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VSDS

VSDS

(OP)
Hi guys I’m new to this forum and fairly new to the electrical industry. My understanding of VSDS are very limited other than the basic principles of the AC being converted to DC then transferred back to an artificial AC (sorry for my crude explanation)

Now to the question what happens if the parameters of the motor are incorrectly entered into the VSD so say it’s a 50HZ 415 delta connected motor but we enter the drive as 60Hz 415?

Thank you for your help guys

RE: VSDS

This is more motor theory.
In your example you will be throwing away about 20% of the motor HP.
If you go the other way if the motor is 60Hz, 415V and you enter 50 Hz, 415V you will exceed the Volts per Hertz ratio of the motor and be in danger of saturation.
In your example motor, 50 Hz, 415 V the Volts per Hertz ratio is 415V/50Hz = 8.3 V/Hz.
As the VFD is dropping the frequency, it is also dropping the effective voltage in the same ratio.
If the V/Hz ratio is too high there is a danger of saturation and rapid motor burnout.
If the V/Hz ratio is too low there is no danger but you are throwing away motor HP capacity.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: VSDS

Depends on which parameters are not correct. You can not depend on an the VSD to recognize incorrect parameters. Some incorrect settings will result in short motor life. Some can result in drive trips with confusing messages, or poor performance.

Think of the parameters as the instructions the control block uses to instruct the power section what is to be sent to the motor.

It is better to Take Time to get the parameters correct.

Fred

RE: VSDS

Quote:

Now to the question what happens if the parameters of the motor are incorrectly entered into the VSD so say it’s a 50HZ 415 delta connected motor but we enter the drive as 60Hz 415?

The motor (and the connected load) will run at 20% higher speed.

Muthu
www.edison.co.in

RE: VSDS

(OP)
So it’s a balance between the volts and HZ for a motor that is actually 60Hz 415v and we enter 50Hz we are then not giving the motor the amount of voltage for its frequency?

But I’m my example the motor is 50Hz 415 but been set to 60Hz will this not mean the motor will run faster but not have the correct voltage applied to the windings to increase the motor torque?

I may be confusing myself

RE: VSDS

Yes, at 415 V/60 Hz, the motor will run at higher speed with a reduced torque, which may or may mot be sufficient for your load. It all depends on your type of load. A decoupled motor does not require much torque to achieve full speed.

Muthu
www.edison.co.in

RE: VSDS

A 415V, 50Hz motor should have (60Hz/50Hz x 415V) or 498 Volts applied at 60Hz for best performance.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: VSDS

(OP)
Thanks guys so does this mean as the motors hz has been entered at 60 as to 50 it could reduce the motors life. The motor drives a pump pumping fluid.

RE: VSDS

Pumping can be demanding and so yes it could reduce the motor's life.

It would seem that someone who would be so stupid and careless as to get the motor frequency wrong would also likely fail to set the drive's overload parameter correctly. Most drives are able to drive larger motors than they're applied to a so the extremely important overload function would not even come close to protecting the motor. That could easily result in a motor melt-down. If it's an unattended motor it could burn the whole place down.

Keith Cress
kcress - http://www.flaminsystems.com

RE: VSDS

Yes, if you entered the motor DATA into the drive as 415V 60Hz, the output of the VFD will base everything on a V/Hz ratio of 6.92:1 (415/60), instead of what the motor was designed for, 8.3:1 (415/50). That means that the motor will be under excited on a continual basis and that will result in 17% less torque at any speed. Then when the motor needs PEAK torque, it will be reduced by the SQUARE of the delta, so it will only get about 70% of what is otherwise possible and recovery after a step change in load will be more difficult..

The speed that the motor runs at is still determined by the frequency output of the VFD. If you tell it to run at 50Hz, it runs at 50Hz. It's just that at 50Hz, it will be getting less torque and may run at higher slip, pull more current to do the same work, which will increase the thermal stress on the motor.

Alternately if the motor is capable of >17% more torque than the load needs anyway, you may never notice the difference.


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

RE: VSDS

(OP)
Thanks again guys had another look today it’s been set to 60HZ for max frequency as well as motor frequency. As I said I’m fairly new to VSD drives but I take it the name plate frequency should be set at 50Hz so the drive then knows what motor is connected? What will then be going on with the drive if these parameters are correct (motor nameplate values) but we play about with max frequency would it just be the same as above?

RE: VSDS

If this is a centrifugal pump, the pump speed is critical.
The speed/torque curve of a centrifugal motor is a square function, (HP is a cube function).
Just a little bit over rated speed for a loaded centrifugal pump may result in motor burnout.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: VSDS

(OP)
Right yeah the motor has its FLC set for 50Hz FLC. Just trying to get my head round if you program it to the data plate then change the maximum frequency to higher will the VSD act in a different manner as it knows it’s a 415 50Hz motor and we’re running if it for example 60-70hz?

RE: VSDS

The motor is constant horsepower for the original setting of 415V/50Hz. As you exceed that setting the torque will drop off since HP = torque x speed. As the speed increases so then the torque decreases.

If you're running a centrifugal pump as the speed increases the hp demand climbs very rapidly yet the motor's torque would be dropping. This would quickly lead to overload or stalling.

Keith Cress
kcress - http://www.flaminsystems.com

RE: VSDS

(OP)
Does the increase in frequency do anything to the current drawn by the motor?

RE: VSDS

A an induction motor draws magnetizing current. This is mostly reactive current.
The real current is mainly dependent on the torque demanded by the load.
As the frequency rises, so also does the inductive reactance and as a result the magnetizing current drops.
With less magnetizing current the available torque drops.
Below rated frequency and voltage, the frequency and the voltage rise roughly in proportion.
This allows or causes full magnetizing current and full torque as the Volts per Hertz ratio is maintained.
Assuming that the supply voltage is appropriate for the motor, the motor reaches full HP at rated frequency and rated voltage.
As the frequency rises further, the voltage is unable to increase above the supply voltage and the magnetizing current and torque drop off.
Below rated frequency: Full torque and variable speed, Constant Torque motor characteristics.
At rated frequency: Maximum HP and Full Torque.
Above rated frequency: Speed increases as torque drops, HP = speed x torque so the motor characteristic is Constant Horse Power.
Note:
There are second order effects causing some interaction between real and reactive current, hence the use of the words "mostly", "roughly" and "mainly".

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: VSDS

Quote (Does the increase in frequency do anything to the current drawn by the motor? )


Yes.

Muthu
www.edison.co.in

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