# AC-Drive frequency question



## Siwastaja (Aug 1, 2012)

nesto said:


> Hi
> 
> I asked Zapi about the Output switching fr.
> 350 HZ they said.


This probably isn't the switching frequency (which is pretty much unimportant) but the fundamental frequency (the one you want to know).




> So the Emrax is a 10 polpair motor


Typically, we talk about number of poles per phase. 10 pole pairs per phase would be a 20 poles per phase motor. Are you sure? That's pretty extreme. Sounds like it's designed for direct drive to wheels. What are the power and torque ratings?



> witch means that i could
> not run it higher that 2300 rpm?(until max fr. is reached)


Synchronous speed for a 20-pole motor at 350Hz would be 2100 rpm.



> My question now is when frequentmax. is reached and voltage still rising what happen?Does the Controller reducing the switching points in one phase and motor still increase rpms?


Probably no, if the maximum fundamental frequency for the drive is 350 Hz, then it is the max. You can't go higher. 350 Hz sounds very typical maximum.

Generally, frequency sets the speed. Voltage (or pulse width) is scaled linearly with frequency to maintain constant torque. Torque can be adjusted by differing from the constant voltage/frequency relationship.

Typically, 4-pole (1800 rpm @ 60 Hz) motors have the best power/weight ratio and they are used with reduction gears like something around 8:1.


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## nesto (Oct 4, 2010)

The Emrax was also tested at Siemens Germany.(Inverter 900 Hz!!!)
4800 rpm 98 kW Peak 
I think there is no Controller who switching this amount???


http://www.enstroj.si/Electric-products/emrax-motors.html


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## subcooledheatpump (Mar 5, 2012)

That is pretty extreme. I can't be of much help as far as made for EV components go, but I do know a number of modern industrial AC drives can go up to 1500 Hz output frequency.


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## frodus (Apr 12, 2008)

Isn't the emrax meant to be bolted to a propeller? It's not MADE for high RPM, it's a fairly low RPM motor. Maybe it's been tested that high... but the specs are still under 3k rpm.

And the ACE-5 is for asynchronous induction, you need the BLE-5 made for synchronous BLDC motors.


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## nesto (Oct 4, 2010)

So for my Project i need high rpm above 3000 !
I will go for the unitek bambocar D3, 460 V on the Battery.
Enstroj told me that i will get around 4000 rpm.
but iam not sure about the frequency this is limit to 400 Hz.
Unitek told me something about derating the switching points per sinus at high rpm???

Sorry but i dont want "buy and find out"


thanks for sugestions...


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## frodus (Apr 12, 2008)

Then that's not the motor you want if you need high RPM.

Why do you need high RPM, why not low RPM and use a gearbox? I assume you'll use a gearbox anyway, right?


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## nesto (Oct 4, 2010)

Note: There are two models of the Emrax 
120 Volt 4000 rpm
300 Volt 3000 rpm

I have the 300 Volt Motor using it for a jet-boat project.
geared up from 4000 to 7500 rpm.
I choose this motor because it is light !


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## nesto (Oct 4, 2010)

This Hull is very light, so the motor should be also...


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## frodus (Apr 12, 2008)

wrong motor for the job then. Either you need a different motor, or you need a gearbox. Trying to overspeed a motor will give you little or no torque at those higher RPM's.


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## nesto (Oct 4, 2010)

Yes it is geared up!
But for peak power i need high rotation (4000 better would be 4500 rpm)
If the switching fr. is limiting the speed,how much rotation will give 400 Hz?


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## subcooledheatpump (Mar 5, 2012)

Frequency * 120 / N (where N indicates number of poles)

For example 60 Hz * 120 = 7200 and 7200 / 4 poles = 1800 RPM

Also, "swithing frequency" is how often the transistors in the inverter switch, not the actual output frequency of the inverter


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## frodus (Apr 12, 2008)

Yes, but for big power, RPM is only one part of the equation. If you have low torque at a high RPM, you won't get more power out of it..... Power involves RPM AND torque.

So just because you go higher RPM, doesn't mean torque increases as well.


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## nesto (Oct 4, 2010)

Lets calculate:
i mean the fundamental output Fr. of the unitek is 400 Hz.
the emrax have 10 polpairs ,meaning 20 poles.


400 x 120 = 48.000 / 20 = 2.400 rpm

In testing with 300 Volt they reached 2680 rpm on full load!

The result in picture!


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## frodus (Apr 12, 2008)

That's fine, but look at the output power Pm!

At 2680RPM the Pm is actually lower than at 2500RPM test at lower voltage.

At a certain point, the torque drops off above a certain RPM....


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## nesto (Oct 4, 2010)

But Idc is also more weak at 2680! he had to put more throttle.

I just read in the Unitek manual:

The EC Motor rises speed with Voltage!
The AC asyncron rises only by frequent!


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## frodus (Apr 12, 2008)

Motors only ask for as much current as they need. Controllers don't force current. They loaded the motor at 2680RPM to different torque loads, it still couldn't get past 30kw.

The torque falls off at a certain RPM, period. 

I have no doubt you could get to 3, 4, 5000RPM, but the useable torque at that RPM would be less than the peak torque. i.e. power would flatten off, and eventually start to decrease.


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## subcooledheatpump (Mar 5, 2012)

It's voltage that forces current. An increasing frequency, which is needed for increased speed, increases impedance which resists current. 

Since current is needed for torque, and voltage is needed to overcome impedance, you can come to the conclusion that voltage and current make power. 

Controllers are current limited machines, or in a sense they are current control machines. The more battery voltage you have, the higher speed the motor can go because more voltage will overcome more impedance and force current through the motor at higher speeds. 

However, any EV system has a pratical voltage limit, therefore there will be a point where the impedance will exceed the voltage ability of the battery, so torque will fall at some point. 

This is true for all motors, since all motors essentially work the same. Some motors produce their own Alternating current by mechanical means, like a brush and commutator. Others rely on a controller to make AC for them, like a BLDC or induction motor. But they all follow the same principle. 

More voltage, more speed. More current, more torque. Speed limits current, which limits torque.


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## dougingraham (Jul 26, 2011)

subcooledheatpump said:


> Controllers are current limited machines, or in a sense they are current control machines.


Electronic speed controls only control the output voltage. It is the only thing they can do. AC controllers also control the commutation but essentially all they do is change the voltage that the motor sees. An ideal electric motor will try to turn at a given speed when a given voltage is applied. Increase the voltage and the shaft speed will try to increase. The amount of current depends on the load applied to the output shaft. Go up a hill and the load on the shaft increases and the current draw will increase. Down a hill and the load decreases and the current will decrease. Because we don't deal with ideal electric motors there is some cross coupling when the load changes. When the load increases the RPM will drop because the motor effectively sees less voltage due to resistive losses. Decreasing the load will increase the motor RPM because the current decreases and the resistive losses are less so the motor sees an effective increase in the voltage.

Electronic speed controls have a control loop where they look at a number of parameters and change the commanded output voltage in order to keep those parameters under control. Generally they look at the commanded throttle position, battery current, battery voltage, motor current, and motor voltage. For EV use the throttle position usually is configured to command the motor amps because this gives a natural feel. Motor amps will be commanded somewhere between 0 and the controllers limit or a lessor arbitrary motor current limit. This will be overridden if the battery current, or motor voltage goes too high or if the battery voltage goes too low.

It is the voltage that controls the desired RPM and the current follows based on the load on the motor.

Voltage = RPM
Current = Torque


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## subcooledheatpump (Mar 5, 2012)

Right but when you say load, that directly means motor torque. 

Essentially any mechanical load isn't a speed load, it's a torque load. Any driven machine requires torque to drive. The torque required, and the speed at which it is required to produce that torque determines power. 

With no load, the motor then is free to spin up to whatever, almost regardless of the current since the torque is zero (not counting bearing or other friction losses). So since a car, or any other vehicle requires torque to move, and as you say we do want torque control, the controller is essentially a current limiting machine, or rather it determines the appropriate current for the commanded torque from the accelerator. Yes I understand a controller will output voltage, but in a sense that is still the wrong way of looking at it. The batteries output voltage, and if they are connected to the motor, a current will flow. The controller only adjusts how much current is allowed to flow. That might be accomplished by means of limiting the voltage, but the objective is still the same, torque control


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## nesto (Oct 4, 2010)

@ frodus

I looked to Siemens Test and they draw up to 61 kW power at 2600 rpm!
So for High power you Need High Rotation.


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## frodus (Apr 12, 2008)

So whats the peak power at 3000RPM? 4000RPM?

It may peak at 2600RPM, but above *some point* it starts to flatten and fall off.


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## nesto (Oct 4, 2010)

I have no curves of this motor but what you say to Enstroys comment?


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## frodus (Apr 12, 2008)

Still doesn't make my statements any less true, but it does seem that the peak HP of the motor is higher, and the motor could be run at a higher RPM.

Still doesn't mean that there's not a peak of the motor where the torque falls off.

And without ACTUAL TESTING on the motor at 130kw, we cannot trust those claims.


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