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# Variable-frequency drives

Discuss Variable-frequency drives in the Commercial Electrical Advice area at ElectriciansForums.net

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#### speedtronic

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Can Variable-frequency drives be used for driving any 3-phase induction motor at a speed & frequency other than that is mentioned on its name plate? I mean normally induction motors are designed to operate at 50 Hz or 60 Hz at a constant speed as mentioned on their name plates then shouldn't this mean that they can operate only at this frequency as mentioned in their data sheets & name plates.
Moreover, if we operate a 50 Hz motor at 300 Hz frequency using VFD, why is it not damaging to motor? Please explain

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#### Sparky-Tom

Yes, 3-ph motors can be driven at frequencies other than that stated on its name plate. Another factor in determining the speed of a motor is the number of poles it has, there is an equation for calculating this, but i cant remeber it off of the top of my head.

I have never seen a motor being run at 300Hz, nor have I heard about a motor running at 300Hz. Although I wouldnt reccomend it, it may be very damaging to the motor. What cooling system is being used with the motor? Other factors in determining the motor arrangement is altitude and ambient temprature.

What is the motor being used for? What RPM are you trying to get from the motor? Could be acheived using a gearbox?

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#### Andy Smith

Variable frequence drives (inverters) can drive any induction motor. normally people set limits at about 80 H z. You have to take into accout allot of features

Speed loss ,Slipage,Heat,current,Tourge V/f settings.

Pick the right inverter for the KW of your motor.

Key point if the motor runs slow an addition fan may be required for heat contoll.

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#### linus78

Is important the relationship between voltage and frequency (V/f must be constant) and the informations of the supplier, if the motor can running to 300Hz ( I had looked an applications in textile-industries..).
Speed of the motor may be is= ( 60*f)/( n° of poles/2 ). For example f=50Hz, 4 poles, about 1500 rpm ( 1380rpm in main constructors ).

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#### Sparky-Tom

What do you guys think of the Fluke 289? After seeing that article im thinking abour getting one...Any reveiws?

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#### Andy Smith

I use a fluke 123 (industrial scopemeter) which is very good would imagine this one is good too. But i like fluke!

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#### silva.foxx

I use a fluke 123 (industrial scopemeter) which is very good would imagine this one is good too. But i like fluke!
What a guy! A man after my own heart, Andy. The only meters I'd put my life on.

I had the 123 but struggled to find enough use for it! I had some fun with the 876 graphical mutimeter. I have in the bottom of my box a very robust 25...you can bouce that baby! The T3 is very pretty in dark places.

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#### Andy Smith

All my meters are Fluke started with the 77 then 79 and then fluke 99b i think (unfourtunatly someone had that one away while i was recording some data) Thats why i thought i would go with fluke for inspection and testing but now seems like Megger is the way to go. Sligtly of subject i know.

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#### krisboulter

Can Variable-frequency drives be used for driving any 3-phase induction motor at a speed & frequency other than that is mentioned on its name plate? I mean normally induction motors are designed to operate at 50 Hz or 60 Hz at a constant speed as mentioned on their name plates then shouldn't this mean that they can operate only at this frequency as mentioned in their data sheets & name plates.
Moreover, if we operate a 50 Hz motor at 300 Hz frequency using VFD, why is it not damaging to motor? Please explain
the equation is rpm = 120 x freq / no of poles

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#### WILLY

Can Variable-frequency drives be used for driving any 3-phase induction motor at a speed & frequency other than that is mentioned on its name plate? I mean normally induction motors are designed to operate at 50 Hz or 60 Hz at a constant speed as mentioned on their name plates then shouldn't this mean that they can operate only at this frequency as mentioned in their data sheets & name plates.
Moreover, if we operate a 50 Hz motor at 300 Hz frequency using VFD, why is it not damaging to motor? Please explain
Many 3 phase motors have a similar construction in terms of bearings and balance to cover different pole pairs and 50 or 60Hz.
So, its quite common for a small motor (<22kW) to be mechanically OK to 3600rpm, this would be 120Hz on a 4 pole or 180Hz on a 6 pole for example. Of course, vibration increases with speed.
Electrically, you theoretically expect constant torque up to rated voltage and speed, thereafter constant power. In practice, performance in frequencies above the rated (base) varies with motor ratings, particularly peak or pullout torque which is normally higher than stall or pullup torque.
A typical calc for maximum torque is that it reduces as the inverse square multiplied by peak torque at rated speed e.g. if motor TqPk is 240% and you are at 100Hz at 400V on a 50Hz 400V winding, you have half the theoretical voltage for full torque, square this 1/2 and you get 1/4, multiply this by 240%and you get 60% peak torque. This means, constant power would mean 50% torque, if you overload to 60% motor stalls and collapses.
I have run 50Hz windings at up to 200Hz but you have to be careful at higher speeds as torque is very limited and you need to overcome more friction / windage than at lower speeds too.
Also, if fitted with a shaft driven fan, this can be audibly noisy as it is designed to cool at rated speed.
300Hz and more is common for spindles but normally you use a stator wound to a higher frequency.
If you've got something specific in mind, maybe I can assist.
Its quite common to go to 100Hz on 4 pole, obviously you would need to make checks with supplier on 2 pole!
Remember, think too about what's connected to this higher speed as the rest of the drivetrain may not be good for higher speed!

In reply to your last point, I doubt you will reach 300Hz on a 50Hz winding easily, one easy trick is to change to the 230V connection if it exists, then you can calibrate at 400V 87Hz and the 300Hz becomes easier at the cost of 1.732x current (so maybe a larger VFD).

cheers

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