Fan motor with a VFD running at low frequency question 10

[Coco_HueHueHue]

Hello,

I did browse the internet a lot and only found part of the answer to a question i am asking myself.

I keep reading that running a VFD in V/Hz control mode at a low frequency will overheat the motor.
I keep reading that the magnetizing current is significant and the motor can't cool down because of its slow rotation (internal fan).

Honestly, i do not understand why the motor would overheat at a low frequency... we have a constant V/Hz ratio.
If the frequency is 0 or very low, then, the voltage will be 0 or very low.

How can a motor overheat when almost no voltage is at its terminals?
I understand that there is no cooling, but if the voltage is near 0... the current has to be near 0 too...

If a motor nominal current is 20 Amp at 600VAC/60Hz, then, at 0.5V/0.5Hz, the current should be 16mA... maybe a little more because of the magnetizing current. But the magnetizing current is constant (from the motor design) and is very small compared to the load current.

I believe there is a piece of information i am missing.

I AM NOT SEARCHING FOR A ANSWER LIKE : "WHEN YOU RUN A MOTOR AT LOW SPEED IT OVERHEATS".
I browsed google a lot and this is the only thing i found.

Does anyone have a clear explanation, a technical article or a good book reference explaining this?
Thank you very much!

 

[Skogsgurra]

The constant V/Hz ratio is not really constant. And the motor voltage isn't near zero volts at low speeds.
There is always enough voltage to produce the required magnetization current. So, if you measure the voltage that actually exists across motor windings you will find something like ten or twenty percent of rated voltage also at very low speeds.
That voltage drives the magnetization current, which heats the motor.
The heat removal is not adequate when the air flow is low. So the motor overheats.
So, even if you do not like it, I have to anser that moroe overheats when running at low speed. If there is no other cooling, like external air or water cooling. Which you usually find on motors that motors that are used in paper Machines, Rolling mills and other applications where you have a high max/min speed ratio.

Be nicer when you ask questions. No need to start with flames.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Coco_HueHueHue]

Lol, i was not being harsh, i was just asking to not receive the answer i can google anytime and find on any forums, but wanted a more detailed answer.

And you gave me an answer to clarifies one thing:

The V/Hz is theoritical and drive application kind of "lie" when they don't mention the details you just pointed out at me.​

Thank you very much.
I will try measuring that this afternoon.

By the way, isn't the magnetizing current more around 1-2% of the nominal current?

 

[Skogsgurra]

OK, I understand.

No, the magnetization current is more like 50 - 70 percent of rated current. It can be even higher That is what makes the Power Factor of asynchronuous motors so low.

Another thing that may upset you ( ) is that you will find a voltage that is unexpectedly high. That is because the voltage is composed of pulse-width modulated voltage pulses that have a constant and high amplitude. The voltage and the currents you SHOULD measure is the fundamental frequency, the frequency that determines motor speed.

If you measure that voltage with a volt-meter (RMS or not) you will get a high reading. Use either a "1 V/Hz" filter or a system that can give you the spectrum of the voltage. Then look at the lowest peak, frequency-wise. It shows the value of the fundamental frequency.
There is a lot of problems with such a measurement, like extreme EMI being present, and that can make such a measurement impossible if you do not use an isolation amplifier with 60 to 80 dB CMR.

But, OK, go on. You will learn a lot from what you see. And slowly realize that theory is somewhat different from reality. And, in my World, reality has a tendency to be what Counts.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Coco_HueHueHue]

I only have a Fluke 85V here with me.
But, I managed to read approximately:

- 20V @ 0Hz
- 60V @ 3-4Hz
- 300V @ 15Hz

So, i guess that the linear V/Hz ratio only sets in after you passed the V/Hz Mid point (probably around 10 Hz).

I have really been misguided by the fact that in reality the magnetizing current of an AC motor is high.
I always thought it would be low like a transformer after its inrush.

I guess I will set the minimum frequency at 30Hz. This way, i will be sure the motor spins quickly enough to cool itself. As a matter of facts, for our appilcation, i could set the minimum frequency at 60Hz since its the only speed i have interest in.

Thank you very much.
Coco

 

[waross]

A large part of the motor impedance is inductive reactance. Inductive reactance is proportional to the frequency.
As the frequency drops, it takes less voltage to force rated current through the motor.
Heating depends mostly on I²R.
R- Resistance is constant.
At slow speed, if the load demands full load current, the heating will be the almost same as at full speed, but with much less air movement to remove the heat.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[GroovyGuy]

Hi Coco,
The answer to your question is that it depends upon the nature of the load - is your load variable or constant torque?

A variable torque load can run at extremely low speeds with little or no chance of overloading (ie over-heating) the motor. Variable torque loads include centrifugal type fans & pumps.

A constant torque load cannot run at extremely low speeds without some means of external cooling. Generally most motors are capable of proving at 2:1 speed turn-down. Any greater than that an external blower motor is often recommended. Talk to your motor OEM wrt this issue. Constant torque loads include conveyors, constant-displacement pumps.

GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[jraef]

It also depends on the cooling system of the motor. TENV motors are designed to not need cooling fans, they dissipate the rotor heat (which is a bigger problem) through the shaft, so they tend to hold up better in "Inverter Duty" applications. However, you will find they are limited in size; I've never seen one larger than 30HP (22kW).

What you will find, or want to look for, with regard to cooling and speed is what's called the "turn down ratio" when using or specifying a motor for use with an inverter drive. So a ratio of 4:1 is typical for a "standard" motor. That means if designed for 60Hz, you can operate it down to 15Hz (12.5Hz if designed for 50Hz), but lower than that and it will likely over heat. TEFC "Inverter Duty" motors are often rated for 10:1 speed range and TENV motors (within the size range mentioned) or if larger, TEBC ("Blower Cooled") are often rated for 1000:1 turn-down, meaning you can operate them down to .06hz (basically zero speed).


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Sofistioelevib]

Coco Eng,
if you are using a variable speed controlled, you have to consider there are lot of harmonics generated from the VFD.
Lots of these are present either at full speed and no speed due to sinusoidal current reconstruction.
These armonics travel on iron base of the motor and rise the heat of the motor.
Furthermore some harmonics have Positive phase, some other negative phase and some other neutral.
These positive and negative phase are inline or 180° apart from foundamental Hz (50 or 60).
It is quite common in VFD find motor with high temperature at low speed because lot harmonics and no right cooling present.
These type of motors normally need a constant speed cooling fan on it..

Regards

 

[Skogsgurra]

That was more of a concern long time ago. When GTO:s and other slow devices were used for switching and the switching pattern was very "primitive", Think Full Block switching and "Full Block with Edge Notches". The faster switches, like IGBT and others, changed that. There are very few "harmonics" in motor current and flux today. The main problem is that the low speed built-in fan doesn't remove enough heat.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Sofistioelevib]

Skogsgurra,
sincerly in my experiencies the harmonics are present on Motor and can be also found with vibration analysis (this means the electric components stressing all mechanical parts of the motor).
The devices i'm talking about are relatively recent (ABB, SIEMENS...) and they produce as default (but ti is possible to change) 2Khz as switching frequency.
Furthermore consider that these frequency normally is not alone but is carrier frequency for other modulating frequencies combining togheter running through baseplate and generate heating.
As before, the best way adopted from ABB for a slow speed motor vith VFD is to connect a steady state fan as a cooler rear to the motor.

 

[Skogsgurra]

Sure. But it was much worse before. And it is not about harmonics in its correct sense today.

Yes, you can see the vibrations if you do vibration analysis. But those vibrations are not from torque variations, like you had when the switching was crude. They are more a result of magnetostrictive forces (that produces audible sound as well).

The ABB Direct Torque sometimes produces torque variations when switching between the "inner and outer flux circles" and the tolerance is set high to allow for lower switching frequencies. It used to be so. But I still say that this is not of any major concern when it comes to heating in modern VFD:s. The misconception is still live and kicking. Just like the thinking that you only get bearing currents in larger motors and that Insocoat bearings help. Technology Changes. And Old Truths still live on.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

Gunnar: "Technology Changes. And Old Truths still live on. " So true.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Sofistioelevib]

I disagree wit lot of part of your sentence... but this is not right place ...

 

[graybrookie]

The Total Harmonic Distortion (Ithd) is considerably higher when the motor is running at lower speeds than when at 100% load. ie. much higher currents in the 5th and 7th harmonic components creating heat.

 

[ScottyUK]

The THD measured where - in the motor, or in the supply?

 

[Sofistioelevib]

In the Motor supply cables

 

[ScottyUK]

On a type with a plain rectifier that's true, but many larger drives now have an active PFC front end to meet harmonic distortion legislation. Sure there's a little bit of switching noise but it can be filtered out reasonably easily.

 

[Skogsgurra]

greybr. & Sofist

Do you have any waveforms, RMS for fundamental and harmonics? It would be interesting to compare with my numbers from a set of measurements I did just to see the difference between older drives and newer ones. My results do not show any excessive heating from harmonics in the motor current at low speeds.

BTW: You work at the same facility?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[LionelHutz]

With fans, the cooling really is a non-issue. You won't be running a fan motor so slowly it won't cool or it would be pointless to even have the fan running at all.

Now bringing up another question. Why are you using a VFD if you're going to run at 60Hz all the time?