Dumb motor questions

Thread Starter

shortbus

Joined Sep 30, 2009
7,621
In the case of AC induction motors, sometimes you can get different HP ratings within the same frame size. What is the internal difference in the stator or rotor that makes a motor a different HP?

What is changed internally, again AC induction motor, to make a motor for 120V or 240V ?
 

#12

Joined Nov 30, 2010
18,210
The frame size is dictated by somebody that makes up standards. You can build whatever horsepower, rpm, or voltage motor you want, as long as it fits in the standard frame. That is accomplished by using different size wire, different number of turns in different configurations, and different amounts of magnetizable metal. There is no law that says, (for instance) a frame 48 motor, has to contain as much horsepower as it will hold.
 

Thread Starter

shortbus

Joined Sep 30, 2009
7,621
So, by having a motor rewound it could be made to run at a different voltage or even a higher HP? As long as the new winding can physically fit in the frame?
 

Thread Starter

shortbus

Joined Sep 30, 2009
7,621
Thanks for all the answers everyone. My big Idea is to have a three phase motor rewired to operate on a lower voltage, then run it on DC through a BLDC motor driver.

The next thing I have to figure out is whether the squirrel cage rotor will work on its own, or if permanent magnets will have to be added to the rotor? The 'wind power guys' do this(adding magnets) to make three phase alternators. A standard BLDC motor has a magnet rotor, but when a VFD is used on a three phase motor, the out-put of the VFD is actually a pulsed DC.

If the magnetic force can be made high enough to make the squirrel cage produce enough current the magnets wouldn't be needed. But the magnets would/should allow a lower stator voltage to work better? Any ideas?
 

vrainom

Joined Sep 8, 2011
126
the horsepower is limited by the magnetic saturation of the core, so it is usually not a good idea to try to make it draw more power.

I think the problem with a squirrel cage rotor would be the torque loss at low power
 

strantor

Joined Oct 3, 2010
5,226
Thanks for all the answers everyone. My big Idea is to have a three phase motor rewired to operate on a lower voltage, then run it on DC through a BLDC motor driver.

The next thing I have to figure out is whether the squirrel cage rotor will work on its own, or if permanent magnets will have to be added to the rotor? The 'wind power guys' do this(adding magnets) to make three phase alternators. A standard BLDC motor has a magnet rotor, but when a VFD is used on a three phase motor, the out-put of the VFD is actually a pulsed DC.

If the magnetic force can be made high enough to make the squirrel cage produce enough current the magnets wouldn't be needed. But the magnets would/should allow a lower stator voltage to work better? Any ideas?
I've thought about (and briefly looked into) something similar, but the conclusion I came to is that:
1. While a VFD is pulsed DC, it uses both positive and negative pulses, and a variable duty cycle to construct a sine wave. as the current in the winding increases, the PWM duty cycle increases, so that by the time it reaches the peak of the waveform the PWM is at 100% duty cycle, and then the duty cycle starts to go down, from 100% to 0%, and then it starts the negative cycle. The BLDC controller has a fixed duty cycle (fixed as in it does not change continuously throughout the cycle, but will change if you change the speed reference) thereby putting out a Square wave, and only in the positive direction. So getting a BLDC controller to power an induction motor with no mods to the motor I think is impossible.
2. I think it should work if you put magnets on the rotor, but I'm not confident it would work well. I have looked at all the formulae that goes into designing a motor to get a specific power output and there's just too much crap to deal with. I suppose you could just slap some magnets on there and see if it works. My suspicion is that the stator windings would also need to be modified.

Take this with a grain of salt though, as you've recently seen just how much I don't know about motors and controllers in my other posts.
 

THE_RB

Joined Feb 11, 2008
5,438
Lot's of people have built their own motors with magnets, and probably more people have built generators with perm magnets.

But making one that works well with good power and efficiency is really hard, especially large motors. It gets *even harder* if you want good performance over a wide RPM range as needed for say an electric vehicle. :)
 

BillB3857

Joined Feb 28, 2009
2,532
Don't BLDC controllers require commutation signals to be returned from the motor via either resolver, encoder or Hall devices? Would you be adding a method of feedback to your modified motor?
 

Thread Starter

shortbus

Joined Sep 30, 2009
7,621
I guess I'm going to shelve this idea again.:( Its something I was working at a few years ago and just got a new dose of enthusiasm from Strantor. Went to talk to the little motor winding shop that I used to buy magnet wire from, and they are out of business:( Got to many other things on my plate to spend the limited time and money on. Damn AADD (adult attention deficit disorder) strikes again. :)

Thank you all for your answers and time.
 

Thread Starter

shortbus

Joined Sep 30, 2009
7,621
Don't BLDC controllers require commutation signals to be returned from the motor via either resolver, encoder or Hall devices? Would you be adding a method of feedback to your modified motor?
Both BLDC and VFD are available as 'sensor-less vector' configurations. They measure the current in the individual stator winding poles and tell from that when to turn on the next pole.
 

strantor

Joined Oct 3, 2010
5,226
Both BLDC and VFD are available as 'sensor-less vector' configurations. They measure the current in the individual stator winding poles and tell from that when to turn on the next pole.
True, but sensorless is a poor choice in BLDC for anything other than a constant load, because it is synchronous and no slip. and even with a fixed load, a long acceleration ramp needs to be programmed in, otherwise the controller will not be able to overcome the inertia of the rotor
 

BillB3857

Joined Feb 28, 2009
2,532
Don't BLDC controllers require commutation signals to be returned from the motor via either resolver, encoder or Hall devices? Would you be adding a method of feedback to your modified motor?
I guess I had that concept locked into my brain since the BLDC motors I work with are full positioning servo types. Never gave it a thought about trying to run one as anything else.
 

strantor

Joined Oct 3, 2010
5,226
I guess I had that concept locked into my brain since the BLDC motors I work with are full positioning servo types. Never gave it a thought about trying to run one as anything else.
I Forgot to mention that, in some BLDC applications, they can get away without using hall sensors by measuring the back EMF off the 3rd winding. it's call zero crossing back EMF. I don't RC/ plane controllers even use this, I think they just bang out pulses to the motor an assume it's keeping up.
 

strantor

Joined Oct 3, 2010
5,226
On the topic of AC output of Induction VFDs and DC output of BLDC ESCs, I found this:
Unlike the 6-step mode operation where only one upper and one low switches are turned
on at a time, the square-wave mode operation allows more than 2 switches are on at a time as
shown in Fig. 3.1. In fact, each phase voltage is either positive or negative, without allowing
floating condition except during a brief moment during switching to eliminate shoot-through
condition.
I think the VFD makes 3 phase sine waves by closing more than 2 switches at a time, putting current though all the windings at once. In a BLDC ESC (usually, apart from the sinusoidal control I'm just now learning about) current is only passed through one winding at a time (2 switches, one upper and one lower)
 

Thread Starter

shortbus

Joined Sep 30, 2009
7,621
On the topic of AC output of Induction VFDs and DC output of BLDC ESCs, I found this:

I think the VFD makes 3 phase sine waves by closing more than 2 switches at a time, putting current though all the windings at once. In a BLDC ESC (usually, apart from the sinusoidal control I'm just now learning about) current is only passed through one winding at a time (2 switches, one upper and one lower)
Yeah did some more digging and the VFD out puts a sinusoidal wave and the BLDC drive is a trapezoidal wave form. There are other types of BLDC drive that do have two coils on at the same time though.

Strantor,do you want me to continue to link stuff for you? I'm still researching looking for a 'holy grail' to this stuff.:)
 

strantor

Joined Oct 3, 2010
5,226
No man, let me share with you this little nugget of personal experience someone beat me about the head with on another forum:

If you want to be remotely logical, start with a brushed controller build, and gain the skills to make a current control circuit that is fast enough to work with these extremely fast dI/dT times, not over-shoot, and be capable of dealing with the insane flyback and transients on input and output, design a cap bank that doesn't boil it's guts out from ripple, and most importantly, start to learn the biggest part of power electronics, placement of the components to function. A cap positioned an inch away in the wrong direction can be useless or worse than nothing. Your ground plane design may have the copper cross section of 0awg, yet it's got a 30v transient bounce from one end to the other when you scope it, and it's causing your gate drivers to get reversed and clamp on there body diodes and explode randomly.

This is just to make a brushed controller.

To make a brushless is making 6 brushed controllers, 3 of them upside-down and floating, and all of them MUST be in perfect sync or a pass-through event makes the whole thing erupt into plasma in a couple of milliseconds.
So I think I may just go ahead with my brushed controller; may save a lot of heartache.
 
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