Inverter control of Inverter

[TheDemps]

Hi everyone,
I have been asked to replace a hydraulic drive on a centrifuge with an electric motor which will be controlled by an inverter. The existing hydraulic drive is rated at 15kw. I am wondering if anyone out there has any experience with this. Can I run it on a 15kw electric motor or will I have to up size it. Also is there any pitfalls I will need to look for. I know that a centrifuge has a long run-up time and can be very slow to come to a stop if there is no brake. can I use DC injection braking and if so do I need to use braking resistors. Any help will be greatly appreciated.
P.S. The voltage here is 3-phase 400v 50hz.

 

[jraef]

For a centrifuge you should be fine using a 15kW motor to replace that hydraulic motor. Although hydraulic motors are better with low speed torque step-change response, that is somewhat unnecessary on a centrifuge.

Braking can be done, but not DC injection. You will need dynamic braking ability, (and make sure your VFD has it since it is not universal) which is where the kinetic energy of the centrifuge is burned off in braking resistors. DC Injection braking on VFDs is used just for the very tail end of the braking cycle, after dynamic braking becomes less effective.

A better choice for centrifuges if you can afford it is regenerative braking. That will put the braking energy back into the supply line rather than convert it to heat in resistors. There is considerable energy in a centrifuge and dynamic braking resistors tend to burn out rather quickly. VFDs with regenerative capability are much more expensive however.

"Venditori de oleum-vipera non vigere excordis populi"

 

[sreid]

As a sanity check, calculate the torque required to accelerate the centrifuge inertia. Just as F=ma for linear systems, for rotary systems

Torque = Inertia x alpha

Consistance units would be

Torque lb-ft
Inertia lb-ft-sec^2
alpha radians/sec^2

The peak horsepower would be at maximum RPM and can be calculated using HP = Torque x omega / 550 (torque in lb-ft, omega in radians/sec).

1 HP = 746 Watts

Most VFD's allow controlled deceleration with the power being dumped into a Regen Clamp resistor. For a centrifuge, this can be high wattage. Low cost solutions are nichrome heating elements and radiant heating elements (as used in toaster ovens).

 

[Barry1961]

Some of the common problems I run into when switching from hyd. to elect. are the physcial size, low speed torque and ability to stall.

Hydraulic motors produce a tremdous amount of torque in a very small package. You may have a hard time getting a motor and gearbox to fit in the same hole. You may need to use a planetary gearhead to get small enough. Stober and Bayside are good about doing custom inputs.

To get the low speed torque you will probably want to get as high a ratio gearbox as you can. Consider driving the motor at over 60hz at full speed to get more reduction and torque. I would guess with this application you could probably run 80hz (56% torque)at full speed. Going from a 3:1 to a 4:1 ratio makes a big difference.

You may be better off with a DC motor if there is going to be much stalling. Vector drives have greatly improved in this area in the last few years but I don't have any personal experience with them.

If you have some numbers on the weight, diameter and speed I could run some numbers on it.

Barry1961

 

[ischgl99]

Is this a disk type centrifuge or a decanter centrifuge where you are replacing the hydraulic scroll drive with an electrical drive?

If this is for the main motor of the centrifuge, you will definitely need braking resistors if you want to accelerate to a stop. You also have to consider you cannot stop the centrifuge any faster then you can start it to prevent breaking the belts/drive components.

Mike Bensema

http://www.dutchmenservices.com