# Combining two identical 3-phase AC sources

I have a micro hydro setup. The generator is a 3-phase AC motor scavenged from a Segway ES2 scooter rated 36 V, 300 W continuous, 700 W peak. It is being rectified to DC.

I thought that would be sufficient, but it appears I can get more power from the water source than I thought, and the motor may well generate well over it's rated limit as a motor. Great news.

My question are - will running it at RPMs that generate > 36 V & 300 watts damage the motor? I assume it will.

Obviously I can get a bigger motor, but I have access to more of these, cheap.

My second question is can I run two of these motors in parallel? Bolt one on top of the other, off the same shaft, so both are rotating at the exact same RPM, theoretically delivering exactly the same voltage and current.

To do so, can I combine both 3-phase outputs to my DC bridge rectifier? Will that work, or will I blow something up?

• You are following a fallacy. For a generator, the highest RPM is at the lowest power output. Because when there is no electrical load, only the internal friction of the drive remains. If you are concerned about overspeed, use a reduction gear. Sep 17 at 20:42
• i get that. i haven't completed load testing, so I'm not sure where my max power point (V x A) is yet, but I'm also constrained because I need to keep RPM under 100v with no load or blow up my controller. So I'm trying to optimise my runner designs for torque. That said, it still looks like I may have the ability to generate more current AND volts than initially anticipated, hence the question, as I either underoptimise the runners, or add a bigger (or 2nd) generator. Basically I'm an idiot on the electrical side of things, so wanted to see if this was even a viable option. Sep 17 at 21:00

My question are - will running it at RPMs that generate > 36 V & 300 watts damage the motor? I assume it will.

Higher voltage than rated means higher speed. The magnets can peel off from the rotor due to excessive centrifugal force, or the rotor may dislocate. But a water mill is a system with less dynamic stress compared to the e-scooter, for sure the scooter has also to additionally withstand road bumps force with conjunction to the centrifugal force. Also the wheel stands in vertical position, so at the bottom dead center the acceleration is 1G higher as the wheel would spin in horizontal position.

As for current, it should't be problematic if the motor is adequately cooled.

My second question is can I run two of these motors in parallel? Bolt one on top of the other, off the same shaft, so both are rotating at the exact same RPM, theoretically delivering exactly the same voltage and current

It should't be a problem if they are identical and both rotors and stators aligned. But you could also use separate rectifier bridges. More over you could introduce a phase shift, so that you get 12 pulses per 360 deg electrical angle, instead of 6 pulses. Less ripple DC output.

EDIT:

image source

simulate this circuit – Schematic created using CircuitLab

Two motors aligned off 30 degrees. The upper is -30 degrees (A+, B-) and the lower is 0 degrees (A+, B&C-) according to above diagram. You have to use a lab PSU or a SMPS with a current limit that may not be higher than nominal motor current.

• 2 rectifiers is fine. so i can combine two DC sources no problem? phase shift is a matter of offsetting one rotor from the other 60 degrees? Sep 17 at 20:08
• @cemdev Actually if you shift by 60 deg you get the same, you have to shift by 30 deg, but ELECTRIC ANGLE. $\varphi_{mech}=\varphi_{elec}\cdot\dfrac{2}{P}$ where P is number of poles (nr. of magnets) $\varphi_{mech}=\dfrac{60}{P}$ Sep 17 at 20:25
• I second the 12-pulse idea. It's how you get a real profit from using two generators. Sep 17 at 20:36
• excellent! many thanks, this is very helpful. Sep 17 at 20:40

If you are about to parallel the outputs of two motors (generators) on the same shaft you would likely run into a phasing problem when connecting the windings in parallel. If the motors are not mechanically exactly in phase they would be "fighting" and creating loss (heat).

Since you are rectifying the output in 3-phase rectifiers there is an easy remedy: Have independent rectifiers for each of the motors and tie together the DC output. Suddenly you actually may benefit from the motors being out of phase since it will only make the ripple on the DC bus smaller. You may enjoy redundancy too.

However - what other said about exceeding RPM (or other specifications) still holds. BLDC motors are common in two configurations. Inrunner / outrunner. The outrunner has the rotor as a ring outside of the stationary coils. The good thing here is that the centripetal force is pushing the (brittle) permanent magnets out against a solud tubular (steel) rotor that is less likely to fail when the speed is exceeded. Still one needs to be careful since the mechanical energy released during a failure may be rather high.

• thanks for that. These are hub motors, so the coils stay fixed to the inner shaft, and the outer housing with the magnets rotates. They're very solidly built as they're designed to be wheels to support the weight of a person traversing terrain, and the RPMs are really quite low, even with no load (under 1000 rpm i believe) so I don't think I'm going to have a problem with a mechanical failure. Thanks to all the responses, if I can in fact generate more power than the one motor can handle, I will add a 2nd motor, offset 30 degrees, to its own rectifier and then join the 2 DC currents. Sep 18 at 13:22
• @cemdev Just to clarify: Your 30° offset refers to electrical angle. If your motor has - as an example - four poles the full electrical cycle would be 1/4 of 360° and your 30° would also translate to 1/4 --> 7.5° mechanical angle. Sep 18 at 15:31
• each individual pole with coils around it? There are over 20! So basically I want it aligned so each pole in the 2nd motor is halfway between the poles in the first motor? Just a few degrees then. Sep 18 at 20:14
• inside of the motor can be seen in this video, at this timestamp. youtu.be/Rgv4uItKLuA?t=250 Sep 18 at 20:15

I have not done this before, but I will share my thoughts on it.

Over-speeding the motor could cause mechanical problems, but if it runs smoothly then it is probably OK. Bearings could wear out prematurely, I guess. Something to watch for over time. Electrically, if the motor current is high enough to cause the stator to get hot, then it may cause premature failure. But if the motor is not getting too hot after a long time you don't need to worry about that.

Running two motors in parallel as you describe, in your situation, may work provided that the Kv of the motors is well matched. It would probably be best to rectify them separately, but, again, if they are matched, you can probably connect the rectifiers to the same DC bus.

Initially you would want to verify that the two motors are putting out close to the same power (verifying that they are well matched). Once you verify that, you can probably stop worrying about it, or just double-check it periodically.

• Thanks for that. You mean I may be better off rectifying them separately, and combining the DC rectified output (to the same controller)? That would be great too. Sep 17 at 19:25
• You are talking about paralleling two 3 phase ac sources, so you cannot do that directly. You would have to match up phase sequence, voltage and frequency. So turning it into DC is better. As in: if you do it you will blow something up! Sep 17 at 19:47
• ok, so 2 rectifiers and joining the DC current would work fine? That's also straighforward. Sep 17 at 20:08
• Yeah, two separate rectifiers. Like I said I have never done it. But I think it will work. I do have experience working with BLDC motors and motor controllers. So I am not completely out in left field. Sep 17 at 20:45
• thanks very much, very helpful. Sep 17 at 21:08