does an AC motor need sinusoidal current or sinusoidal voltage?

[izzu91]

AC motor need sine wave of current or sine wave of voltage?or both?

 

[Tahmid]

You need to supply AC voltage. The current will not be proportional to the voltage.

An AC motor doesn't really need sine wave. But it's desirable to use sine wave.

Hope this helps.
Tahmid.

 

[goldsmith]

You need to supply AC voltage. The current will not be proportional to the voltage.

An AC motor doesn't really need sine wave. But it's desirable to use sine wave.

Hi Tahmid
An AC motor which has been designed for a single frequency should be supplied just and just with a pure sine wave . as fourier series demonstrates , every distorted or un sine signal consists gang of sine waves with different frequencies ( harmonics ) . so if you use signals which are not sine wave for an AC motor which has been designed for a sine wave many problems will be occurred , like :
1- your motor will provide much dissipation in it's core and also in it's windings . ( most of the dissipation will be in core )
2- it's speed will be a bit changed and of course it's torque will be a bit changed too .
3- when it being under the load it's loss power will be higher because current will be increased and of course if such a current is not sine wave dissipation will be increased too .

( for more information and heavier analysis you can refer to the electrical machine books like : electric machinery by professor bim bahara or perhaps chapman . )

Good luck
Goldsmith

 

[Tahmid]

Hi Tahmid
An AC motor which has been designed for a single frequency should be supplied just and just with a pure sine wave . as fourier series demonstrates , every distorted or un sine signal consists gang of sine waves with different frequencies ( harmonics ) . so if you use signals which are not sine wave for an AC motor which has been designed for a sine wave many problems will be occurred , like :
1- your motor will provide much dissipation in it's core and also in it's windings . ( most of the dissipation will be in core )
2- it's speed will be a bit changed and of course it's torque will be a bit changed too .
3- when it being under the load it's loss power will be higher because current will be increased and of course if such a current is not sine wave dissipation will be increased too .

( for more information and heavier analysis you can refer to the electrical machine books like : electric machinery by professor bim bahara or perhaps chapman . )

Good luck
Goldsmith

That's exactly what I meant (and hence the italic).

It's not that the motor won't run at all if a sine wave isn't used. But for the very reasons you mentioned, sine wave should be used.

 

[goldsmith]

An AC motor doesn't really need sine wave

That's exactly what I meant!

Are you sure you exactly meant it ? but you told something else in your former post . you told it doesn't need really a sine wave !! but it is exactly wrong ! because an AC motor which has been designed for a sine wave should be supplied only with sine wave and nothing else ! ok ?

 

[Tahmid]

An AC motor doesn't really need sine wave. But it's desirable to use sine wave.

An AC motor doesn't need sine sine wave to run/operate. You can use square wave or distorted sine wave and the motor will run. But with losses, reduced torque,etc.

To operate/run efficiently, sine wave is needed. And hence it's desirable to use sine wave.

 

[goldsmith]

An AC motor doesn't need sine sine wave to run/operate. You can use square wave or distorted sine wave and the motor will run. But with losses, reduced torque,etc.

My friend when we say does it need or not we mean if we give it such a wave will it work properly or not . ok ?
I know what will happen if you give it such a signal . it will work but not properly and after some little times it will be distracted .
Your statement is exactly like this : we can pug the ground with our hands . of course we can but our hands will be suffered and then we won't be able to continue the process ! so we'll use the proper tool for it like shovel .

Ok ?

 

[Tahmid]

My friend when we say does it need or not we mean if we give it such a wave will it work properly or not . ok ?
I know what will happen if you give it such a signal . it will work but not properly and after some little times it will be distracted .

For this we would need to know the details regarding the AC motor itself. For example, during summer, in Bangladesh, India, Pakistan, etc in some places, DC-AC inverter will be on for more than four hours a day, even more in some areas. 90% of all inverters used are square/modified square wave inverters. But the fans never die out or get damaged, even after years of use. There is a slight audible noise. The fan speed is slightly lowered (nothing unbearable). But that's about it. The fan never gets damaged neither is its operation hampered. Now the fan too is an AC motor.

However, there is no point in arguing regarding this matter.

The conclusion is: an AC motor does not need sine wave to run. But this does not factor in any efficiency/torque issues and will depend on the motor too. So a sine wave should be used if/when possible to obtain maximum efficiency/torque/speed, etc.

 

[goldsmith]

For this we would need to know the details regarding the AC motor itself. For example, during summer, in Bangladesh, India, Pakistan, etc in some places, DC-AC inverter will be on for more than four hours a day

Oh really ? but why inverters ? i don't know much about your country or pakistan or india . but in iran we never use inverters or UPS s for homes because power will be on whenever we want .
May i ask you where is the problem in your area that you've to use inverters in homes ?
And about damages neither me nor you mentioned the ratio of purity of waves . you told modified inverter which means less THD . by the way cores of motors can create some noises too instead of distorted signals .

But this does not factor in any efficiency/torque issues and will depend on the motor too. So a sine wave should be used if/when possible to obtain maximum efficiency/torque/speed, etc.

I agree with this !

 

[FvM]

Referring to post #3:

as fourier series demonstrates , every distorted or un sine signal consists gang of sine waves with different frequencies ( harmonics ) . so if you use signals which are not sine wave for an AC motor which has been designed for a sine wave many problems will be occurred

It's interesting to calculate the fourier series of the motor current when the motor is supplied with a pure square wave. Due to the inductive motor impedance, current harmonics will be considerably lower than voltage harmonics (3rd harmonic current is 1/9 of fundamental, 5th harmonic 1/25...). If the motor is run at slightly reduced load, it will well tolerate square wave supply without overheating.

 

[BradtheRad]

When I was putting together my home power system (1999), I considered what type of inverter to get. I wanted it to be able to power the refrigerator.

The choices were sine wave, square wave, modified (rectangular) sine wave.

I read several articles. I found that an AC motor can be driven by square waves (although it would buzz audibly).

As to the question of efficiency...

For a given load, the sine wave type usually draws 10% greater battery current. This favors the square wave or modified sine wave type.

However an AC motor runs about 10% more efficiently on sine waves. This means that the sinewave type is not at a disadvantage.

I believe the same applies to startup surge, requiring 2 or 3x running current.

The refrigerator drew 800W running. So I purchased a sine wave inverter that was rated to supply 2500W.

 

[masud58]

see link

**broken link removed**

 

[goldsmith]

It's interesting to calculate the fourier series of the motor current when the motor is supplied with a pure square wave. Due to the inductive motor impedance, current harmonics will be considerably lower than voltage harmonics (3rd harmonic current is 1/9 of fundamental, 5th harmonic 1/25...). If the motor is run at slightly reduced load, it will well tolerate square wave supply without overheating.

Hi dear FvM
Of course i think it is interesting too . for instance if the voltage rating of our motor is about 220 volts . 1/9*220 = 24 volts ( rms ) . and ... etc . as we can see many of the text books are dealing with these issues that what will happen if frequency be higher than or less than the nominal frequency of a motor which manufacturer marked about it .isn't it ?
Best Regards
Goldsmith