They won't turn.

Maybe you figured that out on your own and your wondering if this will hurt the motor.  I suspect you would get locked rotor current with no torque causing overheating in very short time.  If the loads aren't critical it would probably be best to trip the motors before transfering to single phase supply.  This would be kinder to the generator and motors.

Some validation of my locked rotor current comment would be appreciated.

If it's a delta wound motor then current will be zero.
If it's a wye-wound three-phase motor then no current will flow unless you hook up a ground to the neutral.

That part was probably an obvious unstated assumption.

Definitely won't start rotate (although if you started it rotating with external force it might concievably stay rotating similar to single-phase motor).

I believe it's possible that it might draw slightly lower  current than a locked rotor current.

The flux path is much different.  In normal 3-phase we have flux from one pole group a,b',c acting together with max mmf equal to 1.5 times that of a single pole-phase group(i.e. a) and returning through adjacent pole phase groups a',b,c' on either side.  

In single phase configuration, the max flux is smaller (1 times instead of 1.5 times) but the turns per phase remain the same.   

Also the flux path length within iron is much shorter (a returns through adjacent pole-phase groups b' and c' which is 1/3 the normal circumferential length of the flux path) but air gap length remains the same.

Both of the above would likely have an effect on the inductances associated with equivalent circuit parameters Xm, X1, X2.

Also the stator now acts as if it had 3 times as many poles as before. Rotor frequency is three times higher. I believe this factor would increase X2 and possibly R2 for large motors (where skin effect is an important factor in rotor resistance).  

Altogether it's a mixed bag of effects. I'm not sure which will predominate.  Since in a normal motor the series reactances X1 and X2 are the primary factors determinng
LRA ~ V/(X1 +X2), it seems reasonable that the increase in X2 due to increased rotor frequency might be the most dominant effect, resulting in somewhat lower current than LRC.

Just a guess. Any more thoughts?

Thank You Pete, I'm very impressed.

Thank you gordonl and pete. You are very helpful.

hi guys
 electric pete , where do you find this data??? I figured it wouldn't turn but I'm in awe again.
 Now to be serious. It may sound silly but why connect the 3 phase loads to the standby system at all. It's allways been the problem that emergency or standby systems are overloaded by 600% and shut down 2cycles after the mains~ so getting these off the system would be the go. If firewater and smoke extraction / ventilation are required then get a back up for them.

As gordonl suggests trip the non essentials. generally we connect the control cct to an unsupported phase or circuit - not connected to the essential services system.

Regards
Don

What size are the motors? Maybe there is a rotary
phase converter in your future. It could save you
a lot of wireing headaches.

I am with sbc, go to a phase converter unless you have to large of motors. I do know of some supposedly ground breaking electric phase converters are coming on the market that provide up to 300% of FLA for starting purposes on one leg and the others provide starting currents the motor requires (up to eight times FLA). If your interested I can give you a company name. Otherwise, I better leave your question to the real motor expert, electricpete! Hehe

What size generator and motors do you have? The generator HP will have to be at least three times the size of the largest motor to start just that one motor. If the motors are under 10 HP each, I suggest your most economical approach would be to change them to single phase. The three phase motors connected to single phase will draw locked rotor amps on the two single phase legs regardless if the motors are wye or delta connected and will not start by themselves unless you provide a third leg. The rotary converter idea would be my last resort if you cannot convert to single phase motors.

I do not know if this help, if your motor below 3 kW try the STEINMETZ CIRCUIT. Its simply by connecting apropriate size of capacitor, it will be cheaper than change all your motor.

I had a similar problem; (customer had single phase supply but wanted to use a 3 phase motor)my solution was to use a single phase input/3 phase output VSD (variable speed drive; in this case a Danfoss VSD. It would of course depend upon the size of motor and costs!

Suggestion: Apply a single phase to three phase power electronic AC-AC or AC-DC-AC converter to provide the suitable quality power supply for your three motors else they be damaged by some schemes that have an impact on the motor HP shaft outputs.
Visit
http://www.thomasregister.com
and type Converters: AC to AC under Product or Service which will return 44 companies to inquire about 1 phase to 3 phase converters.
Also, search web sites, e.g.
http://www.phaseconverter.com/static-converter.html