SNIP
The single phase drive provides torque which fluctuates between zero and a maximum value twice per cycle of the supply frequency.
This is buffered by the inertia of the rotor which averages this out to very near constant speed.
Dual North American motors commonly have a 220V star winding plus three separate single 220V winding. If you connect for 440v but only supply power to the 220v windings it will roughly halve the HP rating!
Jim
Take a real single phase motor and couple it to a real three phase motor. If you power the single phase motor, the three phase motor will act as an induction generator. You may have to put a capacitor across one leg of the three phase motor to get it to act as an induction generator. ( or you can connect one leg to the single phase power ). With the three phase motor working as a induction generator, you will get three phase power. If you use this three phase power to run another three phase motor, you will get energy delivered at a nearly constant rate from the second three phase motor.
No, the breaker feeding the VFD is 16A. But the supply behind it is fused at
100A 240V with 10kA available fault current. The supply voltage doesn't stutter on startup.regards Mark Rand RTFM
The rotating mass of the two rotors stores and releases energy. A large enough spinning mass cannot stop and start at 120 Hz, so it just coasts along.
Joe Gwinn
Yes, it does. This back EMF is precisely the motor acting as a generator.
I've been thinking about the issue, and there are a number of datapoints to consider:
VFDs will trip (complaining of overvoltage on the DC filter capacitor) if trying to stop a motor driving too much rotating mass. The classic solution is to provide a braking resistor to absorb the energy causing the overvoltage. The VFD manuals all say that the energy comes from the kinetic energy of that spinning mass. The more mass, the more energy to be handled, the hotter the braking resistor gets.
When starting, the rotor is stationary and so the motor draws the locked-rotor current. The difference between current at normal load and with the rotor locked is around six to one. As the rotor spins up, the current drops. What's happening is that the motor is generating a back EMF proportional to rotation speed, so at speed the motor is driven by about 1/6 the supply, and the back EMF is the other 5/6 of the supply voltage. This is true of almost all motors, not just polyphase induction motors.
Induction generators do exist, and are usually based on stock induction motors, most often three phase.
There are many articles on induction generators available. The best article on the theory I've found so far is "Induction Generator Theory and Application", J.E. Barkle and R.W. Ferguson, AIEE Transactions (Power Apparatus and Systems), February 1954, pages 12-19.
The big problem with induction generators in practice is that it's too hard to control them, and it's cheaper to use a wound rotor (allowing control by varying the field current).
Joe Gwinn
Right, and it occurs because at any point in time (while the VFD is decelerating the motor) the motor's speed is greater than its synchronous speed, as determined by the VFD's output frequency at that instant.
Yes. The VFD provides the current to maintain the field the motor uses to be a generator.
If one instead has a resistor across the motor and abruptly disconnects the power, leaving resistor and connected motor to their own devices, the field is maintained by regeneration, and too much load on the "generator" will cause the regeneration effect to collapse.
Pentagrid has mentioned this regeneration and collapse in past postings, and I've also seen it discussed in textbooks.
With a VFD driving multiple parallel motors, some of which are more heavily loaded than others, all these effects will play out simultaneously, and the net result will depend on the details.
I have VFDs for my tools, so no RPC to play with. I may have to get a pair of three phase motors just to experiment upon. My local source of cheap motors vanished a few years ago, so it may be a while.
Joe Gwinn
On Sun, 22 Feb 2009 11:48:46 -0500, Joseph Gwinn
Do the VFD's switch in the resistor when braking; or leave it on all the time keeping the room warm?
They all switch.
Joe Gwinn
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