How to describe PMSM/BLDC base speed?

Electric Motors and Controllers
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qaz33
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How to describe PMSM/BLDC base speed?

Post by qaz33 » Thu Oct 26, 2017 10:12 pm

I would like to pick up the winding for my motor to use it in the most efficient way according to my driving average style. Usually I ride around 40km/h. What I know is that PM motors have the maximum efficiency at around 80% of their nominal/rated speed.

Image

But how can I describe the rated speed for my hub motor if it always depends on the battery voltage? Usually the companies describe the nominal voltage as "24-72V"

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amberwolf
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Re: How to describe PMSM/BLDC base speed?

Post by amberwolf » Fri Oct 27, 2017 1:28 am

I'm not quite sure what you're asking, but the motor speed constant kV is used to tell you how fast it will go at a particular voltage, depending on load. kV is approximately RPM/Volt. If you don't know the kV, you can determine it experimentally, by applying full throttle to the motor and measuring it's RPM. THen divide the RPM by the voltage applied.


If you want to play around with it, and see the relationships, you can go to http://ebikes.ca/simulator and pick a motor, then try different battery voltages, etc., and see what happens. Read the instructions below the graph area for what everything is and does and how to use it.



If you mean something else, let us know and we can try to help.

qaz33
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Re: How to describe PMSM/BLDC base speed?

Post by qaz33 » Fri Oct 27, 2017 2:20 am

Of course I know kV.

I ask how to find rated speed of the motor

In every PMSM/BLDC motors for industry purpose you can find one just next to the kV info. For ebike motors there is a lack of it

Punx0r
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Re: How to describe PMSM/BLDC base speed?

Post by Punx0r » Fri Oct 27, 2017 2:27 am

Look at the manufacturer's rating plate...

qaz33
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Re: How to describe PMSM/BLDC base speed?

Post by qaz33 » Fri Oct 27, 2017 2:28 am

But there is no rating plate on the hub motors for ebikes. That's a point

To describe this problem better let's analyse any motor from http://ebikes.ca/simulator

If we check battery for example 36V we see that max efficiency is after obtaining x speed, now let's change the battery voltage up to 72V. The max efficiency will double too. Does it make any logical sense? Not at all
Every motor has some efficiency map which is strictly correlated to it's speed not to the applied voltage. What's more those graphs from ebikes.ca don't show any efficiency graph after using flux weakening

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Re: How to describe PMSM/BLDC base speed?

Post by Njay » Fri Oct 27, 2017 4:15 am

qaz33 wrote:In every PMSM/BLDC motors for industry purpose you can find one just next to the kV info.
Because voltage "doesn't" change (mains), I guess.

qaz33
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Re: How to describe PMSM/BLDC base speed?

Post by qaz33 » Fri Oct 27, 2017 5:02 am

That's true, but we can easily decrease the voltage by PWM

Here is a typical PMSM efficiency map

Image

I would like to find a way (formulas) to find this base speed.

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