Synchronous motors

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Tony S

Senior Member
This is a bit of a follow on from the “slip” thread.

Something I’ve never understood is how a self starting synchronous motor gets itself locked in to synchrony.

I can understand how a five ring motor starts, it’s just a slipring motor when it starts and then the salient pole current is introduced on the separate rings to pull it in to synchrony.

It’s the three ring motors that confuse me. To start a slipring motor you gradually short the rotor how then do you apply DC to shorted rings? Is there a break in the rotor circuit during transition? If there is, locking in to synchrony must cause one hell of a mechanical jolt.
 

Jraef

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This is a bit of a follow on from the “slip” thread.

Something I’ve never understood is how a self starting synchronous motor gets itself locked in to synchrony.

I can understand how a five ring motor starts, it’s just a slipring motor when it starts and then the salient pole current is introduced on the separate rings to pull it in to synchrony.

It’s the three ring motors that confuse me. To start a slipring motor you gradually short the rotor how then do you apply DC to shorted rings? Is there a break in the rotor circuit during transition? If there is, locking in to synchrony must cause one hell of a mechanical jolt.
The slip rings are not for the same purpose as in a wound rotor motor. They are for applying DC to the rotor poles. You don't accelerate it the same way as a WRIM, so you don't involve the slip rings in that process until the very end. The motor is accelerated (usually) with a secondary winding in the stator that is used when running to dampen any load pulsations, but can be used as a regular induction motor. It's called the "amortissuer" winding (from the French for "deaden"). Just like any other asynchronous motor, it runs at a slip speed (Oh Oh, here we go again), about 97% of sync speed.

The very end of the starting process is called "pulling in" and basically when you get to the limit of the amortisseur winding's slip speed, you apply the DC through the slip rings to the rotor field via the Field Application Relay and the magnetic poles in the rotor sync up to those in the stator. "Hell of a mechanical jolt"? Well, I don't know what that means. It's noticeable, but not a big hairy earth shaking deal. You basically CANNOT start a synchronous motor with any kind of appreciable load on it, they are almost always used with a clutch, or if on a pump or fan, with no flow yet. The synchronous motor will have a "pull in torque" rating, and that's what that rating is telling you; the amount of torque the motor is capable of delivering at that crucial moment. You have to make sure in your design that you have that torque in reserve (against your load requirement) so as to not prevent the motor from pulling in. Conversely, after the sync motor is running under load, there is a "pull out" torque rating, being the amount of extra torque the motor can handle before the load causes it to slip a pole, which is a bad thing. So a synchronous controller / protection relay will be monitoring all of this as the process takes place and shut down before anything bad happens.
 

__dan

Senior Member
I had a student labor job in the basement of Castleman, Eng I, before the renovation when it had the DC lab in basement.

Pretty sure everything at that time was Depression era, built in the 1930's. The DC lab had several engine dynomometers, all unused as they had not had anyone to clean or maintain. They had trouble and it was not safe to operate. It was a space no one went into. My boss, an EE, was across the hall and he had me go through the place.

So in a different room they had the DC power plant, two big MG sets and the DC distribution board, all exposed heavy black slate with the old contactors and copper knife switches mounted right to the face of the board. Board was ~ 8 ft tall 40 ft long, no guarding, cages, blast shields, nada. Just slate, copper busbar, and a little insulated place for your hand to throw the switch.

DC MG sets had to be in the 200 kW range. Start winding, run winding, and a synch field. They must have been started as induction motors and then swapped over to synchonous when the synch field was applied.

So to start them there were two huge mechanically interlocked handles, a two handed full body operation. You would throw the start winding switch and watch the gauges, peak current in the 300 range. When current stabilized you would simultaneously return the start handle and throw the run winding switch. You had to practice the throws with the mechanical interlock, this was stuff from the 30's, pretty cool and very dangerous. When people asked me about it I told them not to touch it.

After the run winding had stabilized you could apply the synch field and there would be another much smaller current surge as the slip went to zero, it would go bvzzzzzt. It had an adjustment for the field current and you could run it leading or lagging power factor, rotating capacitor or inductor.

I made repairs and got the dynomometers working. Never saw sparks but my boss closed an old DC molded case breaker into a dynomometer that had a V 8 gas engine attached to it. The dnyo had lost its field so the armature was just a direct short between the brushes. All he could talk about was the blue flame that lept out of it, heard stories about the look on his face.
 

Sahib

Senior Member
Location
India
"Hell of a mechanical jolt"? Well, I don't know what that means.

The synchronous motor pulls into synchronism when, for example, the rotor N poles are very close to stator S poles. But imagine the field supply is switched on at a moment such that the rotor N pole is opposite to the stator N pole. This would result in a "Hell of a mechanical jolt".
The starter controller should switch on the field supply at precise moment to avoid this.
 

Jraef

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The synchronous motor pulls into synchronism when, for example, the rotor N poles are very close to stator S poles. But imagine the field supply is switched on at a moment such that the rotor N pole is opposite to the stator N pole. This would result in a "Hell of a mechanical jolt".
The starter controller should switch on the field supply at precise moment to avoid this.
Hence my statement at the end. Synchronous controllers are complex pieces of equipment. In days gone by much of that was done with electromechanical and magnetic relays. Now it is done with specialized electronics. But either way, there are numerous bad-outcome scenarios that needed to be monitored and protected against.
 

Tony S

Senior Member
So I need to look up amortisseur windings, thanks JR.

I worked on a cement plant where the old plant had 4 x 1500HP synchronous mill motors. These were five ring with conventional liquid resistors for starting. Because the new plant had larger more efficient mills I never got to see the synchronous motors starting, mechanically three were beyond economical repair.

My only dealings with working synchronous machines has been rotary converters. Starting them used to put the fear of god in me as the switchgear was out of the ark.
 

Jraef

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Gentlepeople,

Has anyone on this forum "fiddled-around" with a Super-Synchronous Motor?

Regards, Phil Corso
Isn't that the same thing as a doubly fed wound rotor INDUCTION motor? As far as I was aware, the term "super synchronous" is not related to it being a "synchronous" motor as per the topic of this thread, but rather an induction motor that is USED at super-synchronous speeds, meaning ABOVE the base motor speed.
 

Phil Corso

Senior Member
Jraef...

No.. it had nothing to do with induction motors! Instead it's a self-starting synchronous motor. Following is its desciption taken from Prof. I.L. Kosow's book, "Electrical Machinery and Controls"! I "fiddled" with two in my very 1st job as a EE... a long, long, time ago!

Super-Synchronous Motor (Prof. Irving L. Kosow)

The super-synchronous motor does not operate at hyper-synchronous speed, but its title is a misnomer. It would have been better to have called it a super-torque motor. The motor was developed by General Electric in order to provide a synchronous motor which was self-starting under heavy loads. The super-synchronous is described as a special construction having five slip-rings and employing a wound-rotor in combination with the d-c field winding.

It is a well known fact that unless equipped with an "Amortisseur" winding, a synchronous-motor has no-starting torque, but, the super-synchronous motor was designed to take advantage of the fact its Pull-Out torque is between 250 and 300 per cent of full-load torque. The super-synchronous motor is capable of developing that torque on starting, but in a unique manner.

It requires a special construction, however, and it is probably the costliest motor of its kind for a given horsepower rating between 400-500 Hp, at 440V. The rotor is the standard cage-type rotor with the d-c field winding brought out two (2) slip-rings on the rotor shaft. It is coupled directly to the mechanical load which it must drive.

The entire stator, however, is free to rotate on trunnions, in the same manner as an a-c dynamometer. But, whereas the latter is limited in its angular displacement, the stator of the super-synchronous motor is free to rotate on bearings at synchronous speed. The stator-armature winding, therefore, is also excited through three (3) slip-rings and is usually started at a reduced voltage by means of three-phase reduced-voltage methods.

Its uniqueness is that a large brake is provided around the outside of the stator-frame to apply a braking action and to secure the stator in its running position. Because the rotor is coupled to the load, when a reduced polyphase a-c voltage is applied to the stator with the brake released, the induction motor torque produced by the rotor poles reacts against the "stator" conductors; this reaction imparts to the stator a torque that is opposite in direction to the direction of rotation of the load

The stator picks up speed as the a-c stator voltage is increased; and, as the stator reaches synchronous speed, full supply voltage is applied in addition to the d-c field excitation. The stator pulls into synchronism with the rotor at a standstill, held by the inertia of the fixed heavy load coupled to its shaft. At this instant, the motor is operating as a synchronous motor without load, generating a counter-emf which limits its stator current.

The brake is now slowly applied to the rotating-stator. Since a synchronous-motor must run at synchronous speed, the reduction in stator-speed must be made up by rotation of rotor-speed in the opposite direction, i.e., for a synchronous speed of, say 1,800 rpm, a stator-speed of 1,790 rpm counter-clockwise requires a rotor-speed of 10 rpm clockwise.

The torque-angle, therefore, increases to provide maximum torque, i.e., pull-out torque, in starting the heavy applied load. The armature-current, although high, is limited by the generated emf in the stator. Reducing the speed of the stator by increased braking increases the speed of the rotor, until the stator is at a standstill and the rotor is rotating with the full applied load at synchronous speed.

And now, you know the rest of the story!

Regards, Phil Corso

Ps: Its application reduced machinery-train length by several feet, because it eliminated an electrical-coupling between the motor, and the drive-machine!
 
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Jraef

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Cool. I had no idea that's what these were called. I've seen this one on YouTube and thought it was really cool to watch, but there is no good description of what it is. Thanks for bringing that to light.
http://youtu.be/fNuI6keQXYA

Not the same, but a few years ago I got a chance to help an old local farmer refurbish his workshop. It was all driven by a single 7-1/2HP antique motor and a complex system of leather belts going to different machines, all custom built by him in his youth. The motor was a huge single phase repulsion start induction run unit made by Century Electric in the 1920ish era. You can see them on videos now, but they always much more interesting to watch in person. The videos can't do justice in capturing the centrifugally driven movement of the brushes that takes place just inside of the end bell, but in person you can watch it. We are all used to seeing the rotor as the only moving part in a motor, adding another coordinated movement in there is a treat. Technically something like that is taking place inside of a cap start motor, but we don't get to see it and it only takes a second. In a repulsion start motor that takes a long time.
 
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donaldelectrician

Senior Member
Cool. I had no idea that's what these were called. I've seen this one on YouTube and thought it was really cool to watch, but there is no good description of what it is. Thanks for bringing that to light.
http://youtu.be/fNuI6keQXYA

Not the same, but a few years ago I got a chance to help an old local farmer refurbish his workshop. It was all driven by a single 7-1/2HP antique motor and a complex system of leather belts going to different machines, all custom built by him in his youth. The motor was a huge single phase repulsion start induction run unit made by Century Electric in the 1920ish era. You can see them on videos now, but they always much more interesting to watch in person. The videos can't do justice in capturing the centrifugally driven movement of the brushes that takes place just inside of the end bell, but in person you can watch it. We are all used to seeing the rotor as the only moving part in a motor, adding another coordinated movement in there is a treat. Technically something like that is taking place inside of a cap start motor, but we don't get to see it and it only takes a second. In a repulsion start motor that takes a long time.


Jraef


That is the coolest Motor I ever saw ….


I thought if he did not turn the wheel fast enuff , it will take off ….



Don
 
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