# Technical questions about engines



## major (Apr 4, 2008)

joamanya89 said:


> Hi, I have two technical questions about engines:


Please call the electric machine a motor, not an engine  Engines are the smelly noisy polluting monsters we seek to send to never-never land.


joamanya89 said:


> The first is about power , I know that all electric motors gives maximum power as soon as you start ,


Wrong. Most electric motors will produce maximum torque at stall (or as you say as soon as you start). Torque, not power. The motor produces zero shaft power at zero RPM.


joamanya89 said:


> I mean a 500w motor will try to give it maximum output power from the beginning not as fuel engines right?
> And also they try to spin at their RPM-volt relation, if the motor is 36v 2500rpm, and you are giving it full acceleration it will try to spin at 2500rpm.


Sorry, I don't follow you



joamanya89 said:


> So, because if begin from stop, then the motor will consume a lot of Amps to get it maximum torque to get as quick as it is possible the 2500 rpm, right?


No, this is incorrect logic for the motor/controller systems used in EVs. It actually is correct for a motor slammed across the battery without a controller. But you end up with a lot broken parts and melted tires from excessive torque.

The PWM motor controller has a current limit function used for launches and employs current multiplication. So the motor current is higher than battery current. At starting, maybe 10 times higher or more. It is not a great stress on the battery.


joamanya89 said:


> So, when it is running at 2500 rpm, it amps consumption will decrease until it reach some needed consumption to move the load that is driving. right?


Yes.



joamanya89 said:


> So if I use a motor of 500W to move a motorbike it will be using a lot of amps to move the load right?


Compared to what


joamanya89 said:


> But if I use an exagerated motor, lets say 15kW motor to move a motorbike, would it be using less amps so the battery pack will last long?
> 
> (of course both cases driving at same speed)


Depends on a number of things. Typically more powerful motors induce heavy foot drivers and use more power but go a lot faster. They can be more efficient in "some" cases.


joamanya89 said:


> Please if I said something wrong correct me so I can understand how electric motors work.
> 
> And my second question is about different motors, I just can´t understand the difference between normal dc brushed motor, permanet magnet motor, and etek motor, which is better for what? and why?
> 
> ...


Read a book. Starting with how to count. That's a lot more than 2 questions  Different motors for different folks. None the best. Good points/bad points; study the archives. You can find examples.

Good luck,

major


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## joamanya89 (Feb 13, 2012)

hi, first of all, sorry about all mistakes I have done, some are because translation problems and some others because my poor knowledge. 



I would like to read about this kind of things, would you please recomend me any web or book to read about it?


And thanks for your time.


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## akseminole (Jan 5, 2014)

Perhaps: http://www.diyelectriccar.com/forums/showthread.php?t=669


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## joamanya89 (Feb 13, 2012)

Major, you are right and thanks for all your answers, but there is a few things..

First, there is a physical relationship between torque and power: P = M.w where P is power M is Momentum and w is angular velocity.

So I guess that what you wanted to explain me is that because at stall there is no w, so there is no Power, but a lot of Momentum, right?

And I´m think that is at the minimum w you get, multipling it by momentum that it consume lot of amps right?

Then, If I follow you, the controllers has a current limiter for that, so if I´m not wrong, the controllers use PWM for not shorting the motor to battery and also it has the current limiter to don´t melt the battery, is that right?

So I use the throttle to handle the speed I want to go, then the controller deals with the current to don´t melt everything, but it sets the PWM at right value to get the speed I´m asking, is it correct?


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## major (Apr 4, 2008)

Forget about momentum. I said TORQUE. Power is equal to Torque times Angular Velocity. Watts = (Newton meters) * (Radians/second).

And high motor torque at low RPM does not require high battery current.


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## joamanya89 (Feb 13, 2012)

major said:


> Forget about momentum. I said TORQUE. Power is equal to Torque times Angular Velocity. Watts = (Newton meters) * (Radians/second).
> 
> And high motor torque at low RPM does not require high battery current.



What you're saying is wrong, a motor generating high torque at low rpm DOES consume a large current .

This is because the rotation of the motor is what induce a current in the opposite direction at the motor windings, causing the current consumed by the motor decrease while revolutions increase.

So at hi rpm the motor´s current consumption is less than a when it spins at few revolutions with the same load.


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## samwichse (Jan 28, 2012)

joamanya89 said:


> What you're saying is wrong, a motor generating high torque at low rpm DOES consume a large current .
> 
> This is because the rotation of the motor is what induce a current in the opposite direction at the motor windings, causing the current consumed by the motor decrease while revolutions increase.
> 
> So at hi rpm the motor´s current consumption is less than a when it spins at few revolutions with the same load.


That's motor current, not battery current.


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## joamanya89 (Feb 13, 2012)

samwichse said:


> That's motor current, not battery current.


But motors current comes from battery pack!, peopple is it for true? where do you think the motor´s current comes from? it is all the same current. 

I know the difference between motors current and battery current at the moment of doing maths for calculating motor size, battery size and controller size, but it doesn´t matter is all the same current.


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## akseminole (Jan 5, 2014)

Unless your motor controller is as intentionally ignorant as you, it amplifies the current at low rpm while outputting a lower voltage to the motor.

When your rpm goes higher, eventually, the motor current and the battery current match.


Yes.

If your "motor controller" were you plopping the jumper cables onto the battery terminal lugs, and then onto the motor lugs, you are absolutely correct. The motor current would Be the Battery current.

As the motor is going through the intermediary of the motor controller, things other than what You seem to Assume, will be going on.

For instance PWM or pulse width modulation is how the controller is able to push a lower voltage to the motor while simultaneously pushing a higher Motor current to the motor than the current being Pulled from the battery.
Magic? No. Science? Yes.


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## joamanya89 (Feb 13, 2012)

akseminole said:


> Unless your motor controller is as intentionally ignorant as you, it amplifies the current at low rpm while outputting a lower voltage to the motor.
> 
> When your rpm goes higher, eventually, the motor current and the battery current match.
> 
> ...


It doesn´t amplifies nothing, it just compensates the battery battery power.

The motor power and battery power must be equal, so if you use low PWM, the to compensate the rest of battery power you will use much more current.

So if you have heavy load, the motor will take more time to increase rpm, so it would not be able to generate the magnetic fild as quik as with no load, so it will consume for longer time that quantity of current


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## akseminole (Jan 5, 2014)

No one here disagrees with you, that the motor Will be pulling high current at Stall.

Or that the motor does pull higher anps at 0rpm than at Max rpm.

What we all have been trying to say, is that the motor controller changes the output of the batteries to be compatible with the motor at low rpm.

Rather than putting a high amp load on the Batteries when the Motor is at low RPM, the motor controller Amplifies the power to the motor "like" a speaker amp amplifies the sound signal to a subwoofer Boosting the available Amps to the motor. 

I'm sure it isn't a completely sound analogy, but even in Uruguay, I assume people use amplifiers to make their car annoying and loud..


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## akseminole (Jan 5, 2014)

My question, if you already "Know everything". Why do you bother with asking questions of us mere mortals?


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## joamanya89 (Feb 13, 2012)

akseminole said:


> My question, if you already "Know everything". Why do you bother with asking questions of us mere mortals?


It is not like that, since I have asked my firts question, major just says NO to everything, so then I started to reed a lot and try to answer him, that it was not everything wrong.

I´m electrical engineering student, so I know a few about electronics, but here is not much knoledge about EV, I wanted to know your point of view, because all of you know about the evs.

The deal is that at my first question the answer that I got was just NO, and some of those NOs doesn´t smells well for me.

Would you please explain me how a big motor could have beter performance than a small motor driving the same load?


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## akseminole (Jan 5, 2014)

http://www1.eere.energy.gov/manufacturing/tech_assistance/pdfs/10097517.pdf

The cliff notes is that an underpowered motor will be out of it's efficiency range trying to push a bigger load than it is rated for, while a motor which is slightly oversized can run at a lower power use for the same size load as the undersized motor.

Read the link for the math.


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## akseminole (Jan 5, 2014)

Also, in regards to motors, apparently, the larger the diameter of the motor the longer a "lever" the armature creates for the "creation" of torque.

I am not an engineer of any sort, student or otherwise, so I would take what I say with the grain of Wikipedia sand that it is worth.


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## joamanya89 (Feb 13, 2012)

I appreciate your answer. I will read the link, really thank you very much for your answers.


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## major (Apr 4, 2008)

joamanya89 said:


> major said:
> 
> 
> > Forget about momentum. I said TORQUE. Power is equal to Torque times Angular Velocity. Watts = (Newton meters) * (Radians/second).
> ...


Please remember who is seeking knowledge here and who is the experienced teacher. A little more respect from the student would be appreciated.


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## dcb (Dec 5, 2009)

motors are basically big inductors, and coupled with a controller (the switch and diode part), they are basically a giant buck converter. Together they can convert high battery voltage into high motor current (while the back emf is low)


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## joamanya89 (Feb 13, 2012)

major said:


> Please remember who is seeking knowledge here and who is the experienced teacher. A little more respect from the student would be appreciated.


Major I have no doubt of your knowledge, but you really are the only person in this whole issue gave no answer, you just say that everything was wrong, and now you are too proud to admit that among others could understand each other well.


Regards


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## joamanya89 (Feb 13, 2012)

dcb said:


> motors are basically big inductors, and coupled with a controller (the switch and diode part), they are basically a giant buck converter. Together they can convert high battery voltage into high motor current (while the back emf is low)




So what you are saying is that while te motor rpm are low it will do have big motor current, so it will consume big current quantity, until it spin up and the electromagnétic fild generates an oposit current so it will reduce the motor current, so the current consumption will decreas? Right?


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## dcb (Dec 5, 2009)

if it applied reverse current, it would be operating as a generator. back emf is counter electromotive force, or voltage, which motors create as a function of RPM.

The battery and/or the controller are often the limiting factor in current delivery. And the motor resistance is low enough that you can deliver full battery/controller current almost until counter emf = battery voltage, if desired. But it isn't required to have high battery current to have high motor current, especially if you are trying to be efficient rather than win a drag race.

Are you familiar with buck converters? Do that.


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## joamanya89 (Feb 13, 2012)

I have readen about back converter, explain me something, what would be the load at this case? the motor resistance?


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## dcb (Dec 5, 2009)

more specifically the motor reactance. it isn't pure dc current.


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## joamanya89 (Feb 13, 2012)

dcb said:


> more specifically the motor reactance. it isn't pure dc current.


is it motor restsistance plus motor reactance (a+bi) where b is reactance? so, it load changes deppending on Switching frequence?


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## dcb (Dec 5, 2009)

c'mon now.


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## joamanya89 (Feb 13, 2012)

Thanks


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## Sonikaccord (Dec 17, 2012)

joamanya89 said:


> I have readen about back converter, explain me something, what would be the load at this case? the motor resistance?


The motor windings would be the load.
The winding resistance is a parasitic load that generates heat.
The winding inductance stores the energy in a magnetic field (ideally no losses.)
The Back EMF source absorbs instantaneous power (E*I) and provides torque.

That's why you can have high motor currents and low battery currents. You may have a 400V pack, but your motor isn't going to see that 400V at X amount of current. It's going to see X amount of volts with tons of current because the back emf source is not countering that initial surge of current (More accurately reducing the voltage difference Vin - Vbemf).

It is modeled as a series RL circuit with an independent voltage source oriented in the opposite polarity of the incoming current.

Power in = power out - losses always.


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