60hz synchronous motor on 50hz?

[paulj]

Hello, I would like to start a tesla coil project with a real synchronous motor, (no modified asynchronous motor)

However, they are rare and very expensive in single phase.
I know that in the USA the teletype motors are used but only work in 60 hz 120v, but with us, it is 50 hz 230v.
``
I can change the voltage from 230 to 120v easily, but will running the motor at 60hz guarantee me a synchronized rotation speed?

for example is it on a synchronous motor of 3600 rpm in 60 hz, its rotation speed will change to a synchronized speed in 50 hz (2500 rpm or 3000 rpm?)

Thanks in advance for your answers, Paul

[klugesmith]

Yes.   
It's like the synchronous motors that rotate MWO turntables
at 6.0 rpm (60 Hz) or 5.0 rpm (50 Hz) with same gears.
So you know what cook-times will return mug handles to their original orientation.

I have a Teletype motor & would test it for you on 50 Hz power, if I could find a source for that.  There might be one at work.  Decades ago, to test a 20,000 watt product for use in Europe, they rented a motor-generator set with belt drive to change 60 Hz to 50 Hz.

My motors collection includes a synchronous unit from Ampex, that turns at 1800 or 3600 RPM depending on which stator coil is energized.

[davekni]

I can change the voltage from 230 to 120v easily, but will running the motor at 60hz guarantee me a synchronized rotation speed?

Probably.  A 60Hz synchronous motor will generally work at 50Hz at 5/6ths speed.  Voltage should be reduced too, theoretically to 5/6ths, or 100V for a 120V motor, to maintain the same current draw and internal heating.  (Due to losses, you may want a bit more than 100V, as long as the motor doesn't overheat.)  If it is a capacitor-run motor, the capacitance should ideally be somewhere between 6/5ths and 36/25ths of the normal 60Hz value, depending on winding resistance and inductance.  If the motor uses a shaded pole or other resistive technique (no capacitor), starting torque may be reduced some at 50Hz.

[paulj]

I am going to order a teletype motor.
To test the sycnhronization I will use the technique of Terry Blake with a colored straw on the rotor and a fluorescent bulb to check the synchronization.
i use a 100-105-115-120v transformer, or variac, if that doesn't work i know its motors are finicky to start.

Thank you for your answers, I will keep you posted next month when I receive it.

[304er]

Hi all,

Here is a couple of pictures of a teletype motor my father and I have.
It was a little hard to get a good picture of nameplate info stamped direct into round motor body, I think it is readable zoomed in...hope this helps some...

https://photos.app.goo.gl/jVKkMUn9dUQM5yAK9

Chris Reeland
Ladd Illinois USA

[plasma]

Hi all

You can get a VFD, variable frequency drive from 1-200Hz to control the speed of induction motors, they take care of the current etc.

Make sure you check if the motor is 2-4-8 pole 8 being the slowest rpm.

[davekni]

Here is a couple of pictures of a teletype motor my father and I have.
It was a little hard to get a good picture of nameplate info stamped direct into round motor body, I think it is readable zoomed in...hope this helps some...

The nameplate lists 50Hz and 60Hz operation, along with the corresponding 3000RPM and 3600RPM.  Should work fine.

[MRMILSTAR]

A VFD is not useful for powering a synchronous motor for a SGTC synchronous rotary spark gap. Even if you manage to find a speed (frequency) setting that yields the exact desired RPM, the spark gap rotor electrodes rotated by the motor will very likely not be in proper phase alignment with the power line waveform voltage peaks when they fire. Even if you got lucky and managed to get it in-phase with the power line voltage peaks, the motor speed will slowly drift depending on the precision of the timing circuitry in the VFD thus eventually taking it out of proper phase alignment with the power line.

You actually need a synchronous motor or a normal induction motor modified to be a synchronous motor in order to remain phase-locked with the power line current. Either motor must be connected directly to the power line.

[klugesmith]

Here's a little synchronous motor whose nameplate has speeds & powers for 60 Hz and 50 Hz.
It has a permanent magnet armature, as does the Teletype motor (but not all synchronous motors).

[paulj]

hello,
I ordered a teletype engine, I'll see what happens.
I'll take some pictures for you. 
If it works, I will use it for a really big sgtc that I have planned.

I really like the spark gap coils for their very simple operation;)

[plasma]

MRMILSTAR

I assumed there were two sparkgaps and a DC high voltage supply, with the two electrodes alternate creating AC.

A one sparkgap system would be easier but need synchroniznation.

The former could work at more frequency range?

[MRMILSTAR]

A DC SGTC with a RSG requires no synchronization and can run at any RPM consistent with the other electrical components.

[johnnyzoo]

A VFD is not useful for powering a synchronous motor for a SGTC synchronous rotary spark gap. Even if you manage to find a speed (frequency) setting that yields the exact desired RPM, the spark gap rotor electrodes rotated by the motor will very likely not be in proper phase alignment with the power line waveform voltage peaks when they fire. Even if you got lucky and managed to get it in-phase with the power line voltage peaks, the motor speed will slowly drift depending on the precision of the timing circuitry in the VFD thus eventually taking it out of proper phase alignment with the power line.

You actually need a synchronous motor or a normal induction motor modified to be a synchronous motor in order to remain phase-locked with the power line current. Either motor must be connected directly to the power line.

I agree, but as usual, I got some additional ideas...

Are there advanced motor/servo drives that can be controlled by giving an angle reference? If mains voltage (reduced via transformer and possibly processed somehow) were fed as a reference signal, such a drive would then synchronise the motor speed to the mains frequency. Some jitter might remain but maybe not too much.

Motor type would not matter but it must of course be compatible with the drive system. 

This would of course be rather complex, such drives are probably not common or cheap anyway and it would probably need an encoder for precise control.

Phasing the spark might not be a big problem as it must be checked mechanically anyway, even with true synchronous motors, right? Adjusting the angle reference electrically might actually be easier.

[MRMILSTAR]

Feedback phase control of a RSG is certainly possible. The potential problem is that Tesla coils, and SGTCs in particular, are not kind to nearby electronics. Sensitive electronics can fail or behave in strange ways.

[davekni]

Yes, phase-locked control is possible.  I built two motor drives for my SRSGTC.  First used a standard synchronous motor.  The second used a DC motor with a home-built speed controller locked to line frequency.  That allows simple remote phase adjustment with a potentiometer.  For both motors/drives, I found it necessary to use a separate outlet for the motor, not shared with the outlets used for SRSGTC power.  (Coil power came from two 120V outlets to get more power, one MOT per outlet, series connected secondaries.)  If the motor ran on one of the power outlets, voltage drop induced by the main coil power changed motor phase, which changed power drawn, etc.  Caused unstable feedback.

[paulj]

Hello, I just got the motor, it's a 5sc19lg3b teletype synchronous motor

voltage: 115v
current: 2.0 amp
rpm: 3600
hp: 1/20
cy: 60
it's single-phase, however it has 3 wires (surely to make it run at high speed) and I don't know how to wire it, I measured the inductances of the three wires:

red / green: 38 mH
yellow / green: 17.66 mH
red / yellow: 21.5 mH

Could you confirm for me the direction of connection without my risking to burn it?

thank you

[klugesmith]

Maybe it needs a capacitor.   Does nameplate mention a capacitance value?



I would suggest C between your red and green wires.   AC power, one side to yellow and the other to green or red (for opposite directions of rotation).

[paulj]

Maybe it needs a capacitor.   Does nameplate mention a capacitance value?

I would suggest C between your red and green wires.   AC power, one side to yellow and the other to green or red (for opposite directions of rotation).

no unfortunately, I have no information, I can try to contact the GE archives to have a datasheet, but I don't think they will answer me (especially with my English translated from google).

[klugesmith]

I can dig out my Teletype motor and report about capacitor and winding inductance.  Maybe not in next 24 hours.

[paulj]

I can dig out my Teletype motor and report about capacitor and winding inductance.  Maybe not in next 24 hours.

I think I found it!
https://www.navy-radio.com/manuals/tty/1167B-ch7.pdf
it is a 43 to 48 mfd capacitor
thank you GE archives!
but no information on the connection of the wires, nor the voltage.

Can you measure the coil inductances from your motor, could this help me with the wiring?