Starting Synchronous Electric Motors and Rotary Converters

This photo is of an old AC generator instead of a synchronous motor but, as discussed further below, they are mechanically equivalent.

DeBruler

Synchronous electric motors were the first common type of motor since their design is equivalent to a generator. (Today, induction motors dominate.) But they are still used today for high horsepower applications or speed critical applications such as steel rolling mill stands.

Andrew Jones posted, cropped
16,000HP roughing mill electric motor at the hot mill of Arcelor Cleveland. Removed during a down turn for cleaning.
Hector Soto: I work in the cold mill at Arcelormittal Indiana Harbor. Our sync motor is 25,000 HP and runs 7 generators.
[Some comments indicate that it is a 450 rpm synchronous motor, probably at 13,200 volts. It helps run a roughing stand in the hot mill.]

When operating, the rotor inside turns at the same speed as the rotating magnetic field generated by three-phase power in the stator field windings. When the rotor and magnetic field are synchronized, these motors can develop a lot of torque. But until the rotor is rotating as fast as the field, they have very little torque. So starting them is a problem.

Steel Plant Museum of Western New York posted
Today we are sharing a photo of the 4000 H.P. motor in the 38" Blooming Mill at Republic Steel.

Jason Sessoms posted six photos of big motors used in a steel mill. The comments include more photos.

Chris Nemanic: As per OSHA, handrail height is required to be 42". That gives it some scale

 

Vincent Avila posted
Here's another from the New Orleans Water Board. As I mentioned the old pumps run on 25Hz. We have steam and gas turbines to generate 25 cycle power, but we also have these monsters. They are frequency changers before we had VFDs. Basically 2 motors on a common shaft. One is 4160 VAC @ 60 Hz and the other is 6600 VAC @ 25 Hz. When the 60 cycle motor is fed with utility power, the 25 Hz motor becomes a generator and we get our 25 cycle power. They are absolute beasts of machines.
Mark Means: Remedial EE class please. Isn't synchronous or rated load rpm of any 60 hz motor wildly different from the rpm of a 25 hz motor? If the 25 hz motor isn't being spun at the proper rpm how can it produce 25 hz power?
Stephen Folsom: So 2.4 poles?
Jonathan Stewart: Stephen Folsom that’s the ratio, now multiply by a number that yields an integer and you can calculate the number of poles each motor has, e.g., if the 25Hz motor has 10 poles, the 60Hz motor will require 24 poles.
Steve Hayes: Jonathan Stewart yes, I think that's the minimum. 25 and 60 share the factor 5. Dividing gives 5 and 12 but you need an even number of poles. So 10 and 24 it is. The shaft will rotate at 25/(10/2)=60/(24/2)=5 revolutions per second or 300 rpm. Quite slow, similar to a Francis hydro turbine, which is why the machine has to be so big for the power required.

Some comments about powering an old feed mill that used line shafts offered the following YouTube video. We are used to the frame of a motor being firmly attached to the ground so that the torque of the motor turns the load instead of the motor. In this case, the motor is free to turn. So when it starts, just enough torque is needed to turn the stator instead of the rotor. After the stator starts turning at the synchronous speed, the brake bands around the stator are tightened so that the load starts turning (note the flywheel in the background) and the stator stops turning. Note the buzzing sound when it starts. The electricity is really unhappy until the rotor speed gets closer to the speed of the rotating magnetic field.

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As mentioned at the top of these notes, the design for a motor is the same as for a generator. It is a matter of energy flow. If the kilowatts put in the wires is more than the horsepower taken out of the shaft, it is a motor. If the horsepower exceeds the kilowatts, it is a generator. The conversion between horsepower and kilowatts is 1 to 0.7457. The difference between the input and output of a rotating device is the loss due to heat and determines the efficiency of the device. An example of exploiting the dual nature of a synchronous rotating device is a pumped water storage facility. Another example is starting a rotary convertor. The rotary convertor in the video below converts 19kv 25-cycle AC @ from Con-Ed to 650 volt DC for use by the subway system in NYC. Rotary converters were made obsolete by the development of sold state rectifiers. [John L comment on another video]

To start the rotary convertor, the DC side is used as a motor to draw current from the third rail to drive the AC side up to its operating speed. (DC current is being supplied to the third rail by other rotary convertors that are already online.) Once the AC rotor is up to speed and the phase is aligned with the grid, the connection to the grid is closed. Then power through the convertor flows in the other direction and the DC side becomes a generator instead of a motor, and it supplies current to the third rail..

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I selected some tidbits from the video's comments. (The indentation has no significance.)

Chris Nicholson posted
Rows of Seimens drives for the main rolling mill motors. Each drive consists of 12 cabinets if I recall correctly. Nice and clean.
Larry Oatman: I remember the 80 inch and the 96 inch mill at National Steel were powered by synchronous motors. Three phase crucibles.
Robert Binius: Larry Oatman synchronous motors are a unique beast.

This view shows how the two functions of the big convertors share a common armature. The commuter and brushes of the DC motor/generator are on the left and the three slip rings of the AC idle/motor are on the right. The converter in the lower-left corner is what I'm used to seeing where each device has its own armature and they are connected with a shaft.

Screenshot

A third way to start a synchronous motor is to have an auxiliary motor to start the main motor.

A forth, and I presume current, way of starting a synchronous motor is to add squirrel cage bars to the armature so that the rotor functions as an induction motor during startup. Once the rotor locks into the speed of the rotating magnetic field, the squirrel cage bars are not cutting the magnetic field anymore and they will not have any induced current in them. That is, the motor switches from an inductive motor to a pure synchronous motor.

Tony Brandt posted four photos with a comment: "that rotor weighed 104T."

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I think these were synchronous motors.

Edward Sivak posted
This was 80” Hot Strip Mill LTV Cleveland Circa 1992

Dan Plute commented on Edward's post
Man I'd like to see that motor room. Biggest I worked in was this. 32 inch mill, Bethlehem Seattle.
Bob Collins: Dan Plute that motor room was 1,000 feet long end to end. It housed the main gear drives for the three roughing mills, F1 and F2 mills, and drive motors for F3 thru F6.

 

Tony Strzalkowski posted two photos with the comment: "120” roughing mill armature got smoked by a 5/8” washer (somehow) 1 million price tag. Nail in the coffin at claymont steel. A mill that has been around since 1910, It is now all black top. (Shut down in 2013) Russians (Evraz) bought us in 2008 and it was the down fall of this 160” mill. Only one on the east coast."

[A comment indicated that paper mills also used motors of this size.]

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Sean Brady posted
Operator checks General Electric duplex control panel which sounds a warning in the event of any malfunctioning of the motor room equipment. Weirton Steel Co. - 1957
[Those are motor-generator sets on the left. The mill motors needed DC so these were used to convert the grid's AC power to DC. I think the synchronous motors are on the left and the DC generators are on the right of each shaft.]
Alan Taylor: Those cylindrical things behind the mans head are probably oil immersed breakers for high voltage (above 1000V).
Motor Generator sets were used to provide DC for mill motors. To control the DC mill motor the field on the generator would be varied as well as the field on the mill motor. The sync motor provided a constant speed over a wide range of loads, Phase syncing was an added bonus to the system.
Chuck Taylor: Just looking at this picture I can almost smell the lubrication oils, the Electrical control equipment and even the wiring. Every motor room I've been in was clean like this you could eat off the floor. I don't recall what those four black panels are? anyone?
Fred Lannert: Chuck Taylor Those are Edwards Annunciators.
Merritt Burrus: it would probably surprise a lot of you to know that there are many control panels just like this one, produced by ge, still in service around the US
Jerome Wasil

A.S In electronic tech worked in Industrial electric power all my adult life.

While the machines on left may be MG sets, large synchronous motors had another common purpose back then...

Nicknamed " rotary capacitors "

Plants with large numbers of induction motors, developed a VERY low ( lagging) power factor.

Not only did Utility Companies charge more for this type of load, many other electrical problems arose if POWER FACTOR below say 60%.

Oversized expensive wiring needed, larger transformers main breakers/ fuses, etc!

Synchronous motors RAISE the power factor, just like big banks of capacitors do.

Also, the large heavy spinning rotors level out surges/ spikes in demand from other motors starting up..

Jerome Wasil

Merritt Burrus In New Orleans La where I live now, many parts of the City, are below sea level.

Therefore after any rain event, or for that matter huge storms, water must be removed by mechanically pumping,

" lifting " billions of gallons to nearby lakes.

Salient point is, this is acomplished with a series of quite old drainage pump stations, the largest of which have up to 5000Hp 25Hz synchronous motors, turning axial flow

" woods screw pumps "

As the grid supplies 60hz, two methods are employed:

The water utility ( S&WB) has very old steam turbine/ generators, and 7.6Kv gets distributed on separate poles, etc to the stations!

2... mechanical frequency converters turn purchased 60hz voltage to 25Hz.

No other motor but synchronous types could operate at the very low RPM and great torque needed to spin the giant horizontal shaft pumps.

Scott Gracie shared
I'm unsure, which dept, this motor room was in but it had to be the cleanest area of Weirton Steel.
Jerry Szczepanski: Things are getting pretty foggy after all these decades but if that really is at Weirton Steel it would have to be the Hot Mill Finishing Mill Motor Room, probably on the day before a tour by a bunch of big shots.
Philip Varner: All motor rooms were that clean at one time….

I'm saving these comments about Steinmetz from a video for future reference. If I am remembering correctly (my undergraduate degree is Electronics Engineering), Steinmetz is the one that figured out how to use complex arithmetic to do AC power calculations. The real number is the resistive power and the imaginary number is the inductive/capacitive power. Thus real power consumed fuel but imaginary power consumed just additional transmission, transformer, power meter, etc. capacity. (Update: this 28:123 video confirms that it was Steinmetz that developed phaser-based computations.)

One of the advantages of synchronous motors is that you could change them from an inductive load to a capacitive load by increasing the current in the rotor. The capacitive load offsets the inductive loads of other motors in the plant and thus reduces the power factor of the plant. And reducing the power factor reduces the electric bill.

 

Richard Thezton posted
And still there to this day. [The comments clarify that a testing building is still there.]
Kevin Carey: I love these old machines. I remember working in a copper mill with machines that were pinstriped. 10,000. amp 6 volts for plating zinc onto steel conduit.
Martin Gaworecki: Kevin Carey The electric arc furnace at Crucible Specialty Metals was 1 volt AC at 100,000 amps. The electrode was 18 inches in diameter. The entire floor shook when they inserted the electrode into the mixed scrap metal. They produce high quality metal for engine valves, tool steel (Vacuum Melt akin to makin freeze dried coffee-they sued and won a suit against a Swedish co. for patent infringement). We also produced the titanium billets that were used to make the 'tubs' for the pilots cockpit in the A-10 Warthog.

 

Facebook reel

 

Nathan Hlz posted
1920's Siemens-Schuckert motor

This video uses a "motor lab" to illustrate startup using an integral squirrel cage. I didn't bother to understand the wiring diagrams. I'm glad he mentioned that VARS is "the potential energy held in the magnetic field" because I had no clue.

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This video is basically a testament to the quality of the bearings and oil. It spends the second half coasting back down to a stop. The 1.5mw rotary convertor in this video was used until 1999. [GeekyGirlEngineer]