Why Ac Motors Cogs Bellow 10hz?

[AB2005]

It is generally said that bellow 10HZ, an AC motor cogs under full torque demand application while a DC motor doesn’t. I could never understand why it does happen.

 

I do understand about Magnetizing and working current in both motors. If we compare, we can find only one difference in both motors that DC motor (I would talk about most resent used motor in industry i.e. compound motor) stator is fully magnetized all the time and armature receives the voltage and a working current passes in Arm, it produces sufficient torque under very low speed. And the other hand, Inverter provides full current and corresponding voltage (according to V/HZ selection) bellow 10HZ then why motor cogging?

 

If the problem is the “Power Factor” then most inverter manufacturers quotes that their inverters can improve power factor even during motor start up. For me, only low power factor is the reason for producing the low starting torque under low speed. Can anyone explain in detail?

 

I am sorry if I could not explain well.

[marke]

That is an interesting question.

 

It certainly was an issue with early VFDs due to output waveforms, particularly at low output voltages, but I am not familiar with it being an issue with modern PWM / space vector drives.

Do you have any more information? references etc or is this "common knowledge".

 

Best regards,

Mark.

[AB2005]

That is an interesting question.

Do you have any more information? references etc or is this "common knowledge".

 

Best regards,

Mark.

 

Yes, this is common knowledge question. May someone have a better explanation for this.

 

[marke]

I suspect that this comes from the days when PAM drives were used rather than PWM, so historical rather than modern designs.

 

Best regards,

Mark.

[yuri]

A conventional 50 (/60) Hz induction motor coggs at 10Hz. But, imagine a 2-poles 10Hz nominal frequency induction motor (with its 600 rpm of synchr. speed and, let's say, 575 rpm of nominal speed). What will be the constructive differences of such a motor to "feel good" at the frequency which makes our conventional motors to "cog" ?

[AB2005]

I suspect that this comes from the days when PAM drives were used rather than PWM, so historical rather than modern designs.

 

Best regards,

Mark.

 

Mark;

You mean that by using the modern PWM drives, we can kick-off that cogging effect. If this is then we can say that now we can install AC motor without any fear of cogging bellow 10HZ.

Have any reader of this thread experienced such case that they installed an AC motor with PWM drive at the installation where starting torque demand was near about 150 % and did not faced cogging effect of motor bellow 10HZ?

 

[marke]

Hello Yuri

A conventional 50 (/60) Hz induction motor coggs at 10Hz

A conventional induction motor has a tendency to cog when the stator slots and rotor slots line up. If there is the potential of alignment of the slots, then the motor can behave like a stepping motor rather than an induction motor. Modern motors are designed with an unequal number of slots on the stator and rotor and the rotor slots are generally skewed to prevent cogging. Cogging is not generally and issue with current designs.

 

Crawling is different. A motor can accelerate to part speed and then "lock on to a harmonic" and run at part speed. This is different from cogging and will occur at particular speeds relative to the supply frequency.

The most common crawl speed is 1/7 of the supply speed, so if the motor was being fed with say 49 Hz, the motor could crawl at the equivilent speed of 7Hz. - One seventh of the correct speed. This is dependent on the voltage waveform applied to the motor and also to the construction of the motor.

see : http://www.lmphotonics.com/InductionMotor/MCog.php

 

Hello AB2005

 

In addition to the reply above, I suspect that there are a number of issues here that are being explained as cogging when that is not correct.

On older VFD technologies, the current waveform was very non sinusoidal and so it was very rich in harmonics. This got worse as the frequency was reduced due to the requirement for the voltage to reduce also.

Typically, the output was made up of a series of positive pulses of equal length and a series of negative pulses of equal length. The time between the pulses was lengthened to reduce the frequency and this also reduced the voltage. This yielded a very distorted current waveform and the flux in the iron was very non sinusoidal. The torque at low speeds was very poor.

Modern VFDs use a form of PWM which results in a sinusoidal current, reducing the harmonics in the flux gap and providing a much smoother torque.

With V/Hz type drives, there is a natural fall off in torque at low frequencies due to the resistance of the motor windings. This is reduced by the use of open loop flux vector technology, but there is still a drop off in torque at low speeds Closed loop vector and DTC technologies are able to deliver full torque at very low speeds without issues.

I suspect that you are hearing interpretations of the drop off in torque at low frequencies as being due to cogging when this is not infact the case. Older technologies were more prone to torque oscillations due to the current waveform, and modern VFDsstill have a fall off in torque at very low speeds.

 

When you use a VFD with an AC induction motor, the torque below full speed is limited to the rated full speed torque of the induction motor due to the iron characteristics. To increase the torque, you need to increase the flux, but this can not be done due to iron saturation problems. A DC motor operates differently and sl iron saturation is not a limiting factor. As you reduce the motor speed, you can get extra torque output, so for applications requiring a high torque at low speeds, a DC machine can be a better option.

I have seen many examples where a DC machine is replaced with an AC machine and the low speed acceleration is severely compromised due to the reduction in torque. To overcome this, a severely oversized AC machine is used.

 

Best regards,

Mark.

[AB2005]

Hi Mark;

 

Nice answer as expected.

Now we can say that this is very difficult to say earlier that what happen with the speed control of the application where we have replaced the DC motor with same rating AC motor. If we can afford, we should install a higher rated motor. Hence the how much HIGHER RAITING is another question mark? Second we can overcome on Cogging effect by installation a Sensor less Vector control drive.

 

Any comments from any other reader? May it become a very useful thread?

 

[Davemell0]

Hi Mark;

 

Nice answer as expected.

Now we can say that this is very difficult to say earlier that what happen with the speed control of the application where we have replaced the DC motor with same rating AC motor. If we can afford, we should install a higher rated motor. Hence the how much HIGHER RAITING is another question mark? Second we can overcome on Cogging effect by installation a Sensor less Vector control drive.

 

Any comments from any other reader? May it become a very useful thread?

 

 

Some higher-end sensorless vector drives can give a large amount of torque at low speeds. I have seen MV drives do 150 % of torque at less than 10% speed. However, it is difficult, and even with the best sensorless vector drive you will see some toque drop as compared to a Vector controlled drive. I think you would may not have a problem with a drive and motor of the same rating as your DC motor if you chose a quality vector control drive, used a speed sensor, and got good proffessional help tuning the VFD. However some DC motors were designed to give more than 200% torque at zero speed, so before you go to much farther with any particular type of drive you should check the amount of torque your dc motor is required to produce at zero speed, and what sort of torque your new ac system will produce at zero speed.

 

 

Hope this is helpful

 

Dave