Whats the most effiecient motor to use as generator?

[bigmark1972]

I have been working on a project recently where I want to turn a stepper motor via an air turbine to generate power. It works pretty well I have 160vdc @ .14 amps on one of my steppers and this is more than enough power for the circuit it needs to run. My question is this, is the stepper the best (most efficient) way to generate power or would some of you use another method perhaps brushless DC or induction motor?

My power needs are somewhat small 20-30 watts but I would like to generate it in the most efficient way possible any RPM is possible but I would like to stay below 2k if possible just for bearing life, balance issues etc.

Input appreciated.

 

[itsmoked]

I see you've finally come clean....

First question is size. Got any size constraints?

How many volts do you want, to get you that 30W?

 

[registigger]

The best way depends on many parameters. If you only make one of these, you have the motor and it works, it is the best way. If you intend to produce hundreds, you might want to look at DC motors or induction motors or servo motors. With the proper drive you will get a high efficiency. Do you need to regulate the output voltage? "A any RPM" is not easy to meet. Very low RPM do not produce much voltage except on a servo motor (because of the drive boosting). You need a lot of tork on the actuator. Do you want to pay $10 or $10K? If the RPM is relatively constant, a DC motor is the simplest and possibly the least expensive.

 

[bigmark1972]

I'm clean thanks


Size is a concern the motor I am using now is approx 2" x 2"
with a .25" Shaft. I would like to stay this size or smaller if possible.

I am going to make the turbines voltage self regulating via a proportional valve that will close off air to the turbine as voltage goes to high. I can have a float of 5-10v on the load without issue.

This is not going to be a single unit, I hope to make many of these so I want to do it right. I was considering a brushless DC motor with Nd-Fe-B Magnets would be a nice small package, but I am unaware of how it would act as a generator compared to a stepper motor.

I can use any voltage between 12-300vdc to get my 20-30 watts
I plan to reduce the voltage to the air valve supplying the turbine via a resistor voltage divider. The load can run on any DC voltage from 24-400vdc (solenoid) I just need 20 watts or so. The coil for the regulating valve has a very small load of .67 watt at 24vdc.

I can get this done now with the stepper I have or any like it, I would just like to know if it is the most practical or efficient way to do it.

 

[UKpete]

bigmark, "in the most efficient way possible" i.e. best possible energy efficiency - I would certainly go for a brushless PM machine with rare earth magnets (SmCo or NdFeB). They have the lowest losses of all motor/generator types, primarily because their rotor losses are very low.

Any motor will have very similar performance as a generator. There are a few points to note however (not an exhaustive list, I'm sure there are other points):
1. an induction motor cannot generate without a supply for its magnetizing current.
2. a commutator motor has higher losses due to brush friction and volt drop.
3. machines with wound rotor and wound stator suffer from two lots of copper loss.

The best power density results from running the machine as fast as possible; up to the point where bearing+windage losses become significant, the power output from a particular machine will be proportional to speed.

A typical 3-phase brushless PM machine will give a near sinusoidal output that can be simply rectified; if you want variable voltage you can use mosfets with controlled switching, the losses are lower than for a resistive divider.

 

[Barry1961]

That does not seem like much power. Would a regular DC tach generator work? I know some are mechanically designed for high RPM servo motors. I looked at a couple and did not see a power rating but it seems something that size could dissipate 30W.

Barry1961

 

[mc5w]

Stepper motors are not intended to act as generators - power flow is basically 1 way. Tachometer generators are not intended to carry andy significant load.

You can get small permanent magnet DC motors and give them a try.

 

[bigmark1972]

Stepper motors are not intended to act as generators - power flow is basically 1 way. Tachometer generators are not intended to carry and significant load.

You can get small permanent magnet DC motors and give them a try.

---

Stepper while never intended to produce power work pretty well, I have done it and many wind power people use steppers for the same reason. They make good usable power at low RP M's.

The DC tach Idea is fine but I would need the brushless type and they are not cost effective.

I thing I am going to go with a brushless PM machine with rare earth magnets (SmCo or NdFeB) as UK Pete suggested as this is where my research has led me as well. Problem is I have to really spin these buggers to get power out of them.

I may change my design and utilize a Tesla turbine as they work well in high RP M's and actually gain efficiency over 30k RPM I'd just need some high quality bearings. Another option would be to gear the turbine to the motor/generator but I want to keep this simple.

I have many CNC machines at my disposal so making a turbine is possible and they are decently efficient as well.

I'm not going to quit on this until I am satisfied it is as close to perfect as possible.

 

[registigger]

Your turbine controller will need to be more elaborate than a simple resistor driving the solenoid valve. First of all, this is likely to consume a large part of the energy you are producing. Then the chances are it will not be stable. You need a something similar to a PID controller. This controller would drive a motor actuated valve with UP/DOWN control. With this system, the power consumption will be almost zero under constant load and air pressure. The trick is to evaluate how fast it has to respond to transients. Slow systems can be very simple. Fast systems are much more demanding.

 

[bigmark1972]

You could be right on that I will find out soon. I have a valve picked out that will demand only 4 watts of power and has a very fast response time. I plan on having some mass in the turbine that should give a flywheel effect.
If you are right I will move in that direction if needed. I only need to energize my load for approx .25 seconds every second or so.

Thanks

 

[mc5w]

Even if you abd other have obtained power output from stepper motors, they are not efficient or well performing generators.

If what you want is to charge a 24 volt battery and run an inverter you can get alternators that are designed for this application. These are used in buses and airplanes. You can get them in just about any power output you could want.

 

[mc5w]

A 24 volt nominal ( 30 to 32 volts actual to get the battery all the way up to full charge ) plus an inverter would eliminate the need for close speed regulation.

You could even use a centrifugal pump running backwards which is often done with cooling tower water when it comes back down from the roof of a skyscraper in order to recover some of the mechanical energy the is in the water. This way, equipment on lower floors does not have to withstand the full head of water. The same thing is also done when pumping cooling tower water from a tower that is on an intermediate floor to an AC unit that is on an upper floor. This keeps the water coming back down from turning its mechanical energy into excessive heat.

 

[OperaHouse]

One of the best motors out there for wind generation is a GE ECM motor that is used in furnaces for the variable speed blower. These became common in home HVAC units in the early 90's. It is a three phase 1/2hp PM motor that can put out a nice amount of power in the 200-300 rpm range that a windmill operates best in. Low amount of cogging! HVAC guys I've talked to replace a couple a week because the speed control on the back of the motor goes bad. They would be happy to save you one for $5. Just add some diodes!

 

[bigmark1972]

That would work, but would be too large for my app I think I have found the one I want rare earth magnets are impressive!

RPM Current Watts

4400 6.9A 42

It's a very durable external rotor rare earth brushless
motor.

I have one on the way to test I hope it works spent a lot of time finding this one

27.7mm (1-3/32") diameter, 30mm (1-3/16") from back to front. The hardened 3.17mm (1/8") shaft is supported by 2 ballbearings for a long service life.


87% efficiency watts shown are with efficiency losses. 60 bucks with high quality bearings not bad.

 

[UKpete]

bigmark, good luck with it. How did you manage to post the pictures into Eng-tips?

 

[itsmoked]

my question too..

And this time no, "I simply used some HTML" answer!!

 

[itsmoked]

This is just a test.. Dis-reguard

http://www.hobby-lobby.com/images/pm2212.jpg

 

[itsmoked]

<img src=" &#104;ttp://&#119;ww.hobby-lobby.com/images/pm2212.jpg" border=0>

 

[itsmoked]

The instructions are only shown in the "Preview Post" window.

You must have the imagine on the WEB accessible by anyone.

Then to include an image in your post do this;

In this case the image is at:

http://www.hobby-lobby.com/images/pm2212.jpg

So you enter:
"img

http://www.hobby-lobby.com/images/pm2212.jpg"

Inside [..]these brackets.

And WA-LAW!

 

[UKpete]

itsmoked, thanks for the tip, all I need now is a web page to enter images into.