# Determining Base Speed of PMAC Motor



## JCC (Mar 19, 2015)

Hi Guys,

First off, nice to meet you all! I'm new to this forum. I'm going through a motor selection process, and I trying to calculate the base speed (or knee) of the torque speed graph relating to the motor so I can have a rough idea of what kind of gear ratio will be needed. I'm hoping someone could take a look to see if I'm taking the right approach. The motor I'm analyzing in this thread is the ME1115, listed below:

Motor: ME 1115
Rated 24V to 96V
Torque Constant: 0.15 Nm per Amp
Max Recommended RPM: 5000 RPM
http://www.motenergy.com/me1115motor.html

At this point, I'm only looking at base RPM, the point where the constant torque region ends and the constant power region begins, so gear ratio should not have an affect yet since I'm only looking at motor RPM. 

Assuming 48 Volts nominal and 200 Amps peak, peak power would be 9600 Watts. Peak Torque = 0.15 * 200 = 30 Nm.

Base RPM = (9600 Watts)/(30 Nm)*(60 s/min)/(2*pi rad/rev) = 3057 RPM

Does this seem right? My thinking would be that Base RPM would occur sooner.

Here's another example with using 96 Volts and 200 Amps. Peak Power = 19200 Watts and Peak Torque = 30 Nm.

Base RPM = (19200)/(30)*(60)/(2*pi) = 6114 RPM

I can't help but notice that is well above the recommended 5000 RPM limit. 

Does all this seem right? Am I missing something?

Thanks again!

Edit:
The controller to be paired with this motor will likely be the Sevcon Gen4 size 4, which can handle 39 to 69V and currents of 450A continuous and 540A peak for 10 seconds.


----------



## dcb (Dec 5, 2009)

not the expert, but I've only heard "base speed" in reference to ACIM, i.e. 60hz - slip. 

I think part of the confusion is that it is using rms volts and amps in the graph, and they are ~.707 * the pack volts/amps in this case.

so the green 68v line represents a 96v pack, and it is producing 3000 rpm no load. So, I am guessing, might be wrong, but 48v should give you 1500rpm.

the max rpm will drop a bit with load, especially if your batteries sag under load.

so 180 battery amps continuous * 48v * .92 (efficiency) =~8kw continuous @ ~1500 rpm. Though it isn't unusual for efficiency to drop at lower voltages, despite what the page says. 

The graph and page are very "fudgey".

it looks like the 12kw continuous rating is based on 125 amps AC (which it says is 180 amps DC) and 96v battery pack, so they are mixing AC and DC values in that figure, 125A (not 180A) * 96V (not 68V) = 12kw.

I'm obviously confused as well  since it is an inverter, the DC voltage isn't constant, so using a factor of 1.4 kinda makes sense. so 5.7kw continuous at 1500 rpm @ 48v is my best guess.


----------



## major (Apr 4, 2008)

We've been through the mess of motor specs for the ME1115 here:http://www.diyelectriccar.com/forums/showthread.php?p=376274#post376274
And make sure you have someone who can program and tune that Sevcon before you buy it. Best advice for AC drives is to buy the package pretuned.


----------



## JCC (Mar 19, 2015)

major said:


> We've been through the mess of motor specs for the ME1115 here:http://www.diyelectriccar.com/forums/showthread.php?p=376274#post376274
> And make sure you have someone who can program and tune that Sevcon before you buy it. Best advice for AC drives is to buy the package pretuned.


Thanks major. That thread included a lot of thoughtful discussion. I agree with dcb that the graph and webpage appear "fudgey". 

So for calculating the base RPM:

Electrical_Power*Efficiency = Mechanical_Power

P_E = Vb * Ib = sqrt(3) * Vrms * Irms * cos(theta)
Efficiency = 0.92 (as claimed)
P_M = Torque*Angular_Velocity = Ib*(0.15 nm/A)(RPM*2*3.14/60)

(.15 Nm per Amp is described in the specs. I'm assuming this is referring to dc current)


So for Vb = 48 Vdc and Ib = 200 Adc
P_E = sqrt(3)(48*.7071)(200*.7071)(.866)
P_E = 7199.7 W 

P_M = (200*.15)(RPM*2*3.14/60)
P_M = 3.14*RPM

So RPM = (7.199.7*0.92)/3.14 = 2109 RPM

If you repeat the above with 96 V instead of 48V, the RPM = 4585, which seems high. Additionally, P_E is quite less than the power delivered from the battery.

Dcb's numbers seem to make more sense, and if I simply calculated:
P_E = (Vb*.7071)(Ib*.7071)
... I'll get the numbers mentioned by dcb, but I don't think I can do that since that's for a single phase system, and this is a three phase Y machine. 

I feel like I'm missing something.


----------



## major (Apr 4, 2008)

JCC said:


> P_E = (Vb*.7071)(Ib*.7071)


That's just dead wrong  

If the dealer can't/won't give you good specs on the motor, I wouldn't buy it.


----------



## JCC (Mar 19, 2015)

major said:


> JCC said:
> 
> 
> > P_E = (Vb*.7071)(Ib*.7071)
> ...



I was referring to dcb's post pertaining to the scaling of 1.41 with that statement. What do you think of the calculations before that? Can I take that approach, or is there a better way?

Unfortunately those specs are not from the dealer, but from the manufacturer; however the dealers aren't much better.


----------



## major (Apr 4, 2008)

Sorry, I can't follow what you are doing or what you want. If it is base speed at 48V battery, use 1500RPM. And why would you want a motor from a manufacturer who can't even spec the darn thing  Think he can build them any better?


----------



## JCC (Mar 19, 2015)

major said:


> Sorry, I can't follow what you are doing or what you want. If it is base speed at 48V battery, use 1500RPM. And why would you want a motor from a manufacturer who can't even spec the darn thing
> 
> 
> 
> ...



I simply want to know how to determine the base speed of a brushless motor, and I hope to be able to use such a method as I look at other brushless motors in the future. So at this point, curiosity is the reason I'm wanting to know. If I was dealing with a brushed motor, I'd simply use the Kt and Ke constants or even the power curve to determine this base rpm. Yet to my knowledge, the rpms for a brushless motor is not directly proportional to voltage, but rather frequency. 

It doesn't help that I included in this discussion a motor with specs that turned out to be messy. I initially included it because I had a source of where to get one at a decent price, but with this discussion and the one you linked in post #3, I think I'm going to consider better motors.


----------



## dcb (Dec 5, 2009)

without getting into field weakening (or advanced timing in pmac) as power falls off rapidly, there is a speed where BEMF cannot be overcome by the battery, so I would consider that the base speed (voltage dependant). In the real world, there is a bit of loss at higher currents in that graph, plus with a battery supply there will be voltage sag. Since the graph is gradually losing rpm with increases in current, it is probably at its rpm limit for the applied voltage.

Three phase voltage is measured between two phases, and is .707 of peak. 

with a full wave inverter the max peak to peak between phases is +-vBatt (ignoring switch losses and otherwise idealizing the controller). So vrms is just vbatt * .707. (and rectified 3 phase is just vrms/.707 fyi)


I believe when they say 68v in that graph, @ 3000 rpm they mean a phase 2 phase rms, which is a 96v battery. which gives you, in terms of battery voltage, 31.25rpm per 1vdc.


I believe in the text when they say 30kw max (@ 96v) they mean a 136v battery minimum, and a base speed of 4250rpm. 

and so with a 48v battery I would only expect ~1500 rpm with full available power. 

And I could be completely wrong of course, but I would plan on a ~144v battery and 250+A capacity to get close to max output. The motor specs are not the same as the controller specs. but you need more than 30kw from the battery to make 30kw obviously, and the graph is showing like %82 efficiency (controller in that figure?)

You need lots of volts and/or lots of amps to get lots of power, volts are easier to manage.

you might even want to look at some existing diy inverters to see if you can put a controller together that will suit your needs, though the difficulty level/time commitment is high, it is quite educational. i.e. http://www.instructables.com/id/200kW-AC-Motor-Controller-for-Electric-Car/ or http://johanneshuebner.com/quickcms/index.html?de_electric-car-conversion-site,14.html I think both are induction only at the moment though mpaul is planning on sorting out bldc. there are very long threads here and elsewhere on both controllers also.


edit: it could be that the 68v is battery voltage, but that is such a strange voltage to pick, so I kinda lean towards 96v battery. just a guess.

edit2: apparently the me1115 has a sine/cosine encoder to determine exact rotor position. otherwise it is identical to the hall sensor me0913, if you want to look for other performance data points, i.e. https://www.google.com/search?q=ME0913+evalbum

edit3: it *may* be possible to trade lower voltage for higher current with a delta-wye switch, but you have to know what you are doing obviously. Generally higher voltage is better, till you start exceeding standard parts availibility.


----------

