Electrical motors and generators

[Suyash Sapkal]

Hello,

I am an engineering student. I have a question for you.

Let us consider, we have an electric motor and it is being supplied the electric current. Thus it is converting the electrical energy into mechanical energy and we are getting the rotational motion at the end of the shaft. Now, what if we connect another motor(to be specific a generator) to the end of the shaft and it is further connected to store the energy in batteries, is it possible to convert the energy completely and store it? maybe not due to the core losses in both motors. But, what if we attach a set of gears at the shaft such that, the speed of rotation of the second motor is greater than the speed of the motor of the first motor and it will overcome the losses, is it possible? 

How can I study more about electric motors and power electronics online? or Where can I find the resources online?

I am currently working on a project for regeneration of energy and I need resources to perform calculations and to study the motors and generators mathematically. I need help to know where can I get these resources and how to get them. I would like to get to know few people who would help me.

Thanks.

[Bufofrog]

38 minutes ago, Suyash Sapkal said:

, is it possible to convert the energy completely and store it?

No it is not.  Look at Newton's second law.

Edited July 4, 2020 by Bufofrog

[Danijel Gorupec]

As Bufofrog said, you have no chance with the proposed motor-generator-battery setup. A certain amount of energy will be 'lost' in every step:

- your motor has efficiency less than 100% and will 'lose' some energy as it converts electrical energy into mechanical energy

- your generator has efficiency less than 100% and will 'lose' some energy as it converts mechanical energy back into electrical energy

- your battery has charge/discharge cycle efficiency less than 100% and will give you less energy back than you previously stored into it.

Increasing the rotational speed of the generator does not make any essential difference (although the gearing might give you the ability to keep the rotational speed of your motor and your generator at their best-efficiency range, but you will still lose energy. In fact, the gears will introduce additional energy loses, so it might not even decrease the overall loses).

 

BTW, I did it when I was a kid - it did not work

[Suyash Sapkal]

2 minutes ago, Danijel Gorupec said:

As Bufofrog said, you have no chance with the proposed motor-generator-battery setup. A certain amount of energy will be 'lost' in every step:

- your motor has efficiency less than 100% and will 'lose' some energy as it converts electrical energy into mechanical energy

- your generator has efficiency less than 100% and will 'lose' some energy as it converts mechanical energy back into electrical energy

- your battery has charge/discharge cycle efficiency less than 100% and will give you less energy back than you previously stored into it.

Increasing the rotational speed of the generator does not make any essential difference (although the gearing might give you the ability to keep the rotational speed of your motor and your generator at their best-efficiency range, but you will still lose energy. In fact, the gears will introduce additional energy loses, so it might not even decrease the overall loses).

 

BTW, I did it when I was a kid - it did not work

How do I calculate the losses and do the mathematical calculations? or do you know any resources to study them?

According to me the main losses here will be the core losses of the motor, frictional losses at the gear set and the heat losses.

Actually, I don't know about the battery losses. Assuming the battery to be lithium ion battery, what is the loss in continuous charging and discharging of the battery?

[Danijel Gorupec]

If you will be using your motors and generators near their intended regime (rotational speed, voltage, load), then it is often good enough to just look at the motor/generator nameplate - the efficiency should be stated there. It very much depends on the motor type, class, size... A small asynchronous motor (around 1kW) might be 80-90% efficient. Larger motors have higher efficiency than smaller ones. Technology makes difference too - a permanent magnet brushless DC motor are usually more efficient that equal-power common squirrel-cage asynchronous motor. High-efficency motors can cost significant money.

However, if you will be using your motor in wide range of working regimes, just looking at the nameplate efficiency might not be good enough. For more expensive motors you will probably be able to obtain efficiency diagrams from the motor manufacturer. This would be perfect, because calculating it from first principles is difficult - you would need to know everything about motor construction to calculate it properly (in this case, it would be much easier to just measure it than to try the calculation).

For qualitative analysis and orientation, you will be able to find various efficiency curves by googling. Some general efficiency notes:

- the core loses are usually divided into two parts: hysteresis loses (depends on quality of silicon steel used in your motor - hysteresis loses are roughly proportional to the frequency, keeping other things the same) and eddy-current loses (depends on the resistance of silicon steel and lamination thickness used in your motor - these are roughly proportional to the square of the frequency). In a typical motor, at its nominal speed, the hysteresis and eddy-current loses are about the same.

- the copper loses might be about the same as the core loses (in nominal regime). In some types of motor, these can be relatively easy to compute if you know motor current and winding resistance.

- mechanical loses are usually divided into bearing loses and (in some constructions) cooling fan loses. Normally are smaller than copper loses and core (iron) loses.

- motor efficiency very much depends on the load - the best efficiency is often when your motor gives 70-80% of its nominal load. At low loads, of course, the efficiency drops drastically.

- you usually regulate your motor using some circuitry (motor driver). Modern semiconductor drivers (like inverters) can have efficiencies well above 90%. Still, take it in account.

Only if you will be designing motors themselves, you will need to dig deeper than simply analyzing motor efficiency curves. Today you would be typically using computers and numerical-analysis software to compute your motor efficiency.

 

Batteries are chemistry and not my area, but of course they have limited efficiency (as anything else). I see that a typical lithium-ion battery might have charge/discharge efficiency of about 80-90%. Of course, if you are not actually charging/discharging your battery, like in the case when you are generating just as much power as you are spending, then this efficiency is not a concern. But you will occasionally charge/discharge your battery (otherwise, you would not even have the battery in the first place).

Another source of battery loses is the self-discharge. This means, a battery will lose some percentage of stored energy with time - even if you don't charge/discharge it. I understand that lithium-ion batteries have fairly small self-discharge, so unless you need a long-time storage it is not so much a concern.

As everything else, I expect that battery efficiency depend on quality of construction, and this affects the battery price.

 

Gearboxes can be made to have very high efficiency, fairly over 90% - but it again depends on the construction (worm gear is not very efficient, while helical gears can be). Unfortunately, a good gearbox can be surprisingly expensive as it requires high precision.

 

A trivial example: you have an inverter (efficiency 95%) that runs a motor (efficiency 90%) that drives a gearbox (efficiency 95%) that drives a generator (efficiency 90%) that charges a battery over a rectifier (efficiency 97%). Later you discharge the battery (efficiency 85%) to produce electricity for something useful... The fact is that the electrical energy that you obtained from this battery is only about 60% of what you spent in the first place:  total_efficiency=0.95*0.90*0.95*0.90*0.98*0.85.

 

I cannot recommend any good source on the Internet - I am sure there must be many, but I just tend google for anything specific and usually find something useful. Hopefully somebody else will provide a good link.