Changing the V/F characteristics
Changing the V/F characteristics
(OP)
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
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





RE: Changing the V/F characteristics
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
www.gke.org
RE: Changing the V/F characteristics
Drivesrock.
RE: Changing the V/F characteristics
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
RE: Changing the V/F characteristics
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
RE: Changing the V/F characteristics
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.
RE: Changing the V/F characteristics
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
RE: Changing the V/F characteristics
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
RE: Changing the V/F characteristics
Reandy
RE: Changing the V/F characteristics
Any results? Better with boost?
Gunnar Englund
www.gke.org
RE: Changing the V/F characteristics
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
RE: Changing the V/F characteristics
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.
RE: Changing the V/F characteristics
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
RE: Changing the V/F characteristics
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
www.gke.org
RE: Changing the V/F characteristics
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
RE: Changing the V/F characteristics
Gunnar Englund
www.gke.org
RE: Changing the V/F characteristics
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
RE: Changing the V/F characteristics
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.