Braking three phase vibratory motors - inverter - resonant vibration 1

[fredUK]

I am looking to find a way of braking three phase vibratory motors quickly, as the motors have excessive levels of vibration on shutdown. An example spec of one of our vibratory motors is as follows:

3-phase, 0.9KW, 1.45A flc, 400V supply @ 50Hz

These vibratory motors are usually just run DOL, so deceleration may take between 5-10 seconds. During this period, the motor runs from 25Hz running vibration down to 0Hz, with resonant vibration typically occurring at 10-15Hz. It has been found that by stopping the motor in 0.5 seconds, the vibration is virtually eliminated. I am looking at a low cost solution for achieving this braking time.

Initially, I investigated DC injection brake modules as a possibility. These were found to reduce vibration, but could not achieve a quick enough stop (probably due to the delay in dc injection). I then tried inverters with ramp down and dc injection for braking. This gave improved shutdown times, but for larger motors, it was found that the inverter could not cope with the over-current. As a result, I wired a braking resistor across the inverter, to dissipate the energy as heat. This enabled me to stop the motor in 0.25 seconds and was very effective in reducing vibration. The inverter and braking resistor can cost £300-£400. Does anyone have any ideas on how to stop these motors in 0.5 seconds, but at a lower cost?

 

[ozmosis]

I'm surprised a DC injection brake unit does not work too well to be honest. FOr this size of load, a VSD with brake resistors and DC injection brake seems a little overkill as a similar function can typically be achieved in stopping time with a good quality brake unit.
Was it sized correctly?
Many years ago I used to use the Crompton brake units (as you appear to be in the UK with the £ sign). Try their S35

http://www.cromptoncontrols.co.uk/s10.html

for a bit more umph. I see there's a new unit called smooth brake but no idea what that is about.

 

[ScottyUK]

Motors are available with purpose-built brakes, but the cost of the motor plus the modifications to the control gear may cost more than the VFD solution. Here's the Brook Crompton range of brake motors.



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[DickDV]

I would have chosen the DC injection brake also. You can inject up to 1.5 times FLA in DC braking current without damaging the motor.

Is it possible the DC brake module was undersized?

 

[fredUK]

Thanks for the input. The S35 crompton DC injection brake may well be worth a try. I think the one I used was similar to the s10:

http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=2096384&cm_vc=av_uk

This DC brake is rated at 2.2KW and the motors I tested were 0.3KW and 0.9KW, so I would be surprised if it were underated.

Whilst I’d expect a DC injection brake module to stop the majority of motors rated under 3.6KW (which is what I work with) in 0.5 seconds, surely vibrator motors require a greater DC injection braking current. These vibrator motors can have quite large weights fitted to the motor drive shaft, significantly increasing the moment of inertia and thus requiring greater stopping force. Here is a link for an example of the motors used:

http://www.vibtec.com/pdfs/Leaflet-18.pdf

In terms of using a Crompton brake motor. I’m pretty sure they don’t offer vibrator motors and I wouldn’t want to be adding the motor weights myself if we can already buy in standard vibrator motors.

 

[jraef]

DCIB will not work for an application that requires stopping that fast. A DCIB must wait until the motor field collapses before injecting, otherwise it regenerates and blows the SCRs. So because every motor is different, some mfrs chose to measure the motor voltage before energizing, some just build in a fixed delay, usually no less than 1 second.

People like Stearns make mechanical brakes that can be added on to existing motors, but the motor has to have an NDE shaft. If yours does not, that becomes a much more involved process, probably exceeding the cost of the VFD as others have said.

I think this is something that can be done with a properly engineered VFD and Dynamic Braking package, but it seems that you costing for something that small is a bit high (by US standards anyway). I would look around more.


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[bklauba]

You say these motors are vibratory. Do you mean they are rotary electric vibrators? If so, they might play havoc with a Crompton brake. DC injection is likely your best bet.

Also, if these vibrators are being used to empty a bin, their most common app, they will draw excessive current when the bin material drops below height of the vibrator, typically going from 60-80% FLA to 2 - 3 times that. If this is the app, then a bin level indicator positioned wisely would disable the vibrator before excessive current / excessive amplitude occur. This scheme might prevent your resonant condition. An ounce of prevention . . . .

BK

 

[BobM3]

Or, hook dampers across the springs of the vibrator and use solenoid flow control valves on the dampers so they are only active when the power is turned off to the vibrator motors.

 

[MIK2000]

http://www.miksanmotor.com/en/vibrasyon/vibrasyon_motorlar.asp

 

[itsmoked]

I'm with jraef. DC injection will not work. Altering your motor to fit a brake is bogus since the brake will be thrashed to death and will cost as much as a VFD that size.

You have proven that a VFD with a brake does exactly what you want. Why spend time and money on any other non-solution?

Find some reasonably priced VFDs and brake resistors. Your 400L price is horrible.

Examples:

http://www.factorymation.com/s.nl/sc.2/category.13/.f

http://www.factorymation.com/s.nl/sc.2/category.34/.f

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

Do you really need the braking from full speed to a stop or is there some critial speed range you need it to be braked through quickly?

A DC injection brake does have a delay time from the motor stop to the DC being applied. If the vibrations get excessive withing this time (and you sem to indicate they do) then DC injection braking won't work. If the vibration level is acceptable for the first second then it could be possible you just haven't found the right DC injection brake.

Just out of curiosity, what level of current was the brake injecting? You'll need to get the current up to around 4.5A to 5A to have any hope of a quick braking time.