Easiest way

Thread Starter

shortbus

Joined Sep 30, 2009
9,047
Rb, see prandevou's reply :). Its really just an experiment to see if it can be done. Eventually it would be put into an electric vehicle.
 

THE_RB

Joined Feb 11, 2008
5,438
Ahh ok, it's for a mains voltage 2 phase VFD... Good luck with that one! The first thing might be to see what is involved building two h-bridges capable of continuously PWM switching >300v at high amps into an inductive load.

The part that generates the simple stepped voltage control waveform is the least of your worries! :)
 

strantor

Joined Oct 3, 2010
5,752
shortbus, I am curious; why have you chosen 2 phase? is it just to be different, or have you realized some advantage to it over 3 phase? The only advantage I can think of is less parts in the VFD. only need 2 IGBTs instead of 6, but does that make up the difference of the increased price of the oddball motor? I'm thinking from a production standpoint, if you were going to market this. If it's just for your own little one time project and you do the rewinding yourself, I can see how you would save.

Another thing I was wondering about is the actual rewinding of the 3 phase motor. are the stator slots going to line up properly for the different amount of phases and poles? what will you need to do to the rotor?

EDIT: just thought of the fact that not only do you only need 2 IGBTs, but they can both be low side switches, which makes things much simpler. I guess you're on to something here.
 

praondevou

Joined Jul 9, 2011
2,942
only need 2 IGBTs instead of 6,
You mean 2 for each leg?

If we have two separated coils we will need 4 IGBTs for each coil in case of a fullbridge.( that's 8 in total). If it was a halfbridge it will be 2 IGBTs for each coil (don't know how well or if a halfbridge would work). If we somehow are able to join the coils on one end it will still be 6 IGBTs.
 

strantor

Joined Oct 3, 2010
5,752
You mean 2 for each leg?

If we have two separated coils we will need 4 IGBTs for each coil in case of a fullbridge.( that's 8 in total). If it was a halfbridge it will be 2 IGBTs for each coil (don't know how well or if a halfbridge would work). If we somehow are able to join the coils on one end it will still be 6 IGBTs.
oh yeah. DOH! I was imagining 2 low side switches, one for each winding, like 2 DC motors. Somehow it escaped me that is this is AC.

ok, so you still need 6, or more likely 8 IGBTS, so what's the advantage?
 

praondevou

Joined Jul 9, 2011
2,942
Rb, I have IGBT switching modules for that. http://www.datasheetarchive.com/2mb1200*-datasheet.html I'm rewinding the motor for 100V, so the DC bus will be ~140V.
That means unipolar PWM?

FYI, I was looking for this info, it's from AN1958:

3.2 Volts per Hertz Control

The Volts per Hertz control method, the most popular technique of Scalar Control, controls the magnitude of such variables as frequency, voltage or current. The command and feedback signals are DC quantities, and are
proportional to the respective variables.

The purpose of the Volts per Hertz control scheme is to maintain the air-gap flux of AC induction motor in constant, achieving higher run-time efficiency. In steady-state operation, the machine air-gap flux is approximately related to the ratio V /f, where V is the amplitude of motor phase voltage and f is the synchronous electrical frequency applied to the motor. The control system is illustrated in Figure 3-4. The characteristic is defined by the base point of the motor. Below the base point, the motor operates at optimum excitation due to the constant V /f ratio. Above this point, the motor operates under-excited because of the DC-Bus voltage limit.

A simple closed-loop Volts per Hertz speed control for an induction motor is the control technique targeted for low-performance drives. This basic scheme is unsatisfactory for more demanding applications, where speed
precision is required.

 

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Thread Starter

shortbus

Joined Sep 30, 2009
9,047
shortbus, I am curious; why have you chosen 2 phase? is it just to be different, or have you realized some advantage to it over 3 phase? The only advantage I can think of is less parts in the VFD. only need 2 IGBTs instead of 6, but does that make up the difference of the increased price of the oddball motor? I'm thinking from a production standpoint, if you were going to market this. If it's just for your own little one time project and you do the rewinding yourself, I can see how you would save.

Another thing I was wondering about is the actual rewinding of the 3 phase motor. are the stator slots going to line up properly for the different amount of phases and poles? what will you need to do to the rotor?

EDIT: just thought of the fact that not only do you only need 2 IGBTs, but they can both be low side switches, which makes things much simpler. I guess you're on to something here.
It is the way that I think all of the EVs will eventually be made. The Remy motors in the GM Volt and from what I can find out the Tesla are using 2phase now. Ac motors will eventually be what will win in Ev's.

While most of the controllers available are 3phase, but its only because 3phase won out back in the starting days. 3phase only takes three wires plus ground and a lower voltage single phase can be tapped off. 2phase takes 4 wires plus ground and no lower voltage single phase was available.

In rewinding the motor for a lower voltage you need a larger gage wire. And with the available motors theres not enough slot area to rewind as 3phase and keep the horsepower and speed. So by switching to 2phase I can put in bigger wire and keep the needed amps for the horsepower and the rpm.

The 2phase takes 4 modules instead of 3 for 3phase.
 

THE_RB

Joined Feb 11, 2008
5,438
...
Another thing I was wondering about is the actual rewinding of the 3 phase motor. are the stator slots going to line up properly for the different amount of phases and poles? what will you need to do to the rotor?
...
If the stator slots divide evenly by 4 (like if there are 24 stator slots) then it should be possible to rewind as 2 phase.

But the rotor is bugging me too! What type of rotor does it have and how many poles does the rotor have? Depending on rotor type it may be totally unsuitable for 2 phase operation...
 

Thread Starter

shortbus

Joined Sep 30, 2009
9,047
If the stator slots divide evenly by 4 (like if there are 24 stator slots) then it should be possible to rewind as 2 phase.

But the rotor is bugging me too! What type of rotor does it have and how many poles does the rotor have? Depending on rotor type it may be totally unsuitable for 2 phase operation...
As long as you rewind the motor as the same number of poles the phase number makes no difference. A four pole motor is still a four pole motor.
 

strantor

Joined Oct 3, 2010
5,752
What's the advantage of having an AC motor instead of a BLDC motor in an EV?

Cost? Robustness? Efficiency?
I've been down that road of questioning and all I found was availability and cost (to an extent)

There are only 2 BLDC motors I have found that are big enough to be suitable for an EV. Both are very expensive and I have yet to find a controller for either of them. The motor and controller technology seem to both be stagnating, waiting for the other to make a surge forward. Nobody is going to make a 300kw controller for a motor that doesn't exist, and nobody is going to make a 300kw motor for a controller that doesn't exist. I think we are waiting for one of the big guys in auto industry to shell out the big bucks to develop both at the same time.

Making a BLDC motor big enough for an EV means a bunch of very expensive magnets, and resultant very expensive motor. I think there is a breaking point somewhere below an EV sized BLDC at which point it is no longer cost efffective, and the induction motor starts to become the logical choice.
 

Thread Starter

shortbus

Joined Sep 30, 2009
9,047
Like Strantor said. The only real difference between the BLDC and induction motor is the rotor. Bldc - magnet rotor and induction - squirrel cage rotor. The stators are wound the same.
 

THE_RB

Joined Feb 11, 2008
5,438
As long as you rewind the motor as the same number of poles the phase number makes no difference. A four pole motor is still a four pole motor.
Are you sure it will work like that? How will you go from 3 phase stator to 2 phase stator and keep the number of poles (the stator pole angle) the same?

If the stator has 24 slots, and was three phase the stator pole angle was 3/24 or 45 degrees.

If you rewire the stator to 2 evenly spaced phases you get 2/24 or 30 degrees. It becomes a 6 pole motor.

How many stator slots does the motor have?
 

Thread Starter

shortbus

Joined Sep 30, 2009
9,047
RB, in none of the old books I have on rewinding motors do they mention changing rotors when rewinding. When changing from one number of poles to another there is no mention of the rotor as being a problem.

Most motors, from what I can see, are not wound as full poles, but 'chorded' poles. Since I haven't got a motor yet, can't tell you how many slots. Used to be 3phase motors were almost free. But since VFDs have came down in price, people are using their 3phase motors instead of changing to a single phase motor. A VFD is cheaper than a new motor!

I have several places promising to save a motor from scrapped equipment. So in the mean time, trying to get a controller worked out. not going to waste time rewinding a motor if can't make a controller.
 
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