Inverter drive on 3ph S/C 1

[minger]

Hello,
I have recently come across a situation involving a 3ph s/c induction motor, controlled by an Inverter drive, the motor is 415/240v 50Hz coupled to a gearbox, but the inverter has been set up to run at 60hz when upto full speed.
Now its been a long time since I have done any calculations of this nature so I would like a little help please.
Firstly from what I can remember it is not good for the life of the motor to run it at high hz, secondly assuming the loading was calculated correctly when installed will it actually run any faster? or if the hz was increased any more what effect would it have?
Basically its required that the gearbox output rotation speed is increased. I believe the easiest way would be to change the gearbox ratio, assuming the motor & inverter can accomodate this, but I would like a second opinion and some calculations to back up my view. Thanks

 

[peebee]

See:
thread237-31684 & thread237-21451 & other threads that pretty much already cover this.

 

[CB2]

The two threads that PEEBEE referenced do not address VFD operation of 3 phase AC motors. Since you did not provide the input voltage for the VFD, I will limit my comments to suggest you make sure that the motor will have enough torque/power through the speed range you will operate. The machine manufacturer should be consulted.

Similar questions need to be raised about the gearbox, increased speed/load, and lubrication.

You probably have a common package of equipment that will work, just not enough details to be specific here.

 

[jbartos]

Suggestion: If the motor is not designed for the higher RPM (higher Hz), one has to proceed with the caution. If the motor is designed for the higher rpm, it may be possible to find out the higher RPM limit, e.g. from the manufacturer.

 

[bobby69]

The motor will run on 60Hz. However, you will loose a small amount of torque. Look into the parameters to see if you can change the max. output frequency to 50hz.

Bobby69

 

[tmahan]

What is the frequency that the drive is actually outputing to the motor and what is the supply voltage. Just because the base frequency is set for 60 Hz does not mean that 60 Hz is being delivered to the motor.While it is likelythat it was set incorrectly, the higher(60 Hz) base frequecy may have been set deliberately to adjust the Volts per Hertz pattern used by the drive.


An example that I have encountered in the past is setting the base frequecy for 120Hz on a 460 V network so that a 230V motor could be used on the 460 network without rewinding.


While doubtful that this is the case here it does illustrate that one should not go monkeying about with drive parameters unless you are sure why you are doing so...

Using the VFD it is possible to overspeed (hesitate before going more than 50% overspeed) the motor. As pointed out above this will result in a drop in torue but since RPM has increased the output HP remains the same. I suspect though that you will take a hit on eff. but others here can better adress that issue.

 

[Lewish]

You haven't said what the horsepower of the motor is, or whether or not it is Inverter Duty rated. Take the following with the caveat that I only work with VFD motors under 5 HP. But, all of the Inverter Duty rated motors with which I am familiar will easily run up to 5000 rpm and most are rated to run up to 6000 rpm. They have the rotor balanced and bearings strong enough to run continuously at these speeds. I don't know about larger horsepower motors.

 

[jbartos]

Suggestion to the previous postings: Besides bearing there are also losses due to Eddy Currents and Remagnetization losses (Steinmetz Equation). These can pose a motor cooling challenge at the high frequency (high RPM).

 

[Lewish]

Hi jbartos, could you explain the cooling issue a little more. I have run motors continuous duty, heavy load at 120 Hz for an Inverter Rated motor and have not ever seen a cooling problem. We have all our cooling problems at very low rpm, at 10 Hz for example.

 

[jbartos]

Suggestion: Visit

http://www.anorad.com/applicationAnalysis/competitiveAnalysis.htm

for "anti Eddy Current design" at linear motors:
Velocity - Linear motors are capable of very high velocity in excess of 10 m/s. Anorad’s proprietary anti eddy current design assures negligible eddy current losses at high speeds.

 

[jbartos]

Small encore: Visit

http://www.esmagazine.com/CDA/ArticleInformation/features/BNP__Features__Item/0,2503,13058,00.html

(for "output waveform effects&quot

http://www.theaustin.com/html/ra-variable-frequency_drives.html

At higher frequencies, they lead to eddy currents and skin-effect losses that produce no usable work, but do produce heat, which must be dissipated.

 

[Lewish]

Hi jbartos. Thanks for the post. However, both articles in the second post refer to rather antiquated ways of implementing VFD. The first assumed all were constant volts/hz type, whereas most new VFDs today are space-vector FOC. The second also assumed constant volts/hz but also assumed the really old six-step driver. I don't think anyone use that these days, as the harmonics will eat you up.

 

[jbartos]

Suggestion to low frequency, 10Hz, motor cooling. Some manufacturers provide a separately supplied motor for fan, e.g.

http://www.lincolnmotors.com/

http://www.lincolnmotors.com/pdf/E70.pdf

that removes heat at low motor RPMs.

 

[jbartos]

Suggestion to Lewish: Please, notice that manufacturers still tend to leave a choice between the vector controlled and Volt/Hertz controlled motor controllers.
Visit

http://www.sea.siemens.com/drivesbu/support/catvc/sec4b.pdf

for more info

 

[Lewish]

Hi jbartos, yes I have noticed that some do provide both types of controllers. Vector controlled is more efficient and gives you more torque, which is why that is what we design for small motors. We also recommend an external fan if the motor is going to be run at 10Hz for more than a half hour at a time. Please refer to the caveat in my first posting. Thanks.