A question for the motor/Inverter guys - Jraef & Besoeker et al

[AdrianWint]

My employer makes educational products... amongst them a full time supersonic wind tunnel. The wind tunnel is powered by vacuum pump driven by a 50kW 2 pole 400V motor (I'm UK, so this is 400V line-line 3 phase 50Hz land)

We buy-in the vacuum pump/motor as a package but the control system is our problem. That's fine, I'm competent & happy to design this, but I do have a question for some of the more experienced motor/inverter guys that reside on this learned forum.

The previous builds have used a soft-start to control the motor start-up current. For practical reasons (the amount of spare capacity we have in the factory being one) I've setup the soft-start to throttle the start current at around 250A. This leads to a run-up time for the motor pump of around 20-25s before the current falls back to its running current of around 60A per phase and the soft start comes off line. Since the supply to the factory is only 400A/phase (and the normal loading is around 250-280A/phase) I've always insisted that we run this machine outside normal hours (by killing non-vital load, we can take the 'quiet' factory consumption to around 60-70A/phase) removing any risk of taking out the main supply fuses.

However my Masters have decreed that the Customer wants training on this product and it must be run during normal working hours. On previous builds I discovered, empirically, that if I throttle the current back below about 200A the motor can't accelerate upto 3000 rev before the max start time of the soft-start is exceeded & the unit trips out. I'm not comfortable pulling an extra 250A on top of the normal factory load (even for that 20-30s or so) and feel that the risk of taking out the supply fuses is too great (If they do open, 90 people will be sat around twiddling their thumbs until power is restored by the Utility).

So, here is the question.... If I swap the soft-start for a full blown inverter drive, what would this do to the mains supply current needed to get this pump up-to speed? Will it be nearer the 60A/phase or so that that is needed during steady running (hence I could fire this up during normal factory hours) or will I still be looking at start currents in the 200-300A region??

(There is no requirement for the pump to run at anything other than 3000 rev.... this is purely a 'need the min possible start current' problem)

Thanks for your input

Adrian

 

[Jraef]

My employer makes educational products... amongst them a full time supersonic wind tunnel. The wind tunnel is powered by vacuum pump driven by a 50kW 2 pole 400V motor (I'm UK, so this is 400V line-line 3 phase 50Hz land)

We buy-in the vacuum pump/motor as a package but the control system is our problem. That's fine, I'm competent & happy to design this, but I do have a question for some of the more experienced motor/inverter guys that reside on this learned forum.

The previous builds have used a soft-start to control the motor start-up current. For practical reasons (the amount of spare capacity we have in the factory being one) I've setup the soft-start to throttle the start current at around 250A. This leads to a run-up time for the motor pump of around 20-25s before the current falls back to its running current of around 60A per phase and the soft start comes off line. Since the supply to the factory is only 400A/phase (and the normal loading is around 250-280A/phase) I've always insisted that we run this machine outside normal hours (by killing non-vital load, we can take the 'quiet' factory consumption to around 60-70A/phase) removing any risk of taking out the main supply fuses.

However my Masters have decreed that the Customer wants training on this product and it must be run during normal working hours. On previous builds I discovered, empirically, that if I throttle the current back below about 200A the motor can't accelerate upto 3000 rev before the max start time of the soft-start is exceeded & the unit trips out. I'm not comfortable pulling an extra 250A on top of the normal factory load (even for that 20-30s or so) and feel that the risk of taking out the supply fuses is too great (If they do open, 90 people will be sat around twiddling their thumbs until power is restored by the Utility).

So, here is the question.... If I swap the soft-start for a full blown inverter drive, what would this do to the mains supply current needed to get this pump up-to speed? Will it be nearer the 60A/phase or so that that is needed during steady running (hence I could fire this up during normal factory hours) or will I still be looking at start currents in the 200-300A region??

(There is no requirement for the pump to run at anything other than 3000 rev.... this is purely a 'need the min possible start current' problem)

Thanks for your input

Adrian

Your observations are astute with regard to the soft starter, I would have predicted that outcome. People often misconstrue that just because you CAN set the current limit low, doesn't mean the motor will start with that. There is a minimum amount of current that the motor MUST pull at some point in the acceleration cycle.

A VFD will indeed help. They work very differently because they vary the voltage and frequency together to maintain the full torque capability at all times and as a result, it is theoretically possible to accelerate it to full speed at 100% of FLC. It might take all day, but it can be done and the drive allows that time to be as long as it needs.

The caveat is, the VFD will cost you more, not just to purchase it, but to operate it. The VFD is going to be no better than about 97% efficient. So if your 50kW motor is running at lets say 80% load, that means 40kW of power. To get that output from the motor and drive, the "absorbed" energy through the VFD is going to be 4.25kW, which is what your facility will pay for. Normally, if the VFD is being used to throttle back the speed and replace some sort of mechanical device that was very inefficient, you more than make up for that. But if a VFD is used only as a glorified soft starter and runs at full speed all of the time, then you pay for those losses in operating costs.

If this is just for testing and experiments, that may not be much of a concern, I leave that to you and your "masters".

 

[LMAO]

VFDs also tend to introduce a lot of harmonics into your line, unless you go with active front end or 12 pulse drives which are big and expensive. Harmonic currents can heat up your transformer and harmonic voltage can affect other devices on the line.