Braking with a VFD 1

[Barry1961]

We have a customer that makes small carnival rides that wants to slow down and stop them as quickly as possible. This rides is a horizontal disc that spins with "cockpits" on a 16' diameter that people ride in.

The specs now are roughly....
3000lb on a 16' diameter circle, 6330ft-lb kinetic
9 rpm top speed
120:1, 1hp reducer, 85% efficient, 3494in-lb
1.43:1 secondary reduction, chain & sprocket
Fincor 1hp, 5740 VFD, 115 1ph In, 230 3ph Out
N0 brake resistor option allowed
Supply voltage is 115VAC, 1ph
VFD is at 53hz
20 sec.ramp to stop
I am getting information on expected duty cycle.

He is trying to stop in 20 sec. and is getting an over voltage fault. I did some rough calculations and came up with a 28 sec. ramp being about the quickest I would expect possible with SF and efficiency helping.

So far I have suggested going to a 160:1 reducer and run VFD at 69hz. This would give him roughly 33% more braking torque which would almost get him to the 20 sec. The reduced torque at 69hz should not be a problem with this mainly inertia load.

I was told by Fincor a brake resistor would not help with a 1hp motor at 230VAC so it was not offered.

Are there any tricks or alternate ways to brake quicker with a VFD without causing a fault?
What percent of full load motor torque should I be able to decell at? 115%?

Barry1961

 

[gcaudill]

You will have to work within the maximum bus operating voltage or change the drive to something that accepts a braking resistor.

 

[DickDV]

Barry1961, the Fincor guy is pulling your leg. DC injection, snubber, or regen braking works just as good at 1hp 230V as it does at 1000hp 690V. You just have to have the right stuff to make it work.

First, if the drive has a software option of DC injection braking, you should try that first as it is the simplest and cheapest. The braking energy is located in the motor primarily.

Second, if that isn't available, does the drive have its DC bus accessible on terminals anywhere. Usually, they would be on the same terminal strip as the input power and motor lead terminals. If these are available, there are companies out there that make little snubber brake packages consisting of the appropriate voltage sensing circuit and the braking resistor. Bonitron Inc. is one of those companies. Tell them your motor hp and operating voltage and they will size it for you. About the maximum you can expect is motor nameplate torque down to about 5rpm, then less to a total stop. The braking energy is wasted as heat in the resistor. This system and the DC injection above will not function as a holding brake. For that you need a mechanical brake.

If neither of these are possible, you need a better drive with these features available in it. It doesn't sound like regen braking would be practical on this application because it would not save enough electricity to justify its higher cost.

Don't be tempted to use a mechanical brake to do this job. It's use as a dynamic brake would lead to frequent disc friction pad replacement and an unhappy customer. Use mechanical brakes for holding purposes and let the motor do the stopping.

 

[Barry1961]

Thanks for the info DickDV, the customer is scraping the Fincor for a Hitachi that has a braking resistor. I forget the explanation the Fincor people gave me on why a brake resistor would not help on that size motor but I bought it hook, line and sinker.

The more I learn the more ignorant I realize I am.

Ignorance is bliss.

Barry1961