View Full Version : VFD torque problem with a Bitzer screw

29-07-2013, 08:38 PM
If we connect a 55 kW motor to a VFD with an input on the VFD of 400 V, what's then the voltage at the compressor or outlet VFD?
Today someone told me that outlet will be 375 V.

So given this assumption, torque decreases with square root of voltage.
Torque at indicated 55 kW = 55 x 9555 / 2960 = 177.54Nm
Torque after the VFD will be 177.54 x (375/400) = 156.04Nm
156.04Nm at 2960 t/m equals 156.04 x 2960 / 9555 = 48.34 kW maximaal powere the motor can deliver at 2960RPM
Given this, with a specific compressor TE -25 and 45 TC , motor delivers not enough torque to drive a screw = 51.3 x 9555 / 2960 = 165.6Nm needed and motor delivers max 156.04Nm
So the motor stalls.
Is this calculation correct you think because we start from the assumption that the voltage decreases at the VFD outlet?

29-07-2013, 08:51 PM
A lot of this is way beyond my expertise, however, it makes for interesting reading and possibly answers you question Peter. http://en.wikipedia.org/wiki/Variable-frequency_drive

install monkey
29-07-2013, 09:14 PM
whats the details on the vfd drive?- i take it ur starting the compressor unloaded- what size is the vfd?

30-07-2013, 07:11 AM
Max current for the VFD = 100A, max motor current = 96 A
It's a Vascon drive IM, but my question/concern is not brand related I guess.
As speed decreases by the VFD, DP over compressor stays the same, power will decrease but needed torque will remain the same. So, if we slow down, we have possibility the compressor stalls at a certain load if torque decreases ( U stays the same)
But given this pdf http://controltrends.org/wp-content/uploads/2010/10/VFFundamentals.pdf voltaghe seesm decreased together with the frequency. So torque will decrease also.
I'm confused.
Another one http://www.danfoss.com/nr/rdonlyres/1fde93be-95cf-46f9-b2ae-6f76c69594cb/0/vfdlesson4.pdf

31-07-2013, 12:14 AM
With initial input settings voltage hertz amps etc.., program 460 /480 volts as supply voltage. The drive will output proportionally higher voltages during ramp up and variable frequencies, giving higher torque and overcome stalling problems and overcurrent problems at variable frequencies. From memory Bitzer recommend minimum hertz setting at 30Hz.

31-07-2013, 05:26 PM
This is the answer from Invertek, UK.

The output voltage will be somewhere just below 400Volt, usually within about 2 – 3% in the worst case. What happens in this case is that the current draw for the motor will increase slightly to compensate, so that the power and torque of the motor is maintained. Torque is not proportional to the voltage at all, it’s proportional to power and speed, and power only drops if the voltage drops and there is no corresponding increase in current. In practice, as voltage reduces, current increases.

But torque IS proportional to the applied voltage.
https://www.google.be/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&sqi=2&ved=0CC4QFjAA&url=http%3A%2F%2Fwww.elsevierdirect.com%2Fcompanions%2F9780750647182%2Fsolutions%2F06~model%2520solu tions%25206.doc&ei=Ejj5UZzQHaGN7QaN-IGgDA&usg=AFQjCNHqCOkiRYUzNPRMF8RRcqyJCGoLfQ
So, still confused

31-07-2013, 09:49 PM
Others perhaps can benefit
There is a lot of information on the internet, and it is very easy to read one piece but not fully understand the context with which it was written. The information you link to is for a different purpose. Also, I think some of it is written by people who do not fully understand the subject, and have missed some key points of information.

If a motor runs directly on the mains supply, 400 Volts, 50Hz, with full load or no load, is the current consumed the same? Answer, no. Torque is proportional to current, and since no torque is required for no load, torque producing current under no load conditions is zero, only the magnetising current (plus a small amount for rotating losses) remains. So current is not proportional to voltage in that sense, the motor consumes only the current it needs to generate the required torque.
But the supply voltage does have some effect!

Power = Torque x Speed. This is the empirical rule.

The motor has a torque capacity defined by its rated power, and the number of poles with which it is wound.
You will see on most motors that the nameplate will have different voltage for different countries, e.g. (These are just numbers I made up, but show the example)
380 Volts, 10A
400 Volts, 9.7A
415 Volts, 9.4 Amps
460 Volts…

So the motor power remains the same at different voltage, but full load current changes. So even if the motor voltage is reduced, it can still produce full load torque, but with increased current.

The only condition to this is that the supply voltage is high enough to fully magnetize the motor. For example, if you supply a 400 volt motor with 200 volts, there is not enough voltage to flux the motor, and so full load torque cannot be produced. But anything from 380 volts to 480 volts for a standard motor is fine.

Simple V/F drives maintain a fixed relationship between voltage and frequency, so that as frequency is reduced, voltage is also reduced. For centrifugal fan and pump loads, which have a characteristic when torque is proportional to speed ^ 2, a quadratic characteristic can be used, so output voltage reduces much more in relation to frequency. In the one article you link to, this is why they say current is proportional to voltage, what they are trying to explain I think is the square law characteristic help to save energy.

Loads like compressors and displacement pumps or blowers need a linear V/F characteristic, as torque is the same across the speed range. By maintaining a linear V/F relationship, current for the same load will remain the same at all frequencies up to base frequency, so it is only necessary to control voltage in this linear fashion.

01-08-2013, 03:58 AM
Peter, on some drives you have to lock in constant torque setting. It may have different names in different brands of drives etc.
If it is not locked in drive can try to optimize etc & stall because of hiccup & load of compressor.

Yuri B.
03-08-2013, 08:30 PM
On my view the VFD could be bigger to maintain the voltage at load. As for the torque, different induction motors have differing torque/speed characteristics. Consult the manufacturer of the motors for Bitzers.