Changing the V/F characteristics

[maypot]

Hi,

I have got a 75 k W vsd driving a conveyor and the mechanical set-up only allows the vsd to run at a maximum speed of 25 Hz. The motor is rated at 75 k W 1500 rpm 50 Hz.
The vsd has not got flux vector mode implemented on it.
My problem is that the vsd used to trip on overload at low speed. I am planning to change the V/F curve for the vsds as to boost the voltage at low speed. I trust this should help to alleviate the tripping problem.
Any comment welcome.

Bob

 

[Skogsgurra]

There are two possible reasons why the drive trips: Either too much boost or to little boost. And, of course, the third, obvious one; too heavy load when starting.

Too much boost results in too much magnetizing current and too little boost will increase slip, which makes the motor run hot and probably trip the drive.

So, I think that you should test both. Increase if you don't have a substantial boost already - and decrease if you have.

Gunnar Englund

http://www.gke.org

 

[Drivesrock]

Is the vsd tripping on vsd overload or motor overload? I would guess motor overload. The vsd will have an internal motor overload curve versus speed. At low motor speed the vsd will trip the motor on overload at currents less than nominal motor current as a protection but a high current maybe needed for the torque. The drive recognises that the motor is not able to cool itself as efficiently at low speeds with its own fan. I'm not sure that boost adjustment will help that much. If your motor has an external forced ventilation fan and the vsd programmed for this option then it will allow nominal current at low speeds without tripping on overload. An ultimate protection will be the motor thermostat which can be wired to the vsd to trip it if the motor really is hot.

Drivesrock.

 

[Marke]

Hi Maypot

It is common for materials conveyors to require as much as 160% torque to start when fully laden.
If you are not using a vector drive, you may have difficulty getting a laden conveyor away. At very low frequencies, it is difficlt to get enough flux in the iron to develop the required torque. Adjusting the voltage boost can help, but you need to ensure that the motor is not overfluxed at the speed you operate.

If you are not able to operate the motor at greater than 25Hz, you will be faced with torque and cooling problems. If the motor does not brake the load away, the slip will increase and the current will rise rapidly. This will cause the drive to trip. I would suggest that you consider swaping the motor for one with half the full speed rating. i.e. replace a four pole motor with an eight pole motor.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[reandy]

First, is the drive big enough? You say that it is rated 75Kw, but is that for constant torque, or variable torque? If it is 75Kw variable torque, you can expect approximately 56Kw constant torque, which your conveyor belt application is.

If the motor is connected to the drive with chain and sprockets, or belts and pulley, I would suggest you change the ratio there, and get the motor back up to speed. If however you bought the VSD because this option wasn’t available, I agree with Marke, you need a slower motor.

Reandy

 

[DickDV]

reandy, your comment about the 75kw drive being good for only 56kw constant torque is simply not true. The only difference between a variable torque rated drive and a constant torque rated drive is the amount of short term (usually 1 minute) overload available. It has nothing at all to do with the torque characteristics of the load.

For example, a 75kw rated drive at 460VAC will typically be capable of 124amps continuous with an extra 12.4amps available for short term overloads. You can use this drive on constant torque and variable torque loads equally well as long as you don't need more than 10% overload amps.

That same drive will be rated at 96amps for "constant torque" sizing which drops the KW rating as you indicated. But that is only if you require 150% short term overload rather than 110%. Dropping the continuous rating makes room for the extra overload % even tho the actual overload amp limit is essentially the same either way.

The bottom line here is that drive constant torque and variable torque ratings have nothing to do with torque at all and everything to do with overload capacity. Some careful manufacturers have lately started calling their drives normal duty and heavy duty to get away from the misleading VT and CT ratings. I wish all of them would do the same.

 

[Marke]

Hi DickDV

Yes I agree. The other difference that some manufactures apply is the v/Hz ratio, linear and square law.

The one thing to consider in this case is that the motor is rated at 75KW, but if it is running at 25Hz maximum, it is effectively only a 37.5 Kw machine. Changing the gearing or the motor poles will icrease that to 75KW provided that it is running at 50Hz (or 60Hz if it is a 60Hz motor)

Best regards,


Mark Empson

http://www.lmphotonics.com

 

[maypot]

Thank all for your invaluable comments.
The drive is overloaded on restarting.The system is designed in such a way that the operator must stop the drive from time to time for verification purpose.
On restart, the drive used to trip on overload.The drive is then powered off and on again to withstand the load.
It appears that the drive needs boosting at start up.
Actually there is no boost on the voltage.

Thks.

Bob

 

[reandy]

I guess the point I was trying to make here is that on the slow end of the curve, a variable torque (pump) application isn't "expecting" much of a load at all, however his conveyor belt is. You might say that the load requirements are linear with the conveyor belt, where on the pump application, there is a pretty good curve, and you don't have much load at these slow speeds. It is at the slow speeds where this really comes into play. And the issue here IS a "slow speed"

Reandy

 

[Skogsgurra]

maypot,

Any results? Better with boost?

Gunnar Englund

http://www.gke.org

 

[maypot]

Hi skogsgurra ,

The vsd allows me to change the characteristics of the V/F curve from a constant torque application to a high startup torque one. It has not trip so far .

Rgds.

Bob

 

[mil3]

Maypot,

I must agree with Marke on this one. Save yourself future trouble and consider a changeout of the motor to a low speed motor...or "lower" speed one. It makes no sense driving your current motor at 25Hz maximum. VSD's trip for a reason and most always thinks something is wrong with the VSD. In this case it is trying to tell you something.
If this was my application I would most definately consider a lower speed motor to capatilize on the torque, and save my system both mechanically and electrically. No need to stress the system if it doesn't need to. Reliability will most definately most likely increase aswell.
Just my personal opinion.

 

[maypot]

Hi,

I am now planning to replace the vsd from v/f to sensorless vector. It`s quite complicated to switch to a low speed motor. Will the sensorless drive solve my problem.

Bob

 

[Skogsgurra]

A sensorless drive will be much better. But there are shades of white. Different algorithms behave differently. Siemens has a motor model where EMF is used at high speeds and current at low soeeds. The change-over is at about 5 Hz, but the change-over frequency can be chosen. There are probably refinements to this in the last year(s). ABB has the DTC (Direct Torque Control) which, in my eyes, works better at low speeds.
Describe your requirements and ask a couple of vendors how their drive handles torque at low speeds. Does the drive stop when subject to full load at 1 Hz? Or whatever your lowest speed requires. Get it in writing. That usually sorts out those who can't.

Gunnar Englund

http://www.gke.org

 

[Marke]

Hello maypot.

If you are confident that the power requirement is less than half the motor rating when the motor is running at 25Hz, and the tripping has been just a problem getting the conveyor top breakaway, then yes a vector control system would be an advantage. If the problem is a breakaway issue, then you may need to consider a closed loop vector drive to get the high torque at very low speed. Sensorless vector do not operate as well down to 0 Hz and it depends on the drive just how it behaves at the breakaway frequency. There can be differences due to the effectiveness of the model in the drive. As an alternative to the sensorless vector, you could consider the direct torque control option. I believe that this will give a little better result at the breakaway point. I know of two suppliers of direct torque control, ABB and Emotron.

Interested to see what others think.
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[Skogsgurra]

Hi Mark. What time in Christchurch now?

Gunnar Englund

http://www.gke.org

 

[Marke]

Hi there Gunnar

It is 9PM on Tuesday evening. Winters day, about 11 degrees C max today.
Have a great day,
Mark.

Mark Empson

http://www.lmphotonics.com

 

[mc5w]

At very low frequencies such as below 15 Hertz induction motor torque will falter due to low induced voltage in the rotor. You may need to program the drive to skip the zero to 10 Hertz region when starting the motors. However, operation of a motor at 15 Hertz or 25 Hertz results in more resonable starting characteristics when starting across the line or the equivalent. For instance, a Coilteck sonar control from the Carey Corporation for sensing the depth of a steel coil loop can be set up to do on/off operation at the bottom of the loop so that at low demand a VFD acts like a soft start that in connected to a 15 Hertz power system.

Sounds like you need an unloading valve to ease the starting load of your air compressor. What you need is a check valve between the pump and the tank and a normally open solenoid valve to vent the air line from the pump to the chekc valve. You would put this normally open solenoid valve on an on delay relay that allows the motor a few seconds to accelerate before applying load. You should be able to get a packaged valve and electrical control system that will do this AND maintain minimum pump output pressure when the air tank has no air pressure in it. Where maintaining minimum pump pressure when bringing up the system pressure is important is that rotary screw pumps need air pressure to make the oil circulate.

Permanently running a motor at reduced frequency is a common way of replacing nonstandard motors. For instance, at 1 plant where I worked we had 60 Hertz 10 pole 40 Horespower motors on some thread rolling machines. Some other machines that that were the same model had 100 or 125 HP 4 pole 480 volt motors connected to a 240 volt drive that produced ( nominally ) 24 Hertz 192 volt power to run the motor. The 4 pole motors were a lot easier to buy than a 10 pole motor.