Asynchronous generator or induction motor running in slip

Hi Fellow Wood Gassers
I was wondering if any one has tried to run a 3 phase induction motor in slip, ie tried to turn it faster than the mains is turning it, as far as I can establish it will put power back on the the grid at the exact same frequency that the grid is, in essence it will turn the meter on you pole backwards, it will have to be governed as not to over heat the motor.
Here is a like to a fellow on youtube explaining the concept.
If the grid goes down the motor will also stop generating power back on to the grid, so there is no chance of electrocuting workers on the line. Unless you put capacitors in line with your motor, which will make it self exciting!

What do you think ?
Thanks patrick

Sean French has sucessfully done this. He claimed a GM 350 V8 produced about 72kw on gasoline and about 40 kw on woodgas. They are most certainly unlawful without adequate safety constaints as they could put power to the grid momentarily after a disconnect (I.e. util co shuts off your branch for service and is injured by your generator).

In theory your local branch could have sufficient capacitance to sustain output after the aforementioned disconnect. In all likelihood however your voltage and frequency would drop instantly.

A 27/59 (over /under voltage), an 81 O/U (over/under frequency) and a 46n (negative sequence) protective relay combination set to trip a breaker or ats and disconnect you in the event of an outage or disconnect would disconnect you almost instantly depending on how you set them up.

I don’t know what your laws and regjlations are there but liability is a huge concern. Also, understand that without modification it will not generate power without the grid to excite it (capacitors must be added).

It should however work if you are willing to go rogue and be a scofflaw! Given your earlier reports that rolling blackouts are coming your way a generator would be a better option. Depending on the genset you could rectify to dc and feed a high voltage (string) grid tie inverter to zero your power meter. It would also clean up any messy voltage and frequency from inconsistent quality woodgas. Its worth notinv however that 3 phase grid tie inverters are pricy.

This is another topic Chris might now want on the site, but feel free to PM me.

Patrick, the ‘redneck’ generator in my profile image is a Self-Excited Induction Generator which started life as a 3-ph 480v 20HP motor. The windings have been rearranged in Double Delta fashion with C-2C capacitance to maximize 120/240v single-phase power. If you’re needing 3-phase off-grid, the addition of capacitance is more straightforward but still requires some voltage regulation controls.

We had this generator running on Woodgas at the Kansas gathering a few years back, but the voltage regulation was not automated at the time and we popped a few 400w halogens!

Thanks Henry and Billy
I already have a sand alone 80 Kva alternator with AVR on my system.
I was just interested if it is possible ?
So that if I increase the size of my mill and my transformer can’t handle the load I could supplement the mains supply with an induction motor in slip so I keep in phase and voltage !
As the saying goes many ways to skin a cat !

Thanks Patrick

I had to ponder on this for a bit before responding again…This is by no means a complete solution, but here are a few things that come to mind.

While this is possible, there are a few technical concerns…

Getting the things started:

The easiest option is to use mains AC to start the motor and bring it up to speed. This method usually involves a clutch/freewheel pulley on the motor. The electric motor spins up and rotates freely while the Engine is cranked and ramped up to speed. When the engine RPM exceeds the AC motor RPM, the clutch engages and the motor is driven over-speed by the engine. You then drive it to the RPM’s required to export power. The downside of this setup in your application is that your generator will need to be capable of starting the motor and bringing it up to speed. A large AC motor capable of producing the power you’re looking for will have a large inrush of current when it starts up, and it may have to be the only load present when starting the system in the morning. So long as the AC motor is small enough to be started by your generator when at base load KW (more on this below) and still large enough to drive loads this is the most practical method I can see.

It might also be possible to use a Variable Frequency Drive to slowly spin it up to rated speed with less inrush on your generator, but then it would have to be synchronized with the generator before being connected electrically. This would involve more complicated equipment and controllers - you would be paralleling the VFD with generator momentarily, which could have disastrous effects if done incorrectly. In this configuration the AC motor could also be used to start the engine.

Not sure if it would be possible to spin the ac motor up with the engine and then connect it. As soon as it connects with the mains, the motor will jerk into phase with the generator, likely with great force - possibly enough to damage the mounts and/or engine driving it (or the generator). I don’t know if there is any way to sync the motor shaft with the generator power prior to connecting it electrically, perhaps someone else knows of a way to achieve this?

Managing the system:

This I think is where it gets a tad hairy. Your generator voltage and frequency will be used to excite the AC motor. When you drive the AC motor over-speed it will produce slightly higher voltage and export power back onto the grid (or your local site in this application). Your generator will likely try to lower it’s voltage which it won’t be able to do since the AC motor will continue to produce higher voltage (until it’s RPM’s are low enough to again act as a motor). Your generator engine controller will have to use something other than voltage regulation to control it’s speed, not sure what type of controller you have on it?

In this situation your generator can and will begin to import power and “motorize” driving your generator engine and producing a lot of heat - this is VERY BAD. You would need a set of current transformers (CT’s) on the generator to monitor reverse power flow and trip the generator breaker offline. This will require a powered circuit breaker on the gen (an ATS could also be made to function as a power disconnect device). Likewise you’ll need a set of CT’s and a powered disconnect on the AC motor to allow you to monitor it’s state: importing or exporting power and how much as well as disconnect in an emergency.

I think the only practical way to control the system is to monitor the overall bus voltage, and look at the CT’s on the genset and AC motor. So long as the bus frequency is acceptable and stable, system voltage is within acceptable range, and the generator CT’s show that you are exporting power (base load KW) the generator should stay happy.

The AC motor will need to have some sort of engine controller built to control the RPM of the engine driving it as well as a power breaker/disconnect. Your controller for the AC motor will need to monitor the current produced by the motor, system voltage and shaft RPM as well as being able to monitor the CT’s on the generator. I think it would be preferable to have the AC motor handling the majority of the load. Set the generator base load KW to a percentage of expected load and have the AC motor pick up as much of the rest as it possibly can. Then, if a large load is added, your gen has more to give. When the AC motor engine controller sees the shaft rpm drop and current on the gen increase, it should increase throttle until the genset is again producing no more than it’s preset base load KW. When it see’s shaft rpm increase and genset power drop (or go reverse) it should reduce throttle until it is at or below maximum shaft RPM and the generator is at it’s preset base-load. The reverse power relay (32R) on the generator will need to be set with a delay sufficient to allow this process to occur but not so long that damage occurs to the generator windings (heat). If the 32R monitoring the gen is set too tight, it will trip the generator offline, resulting in uncontrolled output from the AC motor.

You’ll need to set the gen base load to a load value that is always present in your facility so that it can always be at the base load (minimum KW load). This will allow the AC motor to parallel with it producing the power that drives additional loads. The KW required for your facility lighting heat etc with no other machinery running would be an ideal base load KW. Then the AC motor can be started first thing in the morning before equipment is fired up and it will pick up the additional loads as they come on with the generator acting as a buffer and voltage / frequency manger. You may wish to devise a start up order so that the largest loads are powered up first when your system has the greatest headroom and add them incrementally until all equipment is running. This could add a few minutes to your morning routine but could get you to where you want to be.

Your AC motor would then pick up all of the load it is capable of powering. If your facility loads exceeds this capacity (and they most likely will) your genset will have to pick up any additional loads.

The trick is getting both the generator and AC motor controllers to speed them up or slow them down in a complimentary fashion - it will have to be an integrated system. The AC motor will have a minimum shaft RPM to export power and not draw it. It will also have a maximum shaft RPM that, if exceeded will produce no additional power output (max KW). Depending on the load it will need to spin somewhere in that range, going above or below those values will be an alarm condition.

The biggest concern will be the adding and removal of large loads once the system is running. If a large load is removed your voltage will increase and the generator will import power from the AC motor until the AC motor engine controller reduces RPM lowering the bus voltage. When a large load is added, it will drop the bus voltage lugging the generator engine as you are presently used to seeing. The AC motor will also reduce the shaft speed as it starts to work, much like the generator does. If it slows sufficiently it will again become a motor, exacerbating drag on the genset and forcing the gen to work even harder. If it has enough KW headroom, and the engine driving it has sufficient power, the generator should recover allowing the AC motor to speed up and produce power again.

To avoid this scenario, the generator should always have enough headroom to pick up the biggest load in your facility while producing slightly more power than your largest load draws, so that it can be added or removed at any time without disrupting the overall system. It should speed up to power any added loads momentarily while the AC motor is sped up to accommodate them or slow down to reduce output (but not below base load KW) while the AC motor is slowed. This would allow the AC motor to change speed and output at a given rate, simplifying the AC Motor engine controller and allowing the generator engine controller to act as it already does.

Add additional metering and controls to suit your fancy as your needs and budget allow.

“I don’t know if there is any way to sync the motor shaft with the generator power prior to connecting it electrically, perhaps someone else knows of a way to achieve this?”

Henry, the most straightforward and simplest way of doing this is to put light bulbs (that can take phase-to-phase voltage) across the syncing contactor. When the lights are at their dimmest, it is safe to close the contactor.

This trick is also a quick way to measure generator frequency. One blink per second means you are running 1 cycle faster or slower than the grid. The lights in this video were used for that purpose: C-2C Self-excited Induction Generator - YouTube

That’s old school, it works, but is not the best way to do it - use a 25 (sync check relay)

Unfortunately, this won’t work without power on the ac motor! “prior to connecting it electrically”

Yep, kinda like woodgas… old school, but it works!

Yes, this does require some capacitance for excitation.

Hi Henry and Bill
Thanks for your input, I will have to reread your post to try understand it compleatly.
But it sounds like it would easier to have separate alternators on separate circuits and safer, than trying to boost a single unit with a motor running in slip!