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Synchronouse motor with Adjustable speed magnetic drive

Synchronouse motor with Adjustable speed magnetic drive

Synchronouse motor with Adjustable speed magnetic drive


Continuing my understanding of synchronous motor applications as I recently posted above I was in a water treatment plant recently and came across a 4.16kV 840Hp brushless synchronous motor that was connected to a Ampli-Speed Adjustable-Speed Magnetic Drive which then in turn was driving a large water pump

I have never seen one of these adjustable speed "magnetic drives" so I'm curious as to how they work in conjunction with a synchronous motor in this application. Is the speed of these magnetic drives somehow controlled in order to control the speed of the pump?

The one thing that about the application that stuck me as being odd is that I did not see any sort of excitation control for the synchronous motors. These motors were fed from a traditional fused contactor with a basic overload relay. There was no sign of any sort of excitation control or projection for the synchronous motor.

Very close to the Motor start lineup I did see a control cabinet with EMICC controllers that the tech I was with mentioned were used to control the speed of the motors. From looking at these controllers and talking to the tech it appeared that these controllers were used to control the speed of the magnetic drive but not necessarily the motor itself. Is it possible that this control system can somehow provide control to the magnetic drive as well as excitation voltage to the synchronous motor?

Has anyone ever seen an application like this that can explain how it works.


RE: Synchronouse motor with Adjustable speed magnetic drive

It could be a synchronous motor with an amortisseur winding. I could also be a machine with brushless excitation and a thyristor bridge to short the rotor winding during start.

The magnetic speed variator works on variable slip (eddy currents). Less magnetization produces more slip and lower speed. It has as much losses as a wound rotor machine with a rotor resistor for speed control. Shouldn't be used with constant torque loads, but can be tolerated on square law pumps and fans.

Gunnar Englund
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Synchronouse motor with Adjustable speed magnetic drive

The magnetic drive basically works as a slip clutch. They are very durable drives but the slip causes energy to be dissipated as heat. They are appropriate for fans and pumps where a small change from full speed (low slip) produces a large change in flow and power consumption. At 50% of full speed (50% slip) the drive would be only 50% efficient. In some applications, reducing a pump's speed to 90% might reduce the flow to zero.
The type of motor and how it works is a pretty much unrelated issue.

RE: Synchronouse motor with Adjustable speed magnetic drive

If, as Gunnar said, it is a brushless synch motor where the bridge is in the rotor, there must still be a way of transferring power to it. This is typically done with an induction coil on the stator frame, so it basically acts as a transformer across the gap between stator and rotor, then on board the rotor is a static bridge rectifier to make the DC excitation. In the stationary side, you simply do not power the excitation until the motor is at pull-in speed. The is typically what you see og GE Synch motors, so if it is a GE, it's almost guaranteed to be like that. The excitation control components are very simplistic, you may not have known what they were, and certainly if the entire package was built by the magnetic drive mfr, it could be buried in the electronics for the mag drive. Remember, from their standpoint they are not likely doing anything fancy with the synch motor like power factor control, all they need it to do is accelerate and pull in so they can do their magnetic thing. They used a synch motor because it started off being lower speed, which was important for some applications, plus it helped with the overall efficiency issues compared to using a high pole count on an asynchronous motor.

When MV VFDs were virtually unheard of or prohibitively expensive, these magnetic clutch drives were often one of the few viable options for large loads. They make almost no sense now because of the efficiency issues. Another alternative that has become almost extinct now are Liquid Rheostat drives with Wound Rotor Induction Motors. They are still out there for sure, just fewer new installations every year.

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: Synchronouse motor with Adjustable speed magnetic drive

Long ago I read a fascinating account of the patent fight over the Polarized Field Frequency Relay.
Prior to the development of the PFFR an operator would apply the power to the stator of the synchronous motor. When the operator judged that the machine was up to speed, he energized the field.
Sometimes the motor was damaged. If the field was applied at the wrong polarity relative to the angular position in the stator, severe and sometimes damaging transient torques would be developed.
The polarized Field Frequency Relay was energized by the voltage induced in the field winding. As the slip frequency droped, so also did the frequency applied to the PFFR. The magnetic circuit of the relay was polarized by a DC coil. As a result, the magnetic flux in the relay was offset. At the operating point, one half cycle of the magnetic force would be offset to zero or close to zero. At this point, the relay would drop out and a set of normally closed contacts would close and apply the DC to the field. Due to the polarization, the field would be applied with the correct polarity to avoid damaging transient torques.
I was under the impression that a brushless synchronous motor had a rotating circuit that inhibited the application of the field until the field was in a position to avoid transient torques.
Is this no longer a feature of brushless synchronous motors??

"Why not the best?"
Jimmy Carter

RE: Synchronouse motor with Adjustable speed magnetic drive

We have quite a few large synchronous motors 10,000 to 22,000 horsepower, driving centrifugal compressors in service on a natural gas pipeline. We vary the speed on the compressors to meet operational requirements by using Voith Vorecon hydrodynamic variable speed drives. We run the motors at nominal 60 Hz - 1800 RPM and vary the output speed of the drive in the 11,000 RPM range.

The motor-transmission-compressor combination has worked well for us.

old field guy

RE: Synchronouse motor with Adjustable speed magnetic drive

Hey Rockman,
I haven't seen an eddy current coupling (ECC) drive since the late 70's, even then it was 15 years old. It probably should be in the Smithsonian.

The following blurb I found on Wiki:
Eddy current drives
An eddy current drive consists of a fixed speed motor and an eddy current clutch. The clutch contains a fixed speed rotor and an adjustable speed rotor separated by a small air gap. A direct current in a field coil produces a magnetic field that determines the torque transmitted from the input rotor to the output rotor. The controller provides closed loop speed regulation by varying clutch current, only allowing the clutch to transmit enough torque to operate at the desired speed. Speed feedback is typically provided via an integral AC tachometer.
Eddy current drives are slip-controlled systems the slip energy of which is necessarily all dissipated as heat. Such drives are therefore generally less efficient than AC/DC-AC conversion based drives. The motor develops the torque required by the load and operates at full speed. The output shaft transmits the same torque to the load, but turns at a slower speed. Since power is proportional to torque multiplied by speed, the input power is proportional to motor speed times operating torque while the output power is output speed times operating torque. The difference between the motor speed and the output speed is called the slip speed. Power proportional to the slip speed times operating torque is dissipated as heat in the clutch

Is this still in regular use at the facility that you visited? If so, they should consider how much energy (ie $$$) is being consumed; I'm sure if they did a review of the energy costs that the ECC could be available for the Smithsonian.


"Wish I didn't know now what I didn't know then." -- Bob Seger

RE: Synchronouse motor with Adjustable speed magnetic drive

Thanks for all the great responses

Yes this magnetic drive and (4) others like it are currently in use on large Effluent pumps in a waste water plant. These Effluent pumps are large pumps used to pump the treated water some distance out into the ocean. I've attached a picture of the magnetic drive nameplate. There are (5) of these pumps currently in service which all have the same synchronous motor / magnetic drive combination. The plant was built in the 70's so perhaps these somehow remained in service from that time period.

The motors are 840 HP Synchro-Pac Brushless Synchronous motors which appear to be made by E-M (Electric Machinery) part of Dresser-Rand's motor and generator division. I'll post a nameplate picture in following post as I don't know how to post two photos in the same post.

I'm still educating myself on synchronous motors but based on what was said above it sounds like this brushless sync motor had an internal thyristor bridge on the rotor in order to provide the DC field current? It sounds like in this case the power factor cannot be controlled as the excitation current is being provided by the fixed thryrisotor bridge and may be a function of the induced voltage on the "induction coil"? Not quite sure how this works? Is the power factor in this case then determined by loading similar to an induction motor? It sounds like there really isn't much excitation control going on here (no need for fancy motor relay) and the only control is really for the magnetic drives which reside in a separate enclosure from the motor starters.

I'm not sure I understand why a synchronous motor was used due to the low speed application? Are synchronous motors known for being able to produce low speeds rather than getting a high pole count with induction motors?

GroovyGuy - You bring up an interesting point about wasted energy and potential cost savings from getting rid of the magnetic drive. Would the magnetic drive simply need to be replaced with a MV VFD in order to control the motor and pump speed? Are there any references that you are aware of that discuss the wasted energy from the magnetic dries and the cost savings for eliminating? The plant also recently had slip recovery drives on other large pump motors which they recently eliminated to replace with MV VFD's so perhaps there may be a similar opportunity here.

RE: Synchronouse motor with Adjustable speed magnetic drive

It looks like a similar thing is still marketed today... as a "breakthrough technology".

(2B)+(2B)' ?

RE: Synchronouse motor with Adjustable speed magnetic drive

Ah yes, Ampli-Speed... they use what I still think is the coolest, yet corniest, trade name for their control system; "Regutron". The name makes me think of the Transformers comics / movies, with "Regutron" being the bureaucratic robot that determines the rules by which the Deceptacons and the Transformers fight their battles...

I once got in a battle with E-M over their wanting to put one of these systems as a soft starter on a "pulper", a machine in the paper industry that grinds the wood chips (for lack of a better term). They made all kinds of verbal claims to the user about how efficient their system was, but never provided any actual documentation of it. They couldn't of course, because it is not efficient when used as a soft starter... in fact it's not efficient for speed control either, unless you compare it to a gearbox or mechanical vari-drive, which is of the same vintage this technology is. So IN IT'S DAY it may have been a more efficient choice than whatever else was available, but that hasn't been the case for 25+ years now. And yes, electricpete, the Mangnadrive is exactly the same thing, with the same problems. If you scan through their website, you will notice they too are very carefully avoiding talking about the ACTUAL efficiency of their product. They say something like "Up to 70% efficiency savings on some applications...". Yes, if it's on a centrifugal pump and you drop the speed, the energy required drops faster than using a throttling valve. That's the same thing you get with a VFD. But the magnetic clutch is LESS efficient than using the VFD, that's why they avoid discussing it.

E-M was old to Converteam years ago, then when GE bought Converteam, they became redundant to GE's synchronous motor division, so GE sold them again to Weg in Brazil. They are still around, I think as "Weg E-M" or maybe just "WEM". As far as I know, they still sell the Ampli-Speed too, along with Regutron!

You can still control the PF on the brushless synch motor, you just control the AC voltage with a Phase Angle Controlled SCR controller on the stationary side. You may not have that however, because like I said, as just part of the pump control package they may not have needed to worry about that. So all you may have is a small transformer in the control panel, feeding AC through a contactor to the Exciter Field Coil on the stationary side of the motor.

This diagram is as close as I could find for you, but it's very similar.

What's likely different is that you probably don't have the Field Discharge Resistor on the rotating element, instead you have a contactor on the stationary side, and they just don't apply power across the gap to the exciter until after the motor is ready for it.

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: Synchronouse motor with Adjustable speed magnetic drive

"I'm not sure I understand why a synchronous motor was used due to the low speed application? Are synchronous motors known for being able to produce low speeds rather than getting a high pole count with induction motors?"

The motor could be either synchronous or induction, but high pole count induction motors tend to have very low power factor and consequently they have high line currents for the power output. In large installations this requires over-sized supply transformers, cables and switchgear which increases the capital cost, plus whatever penalty the utility imposes for low power factor and the cost of this over the life of the installation. A synchronous machine avoids these problems, at the expense of a more complex and slightly less robust motor.

RE: Synchronouse motor with Adjustable speed magnetic drive

Hi Scotty,
Synch-motors are generally less expensive in low rpm applications, and-or large motor power ratings.


Excerpt from:

"Wish I didn't know now what I didn't know then." -- Bob Seger

RE: Synchronouse motor with Adjustable speed magnetic drive

Of course, cost! smile

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