One for the PMAC motor buffs

[jonescg]

I have placed a deposit on an Evo AFM140 motor. Upon Evo's recommendation I chose the #4 wind, which has a Ke of 1.08 Vs/rad.

For those of us who prefer to work with rpm/V, this equates to 8.842 rpm/V.

The mechanical speed limit of this motor is 5000 rpm, or which would occur at 565 V (RMS). Since most inverters aren't as efficient at turning a DC Bus into AC, multiplying by 1.5 is a safe bet. This means I need to be running a nominal 850 V DC!

Surely the #3 wind would be better for me? It has a speed of 11.79 rpm/V, meaning top speed occurs at 424 V (RMS) indicating a nominal battery pack voltage of 636 V (which is what I have planned).

Why are they recommending the slower, #4 wound motor? I have queried them about it, so I will hopefully find out soon.

Cheers,
Chris

Edit: Should add I will be using a RMS PM150DZ inverter which can tolerate a maximum DC pack voltage of 720 V, and deliver a constant 250 A (RMS) to the motor. It's good for 300 A (RMS) peak, so if the motor is up for it, it will be good for about 125 kW.

 

[Nuts&Volts]

That RPM/V measure is the RPM over the bus or supply voltage. That means that you will need a 565V bus to get to 5000RPM. That is probably based on the fact that the motor inverter used must convert the DC bus voltage into a true sine wave and thus the motor voltage line to line (or AC) will need to be 565/sqrt(2) or ~400VAC. From this you can make a RPM/Vac if that's easier for you to work with.

Also consider that this is without using Field Weakening at all. That means you will have full torque up to 5000RPM

edit: Actually looking at the spec sheet. You should use a bus voltage of 600V nominal to get full torque at 4000RPM. Then you can easily get to 5000RPM with field weakening. At 4000RPM you can thus create about 167kW of power with 400Nm of torque (@220Arms*1.81Nm/A). With Field Weakening you will also get roughly constant power at above 4000RPM, so 167kW at 5000RPM too.

 

[jonescg]

OK, so which wind are you talking about? The number 4? When they say 'Inverter supply voltage' are they referring to the voltage supplying the inverter, or the voltage being supplied to the motor from the inverter...

Field weakening is something which is not discussed anywhere in the spec sheet. And to be honest I don't fully understand it. In simple terms, how does weakening the rotating magnetic field cause the motor to speed up?

Thanks for your patience all, I am a plant biochemist after all...

 

[Nuts&Volts]

I am referring to the 4turn wind.

On the spec sheet inverter supply voltage whens the DC bus voltage input to the inverter and is labeled as such (VDC). The inverter output voltage is never, to my knowledge, labelled VDC on a AC inverter. EDIT: However Ke is in Vrms (or phase to phase) as noted at the bottom of the table. Just saw that a minute ago. Thus 600VDC will be 424Vrms (600/sqrt(2)) and 424Vrms*8.81RPM/Vrms = 3740RPM close to 4000RPM they quote. Sorry for this mistake

The graph on the spec sheet is the power curve you will get if you have a 4turn motor on 600VDC with 220Arms peak (motor current). ie 220Arms*1.81(Nm/Arms)=400Nm. 300Arms will give 543Nm peak. You should be able to get 400Nm at base speed (3740RPM) which turns out to be close to 156kW not including inefficiencies.

Field Weakening. The copper windings are experiencing a rotating magnetic field which induces a voltage in the copper. This voltage is opposing the applied voltage which the inverter is supplying and it increases as speed increases. It is called BEMF (Back Electromotive force) In order for torque to be produced current must flow and it flows according to ohm's Law in this manner I= (V_applied-V_bemf)/R_motor. So as some point the V-bemf will grow large enough so that full current cant flow this speed is called base speed. Above base speed, current drops and less torque is made and less power is made.

A inverter will thus send a current "early"(that is it advances timing essentially). This current decreases the magnetic field, which thus decreases the BEMF which allows peak current to be sent through the motor at higher motor speeds. However some % of peak current has been used to "weaken" the magnetic "field" and thus can not be used to generate torque. So you gain speed and lose some torque but at equal rates and this equates to roughly the same power. Field weakening creates roughly a constant power region (thou motor eff may drop).

Without field weakening or poor field weakening the torque and thus power will drop off quite rapidly.

Let me know if you want me to explain more.

 

[jonescg]

So the #4 motor, on a 640 VDC nominal battery pack won't spin any faster than 3700 rpm unless the inverter can provide field weakening?


OK that makes sense. Grade my understanding :lol: So not only is the stator producing a rotating field, the magnets flying past them induces a current back into the stator (BEMF). In order to go faster you need to limit the induced current. As they are permanent magnets, you can't do this, but you can adjust the 'forward' EMF. Most inverters are good at this I would trust.

Evo still haven't got back to me about this very query, but it seems the source of my confusion is the term 'supply voltage'.

Always learning...

 

[Nuts&Volts]

Note my various edits in the beginning of my last post.

And yes you are mostly correct. You can not induce current. You induce voltage and due to ohm's law current flows. Technicalities I know

So you limit the induced voltage. The magnets have a magnetic field going out of them in a direction. The stator normally creates a magnetic field 90deg to that rotor field (I think 90deg...). This creates maximum torque. To limit induced voltage the stator sends current "early" which sends a magnetic field 180deg/in the direction opposite of the rotors field. This cancels out a % of the rotor field and thus the stator sees a weaker rotating magnetic field and a lower induced voltage. So more current is able to flow. Its like an endless loop


I like to grade with more explanation haha.

 

[jonescg]

Yep, the answer was field weakening I can see why they don't mention it on the spec sheet - too bloody complicated :lol:
Many thanks to Evo for explaining this:

The voltage constant for the AF-140-4 is 1.44Vs/rad, measured in Vpk, line to line. Hence, your back emf is about 150Vpk/1000rpm. Thus, to spin it up to 1000rpm you need a bit more than this in terms of dc voltage from the battery, let’s say 160Vdc. From this point of view you are right that you would need about 800Vdc to spin the machine up to 5000rpm. However, what you would typically do is employ field weakening, which effectively uses current to supress your bemf. Hence, you will find that it is perfectly possible to spin the machine to 5000rpm on a 600Vdc or even lower dc bus. If you dc voltage is about 640Vdc, as I understand from your email, you will be perfectly fine running the AF-140-4 machine.

You are also right that you could potentially use a AF-140-3 machine instead. This would mitigate the need for field weakening but would have two big drawbacks: Firstly, the inductance of the machine is much lower, which translates into a choppier ac waveform and, consequently, magnets which will overheat when operated at higher speeds for a longer period of time. This is difficult to quantify but the risk is very real. Once demagnetised, you will see an irreversible loss in performance, i.e. torque and power will drop significantly. Second, since you have a lower voltage constant on the AF-140-3, you also have a lower torque constant, which means that you need more current to produce torque. For example, the AF-140-4 needs about 350Arms to produce 600Nm whereas the AF-140-3 needs 465Arms to achieve 600Nm.

 

[grindz145]

I'm looking forward to what comes of this....

I wouldn't worry about using field weakening. Not effective for what you want to do. Great for a '125% mod'on a topped-out hub motor.

 

[jonescg]

But the maths says I have to use it, or else I can't go any faster than 3750 rpm... :?

 

[Biff]

The guys at EVO, know what they are talking about, and for your application you really want to be running field weakening (a.k.a timing advance) to get your motor spinning at top speed.

If you had your motor setup to have full torque up to maximum RPM, you would sacrifice the available torque at lower speed. Not only that, but in your case, like most, your system overall will run more efficiently as well.

Typically you spend very little time at full speed, so having peak power at full speed seems like a waste.
Having higher torque / Amp, allows you to put down the power you want to accelerate with less controller current, which keeps your controller happy, at the same time you apply so much torque that you need to use all that available torque for less time which quite often ends up heating your motor less as well.

Designing and gearing your system to deliver peak power at around the speed you can use it to your greatest advantage, and still being able to achieve the top speeds you want is the key to the system, and I think EVO has given you solid advice.

That said, I think the 130 would probably have done the job just fine for you, but it very rarely hurts to have a little more power than you need

-ryan

 

[jonescg]

Sorry to drag a dated thread up again, but there has been some heated disagreement over on the AEVA forum about which motor winding I should use.

One poster is adamant that the #3 wound motor will be better for my chosen DC Bus, and that the #4 will be too slow or underpowered. I suspect this is because the concept of field weakening is essentially one of compromise.

I guess the point is, will the ability to wick this motor up to full speed (5000 rpm) and get useful power from it at this speed, be limited by the inverter, or the limited base speed of the motor? Even if it's stuck at 3700 rpm before entering the field weakening zone, with a 14:41 reduction on 17" wheels equates to ~160 km/h.

It probably has something to do with rotor preservation. Too high a current would overheat the magnets, and that's not good.

I think the only solution is to try it out