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View Full Version : Obscure question re 'synchros' aka selsyn..?



The Artful Bodger
10-06-2011, 09:04 PM
I have a pair of synchros, not the little instrumentation types but bigger ones that can produce useful torque.

They are about the size of a typical 1hp induction motor and are rated at 240V 50HZ.

These are interesting devices, motor like machines with a 3 winding delta stator and a single winding rotor, 5 connections, one to each point of the stator winding and one for each end of the rotor winding. They way they are used, typicially, is(mostly 'was') mains AC is applied to the rotor windings and the three points of one stator connected to three points on the other. With AC power applied nothing visible happens until one rotor is turned at which point the other rotor turns in exact synchronicty. The instrumentation synchros operate the same way but have very little torque capability.


What interests me is that I presume that with mains frequency on the rotor if 3 phase mains was applied to the stator the motor would not turn but if the frequency of the rotor or stator windings changed the motor would turn. So I could put mains on the rotor and use a VFD to turn the motor. The big attraction here is that this would be a variable speed AC motor that would still have torque right down to zero RPM? True?

Now for the question, do all three legs of the stator circuits have equal current?

macona
10-06-2011, 09:21 PM
Basically they are a rotary transformer, just like a resolver.

The position of the motor is relative to the phase difference between the rotor and the field. I am not sure what will happen if you hook it up to a vfd with the rotor on mains. it may be unstable.

loose nut
10-06-2011, 10:23 PM
It's been a long time since I had anything to do with them, 35 years give or take but they use to to be used in analog computers IE: Fire Control computers for one.

When you turn one rotor you are setting up the other rotor in an unbalanced state in regards to its magnetic field so the second one rotated until it's mag. field is balanced with the first one. It's is a simple method of transmitting data.

Using synchros, differentials and resolvers you could do real time calculations ( not like you would do in a digital computer, the voltages applied to these devices were logarithmic so combining the input resulted in the mathematical output needed, some could take multiple inputs,adding or subtracting the voltages to get a real time output which would be fed into an amplifier and out to a drive motor of some type.) to solve Fire Control problems that aim the guns on a ship. Hard to explain after so long but the theory would fill about 10 volumes.

I have know idea if they can be modified to be used as a motor. Some of them work in a similar manner to a stepper motor so maybe some sort of continuous input could be set up to drive them. No idea what there duty cycle would be like.

Have fun.:D

Don Young
10-07-2011, 12:17 AM
A synchronous AC motor is basically similar. It uses three phase power on the stator and DC power on the rotor. The three phase power creates a rotating magnetic field and the magnetized rotor is pulled around it with good torque as long as it stays locked in sync. If it unlocks, the torque decreases because at some points the fields are trying to repel the rotor back in the reverse direction.

Such a motor cannot produce torque when the rotor is stationary and for starting the rotor magnetizing current is turned off. The rotor also has "squirrel cage" windings and is started as an induction motor. I do not see how a selsyn/synchro motor could produce any torque at zero speed with balanced stator currents. There are "torque motors" that do produce continious torque at zero speed.

The Artful Bodger
10-07-2011, 01:56 AM
Thanks Don, that was one of my questions, 'are the stator currents in a synchro balanced?'

Paul Alciatore
10-07-2011, 02:49 AM
A synchronous AC motor is basically similar. It uses three phase power on the stator and DC power on the rotor. The three phase power creates a rotating magnetic field and the magnetized rotor is pulled around it with good torque as long as it stays locked in sync. If it unlocks, the torque decreases because at some points the fields are trying to repel the rotor back in the reverse direction.

Such a motor cannot produce torque when the rotor is stationary and for starting the rotor magnetizing current is turned off. The rotor also has "squirrel cage" windings and is started as an induction motor. I do not see how a selsyn/synchro motor could produce any torque at zero speed with balanced stator currents. There are "torque motors" that do produce continious torque at zero speed.

Don, to the contrary, they were intended for low speed or stationary operation and will produce full torque at zero velocity. This would be a holding torque, similar to a stepper motor that is not being stepped, but has a holding Voltage applied to one set of windings.

I imagine you could use a combination of the line Voltage and a VFD to operate them as a motor, but I suspect a standard motor would be a lot more efficient because the motor is designed to produce torque at speed while the synchros are designed to produce torque while going slow or stopped.

As for the line frequency, if they are military surplus, it may be 400 Hz because a lot of them were used on aircraft.

The Artful Bodger
10-07-2011, 04:02 AM
Paul, these are from a mobile balloon tracking radar and are either 50 or 60Hz. I think it was Marconi radar and most likely 50Hz.***


It seems to me that 50 Hz on the rotor and 50 Hz on the 3 stator windings would be stopped, higher frequency on the stator would turn it one way, lower the other. Maybe a difference of 50Hz (50Hz on the rotor and 100Hz on the stator) would have it turn at 3000RPM.


I just wish I had a VFD, time to look at the on-line auctions..:D

***Do you remember Saddam's mobile bio warfare labs?

Lew Hartswick
10-07-2011, 10:59 AM
As one who used many pieces of equipment that had them, please note that the transmitters and receivers were made different. The ones designed for receivers (motors) had damping the others did not.
There are another version that were called "control transformers" that
were not powered on the rotor but the output taken from it and used as the input to an amplifier to run a motor to "servo" the mechanism.
As in the many PPI scopes on radar sets. Then there is the differentials which had the 3 phase input - 3 phase output and the motor
could thus be phase shifted from the generator.
PS. The name Selsyn was (?is?) a trademark of General Electric.
...lew...

Blackadder
10-07-2011, 11:58 AM
we used to use the selsyn ones on the blast furnaces ( base exchange ) to detect the burden level and display it to the scale car operators so they could keep the furnace topped up they where about 200 yards apart and where arranged to motor the rods out when the large bell was lowered to charge the furnace

if I remember correctly it was 5 small bells to fill the large one and each small bell charge was either 30cwt of coke or 2 tons of ore

as I remember the stators were just connected and ac to the two rotor wires connected to the slave unit and driven units

here is s diagram/info http://www.navweaps.com/index_tech/tech-051.htm

Stuart

Don Young
10-07-2011, 10:08 PM
Thanks Don, that was one of my questions, 'are the stator currents in a synchro balanced?'

I guess I didn't make clear what I was referring to. In its intended operation mode, the currents are not necessarily balanced. It is the unbalance in the currents that transmit the position data to the receiver. My reference to balanced currents was assuming the stator of a single unit was energized by a balanced 3 phase source.

Don Young
10-07-2011, 11:07 PM
Don, to the contrary, they were intended for low speed or stationary operation and will produce full torque at zero velocity. This would be a holding torque, similar to a stepper motor that is not being stepped, but has a holding Voltage applied to one set of windings.

I imagine you could use a combination of the line Voltage and a VFD to operate them as a motor, but I suspect a standard motor would be a lot more efficient because the motor is designed to produce torque at speed while the synchros are designed to produce torque while going slow or stopped.

As for the line frequency, if they are military surplus, it may be 400 Hz because a lot of them were used on aircraft.

Paul, I agree 100%. I was referring to the OP's idea of applying three phase power to the windings to develop a variable speed motor, not to the original intended operation mode.