Sizing an alternator as BLDC servo

[Vog]

Hi everybody I would like to get some feedback on my motor sizing analysis.

I am looking to build a custom traction application that requires 0~800lb pulling force and up to 10m/s pulling speed. The max mechanical output is probably around 5 KW. Both the force and the speed need to be accurately controled. The motor will be powered by a few auto batteries supplemented with household 110V AC. Batteries are needed since household 110V AC can supply only up to 3KW max.

I have considered a few options on the motor:

* DC PM servo motor, $200 on ebay for 5KW range.
* 5HP 12V or 24V auto winch motor, $200~$300
* 12V 200A auto alternator used as BLDC, $150.

The 5KW rated DC servos are quite large and heavy (50+lbs) and they normally require 230V or 460V input so that is not desirable however they have the benefit of low inertia and high acceleration.

Most winch motors are series wound. They are small (15lb) yet proven powerful: mated with 300:1 gear reduction they can pull 15000 lbs at 3ft per minute. The problem is that they are built for low voltage and high current (400amps) uses and their efficiency is low (combined with the gear box they give around 35% efficiency).

In comparison, auto alternators have several advantages:
* they are light at 15lbs
* wide speed range, up to 7000rpm (both servos and winch motors go up to 300~4000rpm)
* they are built for low voltage operation but can be driven to up to 100V.
* they are easy to acquire

However it is not clear if the size of 12V 200A alternator is enough and here is my analysis:

Here is typical 12V 200A alternator performance:
RPM Cold/Hot
2000 65/60
2500 114/105
3000 146/133
3500 165/150
4000 175/162
4500 184/169
5000 189/174
5500 194/175
6000 197/179
6500 199/181

Given these numbers, the electric torque for the alternator:
torque = Amp / rpm * 12v * 60 / 2 / 3.1416 = Amp/rpm * 114.6
RPM torque
2000 3.7Nm
2500 5.2Nm
3000 5.6Nm
3500 5.4Nm
4000 5.0Nm
4500 4.7Nm
5000 4.3Nm
5500 4.0Nm
6000 3.8Nm
6500 3.5Nm

However, understand that the field (rotor) current is regulated for stable 12V output. This means the max field current is used at 2000rpm and down regulated as the rpm goes higher. So if we were to maintain the same field current and apply 200A to the stator we should get the theoretical max torque:

Max (theoretical) electric torque is 3.7Nm/65*200 = 11.4Nm

At this field current and 7000rpm the EMF will be much higher, probably around 40~50V. Given the reports on the web where folks use their alternators as welders at unregulated 110V DC the field current can be driven even higher, giving close to 40Nm of torque. 40Nm + 7000rpm makes a very powerful servo motor!

Overall the alternator's outer limits of 100V 200A should be sufficient for 5KW mechanical output.

Any comments?

Vog

 

[itsmoked]

Have you got a link to BLDC-izing an alternator or are you planning to do this yourself?

I am having a hard time with the concept of getting 5kW out of an alternator designed to supply 2.8kW. That's 6hp out of a 3.75hp alternator.

Usually using a highly optimized device, (such as an automotive alternator), for another application costs you something like; higher voltage => lower current at a worse efficiency..

You seem to be looking to get more energy of a different form out of it! I have my doubts.

 

[nbucska]

You can use a motor with higher than nominal current
for limited time but with lower efficiency due to
saturation. Even if you could get 5Hp of a 3.2 Hp alternator used as motor for a short time (what I don't believe) , you must take the on and off times into consideration.

Let's step back: you want to move 0 to 800lb in controlled the force,speed {position too ?}.

What is the distance? how does the power changes with time ?

Do you need one or many? is the lifetime important?
etc.





<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[Vog]

Note that the current ratings of the alternator is not to be exceeded - that is the stator will always operate under 200A. The overall system capacity of 100V 200A seems ample enough for 5KW output, right?

The field (rotor) winding will operate at higher than stated rating, probably up to 2~3 times of stated rating. With field winding resistence of 2.5ohm, the heat generation from the field winding could be 22.5W (at 3A) or 250W (at 10A). As long as i can ventilate the rotor I should be ok.

The distance is up to 5 meters. The power will peak in the middle of the travel. The peak power is not expected to last more than 4 seconds. The lifetime of the motor is not that important. Replacement is easy.

One of the things I like about using an alternator is that it allows the field current to be controlled, giving a wider dynamic range than normal PM servo motors. Also alternators can operate at higher rpms.

 

[coconutalley]

My gut feel is with the other gentlemen: alternators were
optimized as generators, now you are trying to use it as motor--opposite of its design intention. It is not designed for getting driven but not for driving loads. I would guess that slip is probably a big issue that you don't have to worry about with pm motors. But, don't let our cheap talk and opinions discourage you, go try it and if it works--then we'd know.