and the intensified fields "saturate" the altered rotor and are converted into heat. Grease may boil from the bearings, and we have personally observed instances where the end rings or rotor bars melted from excess heat under normal operation.

Concerning rotor melting: the HR rotor is also prone to damage from high-voltage inductive surges. These can occur on remote single-phase lines when a large load is shut down. The same principle applies here as when the ignition points in a car engine open, and the coil discharges a spark to the spark plugs. In the converter's case, when the switch on a large load down the line opens, the resulting inductive surge, or spark, goes into the rotary motor and opens the end rings of the rotor. Lightning strikes on the line are also a problem, even at a considerable distance from the converter. Any of these conditions may result in an open-circuit between rotor bars. The formerly reliable-starting rotary then may not start.

4. Noise Problems

The High Resistance rotary is electrically unbalanced and noisy. Through the altered resistance of the rotor a phase shift is induced magnetically in the rotor. Additional fields induced by the capacitors modify this intensity. The fields actually "bristle up" so that the operation of the rotary at light load is electrically like a stick being dragged along a picket fence, evidenced by a high-pitched magnetic whine. A single-phase input above 245 volts will further exacerbate this problem, and relief from the noise does not come until several motors are running to absorb the capacitor current of the rotary and reduce the intensity of the fields.

There are people in this industry who confuse the invention of the high-resistance rotary with the invention of the rotary phase converter itself. It is helpful to remember that rotary converters have been successfully used in one form or another for nearly 100 years. It also helps to remember that the people who developed or invented the HR rotor are all dead. With all due respect, I would say that their successors' assertions about the superiority of the high-resistance rotor should also die. Here follows one good reason.

An Analogy

I once built top-fuel dragsters for "Big Daddy" Don Garlits (see Note at the end of this section). When I was building my first chassis with Mr. Garlits, I asked him how much pre-load to put in the cars. Pre-loading a chassis involves twisting the rear axle out of the alignment horizontally, to take advantage of the torque reaction when the car launched. It enabled both rear tires to get equal traction.

His reply--"Zero. I don't pre-load chassis at all." Puzzled, I asked why, since everybody in the chassis business pre-loaded their cars. Tolerating my disbelief, he pulled a large, portable sander--such as you would use to grind welds--out from under the bench. With the sander resting on its back on the floor, he lightly squeezed the trigger, barely offering any resistance to movement of the sander motor when it started.

When this switch closed, the sander jerked violently to one side, in reaction to the starting torque, but then settled in to run without restraint.

"That sander ' torqued over' like a car does when you drop the clutch," said Garlits, "but it didn't keep trying to go in a circle after the initial reaction. Now, think about that chassis. With pre-load, the car leaves the line great. Then you get to drive a twisted car that handles poorly for the rest of the quarter-mile. Before we got good tires on these cars, we had to use pre-load--we needed every advantage we could get. Now, I don't use pre-load."

Now, evaluate the high-resistance rotary based on Garlits' analysis. Like pre-load in a dragster, the High Resistance rotor is designed for the starting line. Years ago, poor dragster tires demanded pre-load in a chassis. Today, tires are better and pre-load is obsolete. At one time, the HR rotary was the only dependable choice for rotary converter self-starting. Today there are better, equally dependable methods. Why build a converter that runs hot and noisy all day, but sounds great for the first three seconds it operates?

Totally enclosed, fan-cooled low resistance rotary phase converter manufactured by GWM

The Low-Resistance Rotary

The low-resistance rotary's only disadvantage is that it is not easily brought up to operating speed. However, heating and noise problems are nonexistent when the rotary's run-capacitor bank is properly balanced to the converter's winding strength. That the rotor has sufficient material to prevent burnout is affirmed by the fact that GWM is the only converter manufacturer to offer an Unconditional Lifetime Warranty against rotor failure.

But starting must be accomplished by equipment added to the converter, and therein lies the rub. Manufacturers of converters using the HR rotor are quick to condemn any competitor who does not follow their orthodoxy--that is, they forbid the addition of a start circuit. But please notice that their arguments then are strictly confined to bragging about what their converters don't have (an extra circuit for rotary starting), rather than any reasoning of real substance. If manufacturers of high-resistance rotary converters were honest, their list of items not present in an HR rotary would include the absence of sufficient metal in the rotor to prevent heating, noise and rotor melting. These are matters that would give customers just cause for concern, were they properly informed.

Rotary phase generator with Line-Tamer™ low-inrush rotary starting feature. This starting method is more costly, but allows a low-resistance rotor to be used without exceeding utility regulations for inrush. Stable operating voltages and no heat losses are featured.

Dishonest Latecomers

In fairness to advocates of the high-resistance rotor--however misplaced their opposition to external rotary-starting circuits may be--I believe that the real target of their indignation is what one would term "casual manufacturers of phase converters." Now, every industry that can be readily copied has its wannabe builders--people who have little or no investment in production facilities, warehousing, or marketing and consumer service infrastructure. Often, literature and instructions are pirated wholesale from other, vested manufacturers (I ran into one the other day that had lifted one of our wiring diagrams for his ad)! Now, the major features of these casual manufacturers are: (A) They can sell it to you for less money and (B) The unit offered is usually (but not always) a "reconditioned" unit--translation: the builder added a single-phase motor starting circuit and motor-run capacitors to a used (or new) 3-phase motor and calls it a phase converter. Now, the user will have no reason to believe that he is not buying a carefully engineered system. But the casual manufacturer has done no research, no tests, used no oscilloscope to determine wave form under load--and he probably will not be in business in another two years to honor any warranty or to assist the buyer in repairing the unit if something fails.

The attitude among most old-line converter manufacturers is this: if these guys had to carve up the rotor in that motor like we did when we started, they would at least be forced into a somewhat technological investment in the Industry. Of course, this assumes that technology in the phase converter industry peaked with the invention of the high-resistance rotor. No engineer worth his degree would concede that assertion. But I suppose that there are still people around who think that automotive technology peaked with the Model T Ford. Simplicity, maybe--like the HR rotor--but certainly not technology.

But there is merit to the idea of a national standard for converters. However, there will always be cash-strapped and easily-led buyers surfing the Internet or searching Industrial Ad publications for a solution to their power supply problem. They won't be affected by local code requirements for a UL or ETL listed converter if no construction permit--requiring codes inspection--is issued. And the buyer who took it in the shorts will call a legitimate manufacturer--we get calls, as others do, every week--after the home-grown converter they bought won't operate their equipment and the "builder" disappears off the radar with their money. So to any readers contemplating such a purchase, listen up: A 3-phase motor connected to any static converter will perform as well as or better than a "rebuilt" converter from an Internet Junkman. Ours comes with clear instructions for this conversion and is easily connected. And we've been here 13 years.

The Newest Technology

So much for the High-Resistance rotor as a litmus test for orthodoxy.

As mentioned earlier, we use our own patented electronic starting circuit which provides stellar performance; in fact, it works so well that several competitors use it in their rotary and static phase converters. As of this posting, there are over 5000 of these printed-circuit relays in service throughout the world. Our standard rotary converter models use this device, with electrolytic starting capacitors. And what, now, shall we say about these electrolytics? Today's electrolytic motor-starting capacitors have Mylar film insulation--not paper tissue--in a sealed, ABS plastic case. Today, the electrolytic capacitors are more reliable than the poisonous, PCB oil-filled motor-run capacitors in use 25 or 30 years ago. Electrolyte is not an inferior substance. Don't forget, computers--with their need for extreme reliability--use electrolytic capacitors!

But for the customer who is still nervous, may we suggest our Line-Tamer® soft-starting rotary option? It can be furnished with no electrolytics, uses proven, garden-variety, wye-start, delta-run motor industry technology, and provides the added feature of reducing rotary-starting inrush current by 67 percent. (See "Light-Dimming and Phase Converters.") This starting circuit design has provided trouble free, low-inrush starting of medium to large rotary converters since 1987 where light-dimming on startup can pose a problem for you or your neighbors.

 Note---Don Garlits is arguably the winingest and most innovative quarter-mile drag racer who ever lived. I believe that the key to his success was Garlits' keen sense to question current theories of design in everything he laid a hand to, and not be afraid to initiate change.