# Running a motor on a VFD past its hertz rating?



## BBQ

I think you have to ask the manufacturer about how many RPM it is designed to withstand.

I had a VFD that was capable of 360 hz, I doubt the motor would have stayed together at 6 times it's labeled RPM ........... but it would have been an exciting ride for the passengers of the amusement ride it drove. :thumbsup:


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## BCSparkyGirl

lowers the life of said motor. We did that in class and it would go faster, but at the cost of reducing the life of it.


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## McClary’s Electrical

BCSparkyGirl said:


> lowers the life of said motor. We did that in class and it would go faster, but at the cost of reducing the life of it.


 

While I agree, somewhat. Most 60 htz motors can run just fine up to 90 htz.


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## Ghandiswrath

Motor would run faster, draw more current, and have less torque. Not to mention degraded equipment life.


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## BCSparkyGirl

mcclary's electrical said:


> While I agree, somewhat. Most 60 htz motors can run just fine up to 90 htz.


this is true, but it will not have the life span of one run at it's nameplate......not by a big difference, but if it's an expensive motor, or critical one, it can make a difference.........and my house is too big a nightmare right now to dig out the specs on the ones we were doing....


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## BBQ

mcclary's electrical said:


> While I agree, somewhat. Most 60 htz motors can run just fine up to 90 htz.


As I understand it the centripetal forces increase squared to the speed so even that relatively small increase in speed really changes the forces on the rotor.

I wonder what speed a typical motor will turn into a grenade. :laughing:


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## McClary’s Electrical

BBQ said:


> As I understand it the centripetal forces increase squared to the speed so even that relatively small increase in speed really changes the forces on the rotor.
> 
> I wonder what speed a typical motor will turn into a grenade. :laughing:


 

I dropped a 10 HP 480 volt motor 125 ft down off the top of a grain elevator to concrete below, it pretty much EXPLODED:thumbup: I wasn't about to carry it back down


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## BBQ

mcclary's electrical said:


> I dropped a 10 HP 480 volt motor 125 ft down off the top of a grain elevator to concrete below,


Cool! :thumbsup:


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## erics37

mcclary's electrical said:


> I dropped a 10 HP 480 volt motor 125 ft down off the top of a grain elevator to concrete below, it pretty much EXPLODED:thumbup: I wasn't about to carry it back down


I did that with my roommate's computer monitor in college from the 5th floor of our dorm building.

It similarly exploded.


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## McClary’s Electrical

erics37 said:


> I did that with my roommate's computer monitor in college from the 5th floor of our dorm building.
> 
> It similarly exploded.


 

Destruction is fun:thumbsup: Had he made you mad?


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## MDShunk

If you're only talking about running a 60hz motor at 65 or 70hz, my experience tells me you can do that for as long as you want with impunity. That's not that much over nominal. 

I've run motors on VFD's at up to 180hz, but I expect a shorter life. Nothing observed so far, but these are "inverter duty" rated motors. I would have to think they've wound them with tougher materials and paid more attention to detail.

One thing I know, for sure, is that the higher up in Hz you go, the more your little insulation nonconformities will show up- both in the branch circuit wiring on the load side of the drive, motor peckerhead, and motor windings. It's a lot easier to jump a spark at the higher frequencies. 

The main reason people start to consider running a motor at greater than 60hz is often because of some primary engineering mistake. Pump too small, ductwork or blower undersized, conveyor gearbox improperly selected, etc. Good design dictates that a system should never be designed with the intention of running a motor at greater than 60hz. Because good design does not always exist, we have the capability of running our drives at greater than 60hz.


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## JRaef

An AC motor is designed to provide a given amount of torque at a given voltage and speed. Speed is only dependent upon frequency, torque is a function of the ratio between voltage and frequency, referred to the V/Hz ratio. If the V/Hz ratio is maintained, the torque remains the same. So for example if you have a 460V 60Hz motor, the V/Hz ratio is 7.67:1. If the VFD cuts the speed in half, it also cuts the voltage in half, so at 30Hz it is putting out 230V and providing full torque.

But if you raise the speed beyond 60Hz and the voltage stays at 460V because you have no more than that, the V/Hz ratio diminishes, and so does torque. So if you run at 75Hz, the motor is now putting out a V/Hz ratio of 460/75 = 6.13:1, which means you will have only 80% of the rated torque available now (6.31/7.67). 

If your motor was over sized for the job by 20%, you are OK. If it was not, you are going to increase the slip, the motor will draw more current to do the same work, which increases the thermal stress and over heats it, which is why people say you will shorten the life. It used to be very common practice to over size motors by 20%, but not so much any more. 

There are other issues to consider, such as the speed rating of the bearings, the effectiveness of the motor cooling system and the added load that may come about from an increase in speed. Classic example of that which is often overlooked by people wanting to do this: centrifugal pumps and fans. 

In a centrifugal load, the power requirement changes at the CUBE of the speed. That's why it's such a good deal to use a VFD on a pump or fan instead of a throttling system on a full speed motor. You save a LOT more energy by reducing the motor speed; at 1/2 speed you are using 1/8th the power (.5 x .5 x .5). But the OPPOSITE happens when you INCREASE the speed. At 75Hz (125% speed), the PUMP is going to demand 195% of the power! So you will need TWICE the motor size you have just to increase the flow by 25%. That causes a near immediate overload.


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## sparkness

I do just that with a lathe and a milling machine that I own. I programmed the vfd for 66 Hz so I could get 10% more speed from the lathe and I use a 60Hz rated 1 HP motor. At the higher speed I'm not using all the motors torque- I just need the speed. The motor is cheap compared to an inverter rated motor. so in my situation it is not a problem for me.


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## JRaef

sparkness said:


> I do just that with a lathe and a milling machine that I own. I programmed the vfd for 66 Hz so I could get 10% more speed from the lathe and I use a 60Hz rated 1 HP motor. At the higher speed I'm not using all the motors torque- I just need the speed. The motor is cheap compared to an inverter rated motor. so in my situation it is not a problem for me.


 Perfect application then. In a lot of tooling applications that seems to be the case, the higher speeds are necessary for finer work, i.e. less loading anyway.


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## Frasbee

BBQ said:


> Cool! :thumbsup:


And if it were your helper that had dropped it?


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## BBQ

Frasbee said:


> And if it were your helper that had dropped it?



It depends .....


A junk motor with no chance of people walking under it ..... Cool!



A good motor dropped to crowded side walk .... not so much.


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## JTMEYER

I dropped a 5 hp 3 phase off a roof about 35-40' onto concrete. But it landed dead vertical, right on the end of the shaft. It shot the whole armeture through the end bell about 6' in the air.


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## John Valdes

Some inverter duty motors are rated at 6000 RPM, with a base speed (nameplate rpm) of 1800. Its not recommended you actually run at 6000 RPM. 
Inverter duty motors are designed specifically for high speeds and high frequencies. Special magnet wire and mechanical enhancements enable high speeds. With special bearings and rotors just the beginning. 
One area where I have seen high speeds (above base) required is in applications where gearing is high. Like Marc said. Wrong gearbox = wrong output speed. This is where a VFD can be alot of help.


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## paulcanada

*vfd current question*

Hi guys, 

I'm just looking for clarification here...so if I take a motor that has for example FLA of 25 and then increase it to 65 hz, my current is now exceeding the rated current right? 

Also, does anyone know some good websites or databases to help me learn more about vfds in general?

Thanks, Paul.


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## mutabi

I do not believe this is correct Paul. And it has to do with how a VFD actually works. I was able to increase the speed of a motor to approx 200Hz from 60Hz, with very little change in current. Someone else may be able to clarify this.


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## Ghandiswrath

paulcanada said:


> Hi guys,
> 
> I'm just looking for clarification here...so if I take a motor that has for example FLA of 25 and then increase it to 65 hz, my current is now exceeding the rated current right?
> 
> Also, does anyone know some good websites or databases to help me learn more about vfds in general?
> 
> Thanks, Paul.


 That is a trick question. It depends on the equipment you have it hooked up to, the service factor, and the frequency specs of the motor and equipment. 

If you have a motor with a service factor of 1, and an FLA of 25, and you are running that motor at 25 amps and 60 HZ. If you increase the frequency to 65HZ you would over-amp the motor.


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## Ghandiswrath

mutabi said:


> I do not believe this is correct Paul. And it has to do with how a VFD actually works. I was able to increase the speed of a motor to approx 200Hz from 60Hz, with very little change in current. Someone else may be able to clarify this.


 Mutabi, what is the equipment your motor is driving?


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## mutabi

11:1 gearbox running a vibrating table. Not under much of a load. 6A FLA, pulling about 3A. Increasing speed to 200Hz kept the amperage about 3A. The difference may have been 0.1A between the two speeds. 

I reckon this discussion could get quite technical real fast lol


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## micromind

At an oil plant I work on sometimes, they have a slurry pump that wears the impeller down over time. It's 40HP, and controlled by a VFD. 6 pole motor, basic centrifugal pump.

They've found that it'll need about 50HZ to reach FLA when it's new, and more like 75HZ when it's worn out. 

The pump will produce about the same flow at the same pressure when the motor is operated at its FLA, regardless of speed. 

Rob


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## CHECKtheE-STOP

psobero said:


> what would happen if i run a motor that has a nameplate rating of 60hz and is on a vfd, to 65 or 70hz???


I work for a beverage manufacturing plant, and I would say about half of our motors are over-clocked. The engineers designed our systems to run at a set rate, and of course, production wants it to run at it's red line limit. The result of running the motors at a higher frequency is early bearing failure, higher temperatures, and loosing the seals on the pumps. I do suggest investing in an inverter rated motor with an electric cooling fan instead of the standard shaft mounted.


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## paulcanada

I remember doing this in quite a few oil field applications. Some inverter rated motors hooked up to pump jacks and some to submersible pumps. 

I am just finishing my last year of school and our modules are explaining the reason for the voltage/hertz ratio etc however they don't really tackle my question and I remember going through the setup of a vfd controlled submersible pump where, for example, the motor was to draw 60 amps at full load and they were concerned about their equipment given the difficulty of servicing if there is a problem. So if we knew that they were thinking of running it at 70 hertz for some time then you would change calculations so that it drew 60 amps at 70hz. If you set it at 60hz in this setup it would draw much less than it would have if 60hz was the ceiling.

Now I don't remember exactly how they did this and i might have missed some steps...it was awhile ago has anyone else done something like this?

Paul.


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## John Valdes

To make this clear. Changing cycles (Hz) and not changing voltage proportionally will eventually burn up the motor. 
A VFD changes Hz in direct proportion to volts. That is the principle behind variable speed (VFD) drives.


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## JRaef

Increasing the Hz output to the motor does not in and of itself cause the motor to draw more current. Current draw is a function of torque and slip. If the motor torque is enough to keep the slip from increasing, then the current draw will not increase. But as I said earlier, increasing the Hz above the motor design point means you begin LOSING torque. So if your motor was designed "close to the bone", it may overload but if it was over designed in the application, that may not be a problem. 

In a lot of older machines it was common practice for mechanical engineers to build in a 20% "fudge factor" in motor design selection. So if you needed X ft. lbs. of torque at 1725RPM, and that equated to Y HP, and the next closest motor was Y+5%, you selected a higher HP size so that you had X+20% torque. That practice has all but ceased in the spirit of energy efficiency now. So if they needed Y HP and the closest standard size is Y+5%, that's what they use. That means if you decide to over speed that motor and you lose more than 5% of the available torque, the motor can no longer perform as designed. The lower torque means more slip, higher slip means more current draw, more current draw means more heat (or over load), more heat means shorter life. But none of this is automatic, it all depends on the load and the machine design.

Centrifugal loads such as pumps and fans are a completely different story and a major pitfall to over speeding. In a centrifugal load, speed = flow = power and the POWER requirements of the load increase at the cube of the speed. So at 75Hz, the speed is now 125% of design. The power now required by the additional flow is 1.25 x 1.25 x 1.25 so the HP requirement is 1.95 x what it was at 60Hz. If the motor was designed in the first place to be twice as big as it needed to be at 60Hz, no problem. If not, it is now 1/2 the size it needs to be and it will over load.


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## Ghandiswrath

JRaef said:


> Increasing the Hz output to the motor does not in and of itself cause the motor to draw more current. Current draw is a function of torque and slip. If the motor torque is enough to keep the slip from increasing, then the current draw will not increase. But as I said earlier, increasing the Hz above the motor design point means you begin LOSING torque. So if your motor was designed "close to the bone", it may overload but if it was over designed in the application, that may not be a problem.
> 
> In a lot of older machines it was common practice for mechanical engineers to build in a 20% "fudge factor" in motor design selection. So if you needed X ft. lbs. of torque at 1725RPM, and that equated to Y HP, and the next closest motor was Y+5%, you selected a higher HP size so that you had X+20% torque. That practice has all but ceased in the spirit of energy efficiency now. So if they needed Y HP and the closest standard size is Y+5%, that's what they use. That means if you decide to over speed that motor and you lose more than 5% of the available torque, the motor can no longer perform as designed. The lower torque means more slip, higher slip means more current draw, more current draw means more heat (or over load), more heat means shorter life. But none of this is automatic, it all depends on the load and the machine design.
> 
> Centrifugal loads such as pumps and fans are a completely different story and a major pitfall to over speeding. In a centrifugal load, speed = flow = power and the POWER requirements of the load increase at the cube of the speed. So at 75Hz, the speed is now 125% of design. The power now required by the additional flow is 1.25 x 1.25 x 1.25 so the HP requirement is 1.95 x what it was at 60Hz. If the motor was designed in the first place to be twice as big as it needed to be at 60Hz, no problem. If not, it is now 1/2 the size it needs to be and it will over load.


 That's what I said.... :whistling2:


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## rick279

psobero said:


> what would happen if i run a motor that has a nameplate rating of 60hz and is on a vfd, to 65 or 70hz???


Motors typically can run safely to about 5000 rpm. Beyond that there is balancing to worry about.

If it's a centrifugal pump or fan, your current will increase significantly as your speed increases.

65 or 70 Hz should be no problem as long as the VFD and the motor can handle the amps.

I work on hundreds and sell thousands of VFDs a year, so I do know what I'm talking about. Email me if you would like to discuss further.


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## Del

*Over 60/50hz*

One application this is often used in is Wind Turbines.

Take a regenerative capable vfd, set it to supply frequency, the added speed of the wind on the blades will take it past the supply frequency, where it then becomes an alternator, and feed back the excess into the grid.


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## Dirceu Dasilva

I have set Vfds to run at 66hz not going above fla ,I wouldn't speed it up more than 10%


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## IMM_Doctor

*Old thread alert*

We woke up a 4 year old thread?

But YES you can.

“Supercharging” a VFD motor application.
The VFD is your friend, it has intelligence, and parameters that allows you to control the motor. Look for CURRENT LIMIT.
We had an industrial manufacturer with an assembly line that returned 1,000 lb. transfer pallets into a three story elevator, once every minute. The 5 HP motor was originally wired and configured for 480v 60hz. If you run a motor past 60hz design, the motor will torque slip, and have reduced capability.
At a later date, due to system mechanical design changes, the elevator needed to operate at a faster speed. The VFD supplier suggested that we supercharge the application.
Step 1 – Intentionally wire the motor to 240v low winding configuration. (Very hard for me as an electrician to do this)
Step 2 – Enter the 240 FLA nameplate information into the VFD Current Limit Parameter
Step 3 – Turn in on and run the motor all the way to 120hz (with full torque)
We did this, and cut the transport time in half. 
Pre-requisites:
Size the drive, and drive supply wiring, for the motors 240v fla
Size the motor wiring for the 240v fla
Confirm with the motor vendor for 3600rpm use

We did all of this, and it works perfectly, without issue. This system runs twice a minute for 24-hrs, 6 days a week. The motor is actually cool-to-touch, less than ambient room temperature, and the fan is blowing twice as fast, and the motor is happily running at FLA or less.


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## Dirceu Dasilva

don't forget to check motor speed ,standard motors run around 1700 rpm ,is you have sealed bearings run a motor too fast can damage bearings ,motor windings ,alignment or affect balance


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