Starting a synchronous motor

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Open Neutral

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We know that motors have a rotor and a stator; the rotor rotates while the stator stays there, RIGHT?
And since synchronous motors have less starting torque, you need a clutch, too.
Or that's what the Professor said......

Another synchronous motor:
BSTJ40-6-1627 describes an M-G set for ATT Long Lines microwave stations.
Go to section 3.1, 2nd paragraph for a DC-less design.
 
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Years ago my company purchased a used 125 HP Ammonia compressor that had a synchronous motor. We sent the motor out to be repaired and went thru controls but this motor would only start if the compressor was unloaded. Refrigeration guy had to forget if it was open or close valves. Once it got up to speed we left it running all day. The other ammonia compressors ran off plain Jane three phase induction motors and cycled on & off. Not sure if newer controls will allow it but some how you could run synchronous motors to produce a power factor over 1.0. ( think 50 plus years ago in Vo Tech school might have called that over exciting the motor ).That was the only synchronous motor that I came across during my 50 years as an industrial electrician. Often wonder if some particular manufacturering process still use them.
 
zbang said:
Synchronous condenser, doesn't have a power factor greater than 1.0, it has a leading power factor, you get this by overexciting the field. Some plants would operate one of these unloaded just for the PF correction.
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This was common in steel mills last century before capacitors became more viable. The motor only powered its own cooling fan.
 
Open Neutral said:
And since synchronous motors have less starting torque, you need a clutch, too.
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Well, it depends. Synchronous motors don’t self start, so they have what is essentially an induction motor cage built-into the rotor, called the “amortisseur” winding, that takes it up to about 90% speed, then the field is excited and it “pulls in”. Once running, the amortisseur winding then dampens out minor speed fluctuations due to load changes. In theory they can build that amortisseur winding to deliver as much torque as you need, but since most synch motors are large, having enough available power to do so is often way more expensive than just decoupling the load, either with a clutch or an unloader valve etc.
 
Jraef said:
Well, it depends. Synchronous motors don’t self start, so they have what is essentially an induction motor cage built-into the rotor, called the “amortisseur” winding, that takes it up to about 90% speed, then the field is excited and it “pulls in”. Once running, the amortisseur winding then dampens out minor speed fluctuations due to load changes. In theory they can build that amortisseur winding to deliver as much torque as you need, but since most synch motors are large, having enough available power to do so is often way more expensive than just decoupling the load, either with a clutch or an unloader valve etc.
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The you-tube's version of "unloading" was what drew my attention. I'd never encountered such.

I've also never heard the word amortisseur before, thanks. But it's been many decades since Machines 403 class.

If you review the BSTJ link, you'll find a explanation of the induction rotor acting synchronous-ly that surprised/confused me.

BTW, my casual understanding is a synchronous motor has a higher efficiency than an induction motor with the same output power; I'd welcome your view on that.
 
Synchronous means it has no slip. The rotor magnetic field is fed in from an external source (in the motor but not induced in the rotor) with brushes and slip rings is possible. Rotor follows (or generates) the field exactly in synch, 1800 rpm for a 4 pole machine and 60 Hz.

The state university, in the basement of Castleman, Engr I, probably built in the 1930's, they had a DC lab in one quarter and a DC powerplant in the middle section. All surface mounted exposed busbar with exposed surface old 1930's or so breakers and knife switches, two 50 kW DC mg sets. like 8 ft tall 40 ft of black slate with everything front mounted. It's all gone now, replaced with debris dry storage. Could have made great antiques or historical artifacts

I did two semesters as student labor job there, had the E2 a few weeks after starting the semester. My boss's office was across the hall from the DC lab. It was unused for years for no maintenance and things did not work but I worked on it and got things working. Two engine dynomometers and a wind tunnel. At that time no one cared the exposed busbar was not grandfathered.

Starting the 50 kW mg sets with the 1930's equipment was a full two handed throw. Two 1930's handles almost as big as axe handles with interlocks. First was the start winding, current would come up to 180 or so, then the other handle was the run winding. There were interlocks and you had to synch the mechanical throw.

That started on induction only and when the run winding was in you could throw a small lever for the (rotor) synch field. And there was another knob to adjust current flow through the rotor synch field to adjust power factor, with a meter that showed the power factor.

I knew the hazards but that was before clothing became a thing. My EE boss gave me a copy of Chapman Electric Machinery Fundamentals and put me to work. Should have been a 3 credit course at least.
 
I know "no slip" but that does not paint the whole case of efficiency to me. I suppose the most obvious way of thinking about it is: how hot does each induction vs. sync. motor get? That wasted heat must be the majority of the loss.

__dan said:
The state university, in the basement of Castleman, Engr I, probably built in the 1930's, they had a DC lab in one quarter and a DC powerplant in the middle section. All surface mounted exposed busbar with exposed surface old 1930's or so breakers and knife switches, two 50 kW DC mg sets. like 8 ft tall 40 ft of black slate with everything front mounted. It's all gone now, replaced with debris dry storage. Could have made great antiques or historical artifacts
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Much of the circa 1915 25Hz/12.5 KV PRR/Amtrak, still in daily use, system between NYC and DC makes your gear look almost new. From what I've been told, you could remake "Young Frankenstein" in their space.
 
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