Asynch Induction Motor / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Asynch Induction Motor

Move Thread

LAN_403

IamSmooth

Tue Oct 16 2007, 03:28AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

When an induction motor is connected to the AC mains the slight slippage allows currents to be induced in the squirrel cage and the motor spins at a constant rate. Say, 1725 rpm when the magnetic field is moving at 1800. If I attach a windmill to the rotor and the wind (through a set of ratioed gears) moves the rotor above 1800 rpm will this feed current *back* into the mains? Will it be in phase? What if the rpm goes to 1900?

Thanks.

Dr. Slack

Tue Oct 16 2007, 06:57AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Yes.

It's called an AC Induction generator.

The many advantages are the same as when it's used as a motor - solid construction rotor, absence of brushes - and as a generator it has automatic synchronism with the AC bus it's feeding power into. On the downside, it's not as efficient as a synchronous machine.

This sort of behaviour is hinted at on the "slip diagram", which plots torque versus speed for constant frequency constant voltage excitation. For a low resistance rotor, the torque is low from stall to around half speed, then builds to its maximum torque at perhaps 90% speed (10% slip). Then the torque falls pretty much linearly, proprtional to slip, through zero torque at zero slip. And yes, at negative slip, the stright line continues, the torque and power go negative, and it becomes a generator.

The current won't be completely in anti-phase, just as the current when motoring is not completely in phase, there will still be some reactive VA, that can be ignored for a small machine or PFC'd for a large machine. However the real part of the current, that representing the actual power flow, will be in the expected direction.

Will it turn your utility meter backwards? It should slow it down for any load, but I understand that meters can have some sort of one-way limit, either electrical or mechanical, to prevent fraud through back-driving. Obviously if your supply contract includes selling power to the grid, then you will have an appropriate meter.

Just like the torque reaches a peak around 10% slip as a motor, due to the high slip frequency in the rotor producing lower magnetic moment due to the inductance of the rotor windings, then I would hazard a guess that as the drive speed increases much above synchronous, the efficiency will fall quickly above a certain overspeed. This sets a limit to the amount of power a machine can generate, quite apart from its thermal limit.

IamSmooth

Tue Oct 16 2007, 06:19PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Neil. that was a good answer. Thank you.

I currently have a bi-direction meter because I am selling solar energy back. I am kind of thinking how to continue on this venture and I am looking to see if there is enough wind across my property. I would very much like to build a small wind generator for myself as a project. Do you have any recommendations for the generation stage? Specifically, do you think it is better to have a slip-induction motor that I can directly tie to the MAINS or should I use a DC motor that would require a grid-tie inverter? I can fathom the pros/cons in that the former requires less hardware, but the latter might generator more power. I don't plan on having a big generator - probably 1kW. Maybe the induction motor is better?

Dr. Slack

Tue Oct 16 2007, 07:20PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

It depends on your arrangement for disconnecting the motor/generator when the wind is low. When the motor is connected to the mains, it will always turn at roughly synchronous frequency. If the wind is light, it will turn slower, acting like a fan and costing you money. In a strong wind, it will turn faster and be a generator. If you can detect the speed or power flow direction, then the AC induction machine is very simple solution.

With an intermediate inverter stage, whatever the form of the generator, you have enough control to always be able to push power back into the grid, even if it's picowatts on a cloudy windless day. Perhaps have a common DC bus on your wind and solar generators, and use a single inverter to sell the power into the grid. Then it's easier to add other sources of power as they occur to you - hydro from the stream at the bottom of the garden, the generator of your stair-climber exercise machine.

Steve Conner

Wed Oct 17 2007, 08:35AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Most of the big wind turbines (in the 100kW range) used induction generators with a two-speed gearbox. An onboard computer would choose the best gear ratio, or just shut the thing down if the wind was so slow that it would be acting as a fan.

With IGBTs and neo magnets getting cheaper, they are starting to go over to a low-speed alternator hooked straight to the rotor, whose output is rectified to DC and then exported to the grid through an inverter.

In hobbyist sizes, there are examples of both types.

IamSmooth

Wed Oct 17 2007, 11:58AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Thanks Steve. I've seen a design on the Web using an induction motor. When the fan speed is less than idle, and the motor would act as a fan, a relay keeps it disconnected from the grid. When the wind causes the fan to exceed the idle fan speed the relay engages the motor (now a generator) to the line to feed power into the grid.

Marko

Wed Oct 17 2007, 10:05PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Those async generators were always a big mystery to me. Especially after I saw some really odd examples like generator with a wound rotor shorted out with two diodes.

One thing I just don't understand is how those generators work when *not* connected to mains.

What determines the frequency if it isn't synchronous?

This particular example we had here had wound rotor, two separate windings shorted with diodes. I have no clue what diodes do.

It had a 7,5uF cap on one winding and two outputs, 230V 50Hz and 12V DC (rectified).

It starts from remanence happily, but had unstable voltage with load because throttle regulation didn't work. We didn't measure the frequncy.

Steve Conner

Thu Oct 18 2007, 10:40AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

When they're not connected to the mains, the frequency is determined by the rotor RPM and the load. It still slips just like an induction motor but in the opposite direction. If the load isn't capacitive, it won't work at all.

Marko, maybe those diodes are there to buld up some DC magnetization in the rotor and make it work more like a "real" synchronous generator, or something.

NeilThomas: Once you get past that torque peak, an induction generator goes into runaway. The power export to the grid falls and the prime mover accelerates off until it breaks or shuts down. It's just like when it stalls as a motor.

Marko

Thu Oct 18 2007, 02:59PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

When they're not connected to the mains, the frequency is determined by the rotor RPM and the load. It still slips just like an induction motor but in the opposite direction. If the load isn't capacitive, it won't work at all.

How exactly is it related? wouldn't the cap attempt to resonate and cancel out with inductive reactance of the winding? Or is that what actually happens?

I do realize it needs to spin faster than it's output frequency.. but what frequency?

Excuse my ignorance...

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob