Current on overhauling motor 3

[Barry1961]

I have an application with a 1/8hp(.09kw), 3 phase, 240V, 60hz motor on a 240:1 gearbox being controlled by a VFD with single phase 240V input. At times the motor is being used as a brake for a large mass. The VFD is rated for 1/4hp(.18kw) and has an internal braking resistor rated for 50% FLT. I am not able to take measurements at this time on the device.

The VFD has a current limit function that is being used as a torque limiter. I am trying to calculate how much braking torque I can get compared to acceleration torque with the same current limit set point.

What is the current curve in relation to slip or torque when being overhauled?

What is the voltage generating curve to slip or torque when braking?

When the chopper transistor starts dumping voltage how does it effect motor current and torque?

I know there is not enough information to get exact answers. I am just trying to get a idea of what to expect.

Barry1961

 

[Skogsgurra]

I think that you can get some result from knowing that the torque vs speed curve is practically a straight line from fully loaded to fully braking. With zero torque at synchronous speed.

The chopper shouldn't affect the working of the inverter (at least not in the first approximation). So your current limit should work as usual - if it is a current limit and not a torque limit.


Gunnar Englund

http://www.gke.org

 

[CJCPE]

If you have a 240:1 speed reduction gear, the inertia reflected to the motor will be 0.0017% of the load inertia. The VFD is not going to be asked to provide any braking torque unless the gear transmits torque back to the motor acting as a 1:240 speed increaser looking from the load back towards the drive. Is that really going to happen?

 

[Skogsgurra]

Oops, missed that 240:1 gear. Since almost all your torque will be lost in that gear, you will probably not come anywhere close to the torque limit while decelerating. As CJPE said.

Gunnar Englund

http://www.gke.org

 

[Marke]

Hello Barry1961

The torque curve of the motor above synchronous speed is a mirror of the torque curve below synchronous speed, so it is possible to get the same maximum torque deccelerating as accelerating.

In this case with the very high gear box reduction, I would expect to see a worm drive and so do not expect that there will not be any overdriving torque.
A good clue is what happens if the power is switched OFF, how long does it take to run down. If you want to run it down quicker than yes there will be some breaking torque required.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[DickDV]

The original poster states that he is using a current limit setting in the drive as a torque limiter so, to answer that the overhauling torque-speed curve is a mirror image of the motoring torque-speed curve isn't really to the point.

The question is about current so we have to be able to translate the overhauling torque into some equivalent current.

When motoring, the motor lead current consists of magnetizing current and torque-producing current. When overhauled, the same two vectors are present but the torque-producing vector has swung around 180 degrees. The total current would have the same magnitude with overhauling torque as when producing the same level of motoring torque. I conclude that the current limit would represent the same torque level in either mode.

As far as the DC bus, the brake chopper, and the brake resistor is concerned, there is an important difference. When motoring, the magnetizing energy and the torque-producing energy both come from the drive. When overhauling however, the magnetizing energy subtracts from the regenerating torque-producing energy so the net energy to dissipate in the braking system is reduced. In addition, whatever small losses are present in operating the drive electronics are also subtracted from the regen energy.

For this reason, the snubber-resistor package can be smaller than the drive kva rating and still produce full regen braking in the motor. Sizing at 85% is conservative and will usually produce more than drive nameplate braking.

Remember also that friction losses in the power train will also add to your braking capacity. A gearbox with 240/1 ratio has huge losses if it can be back-driven at all so I would venture a guess that a snubber-resistor package half the size of the drive would be more than adequate for this application.

 

[LionelHutz]

The VFD has a current limit function that is being used as a torque limiter. I am trying to calculate how much braking torque I can get compared to acceleration torque with the same current limit set point.

Since the origional posted asked the question above I think it's completely the point to say that the overhauling torque curve is the mirror image of the motoring torque curve. The same current limit should produce the same overhauling torque as motoring torque.

The chopping resistor operating has no real effect on the torque. It will vary the buss voltage which can effect torque a bit but it's not a big variation.

The big issue is the dissipation of the overhauling power in the braking resistor. You basically need to calculate how much power the motor is producing when overhauling and then how much power the brake resistor can dissipate.

The power produced by the motor is volts x amps x sqrt(3) x pf.

If you don't know the motor power factor then a rough calculation is possible by using current calculated as the running current minus the the no load current and dropping the pf term. It's not perfect but it will work OK.

The power the braking resistor can dissipate should be 50% of the drive rating if it's sized as claimed. Often, a braking resistor has a duty cycle and is not continously rated. Unfortunately, there are no standards for the resistor ratings. If you can find the wattage of the resistor it will tell you for sure.

In any case, the maximum power the braking resistor can instantaneously dissipate is the buss voltage^2 / ohms. If you go over this value, even a short time, then the drive can't dump all of the energy it's receiving from the motor into the resistor and it will trip on an overvoltage.

 

[waross]

Hi LionelHutz

Since the origional posted asked the question above I think it's completely the point to say that the overhauling torque curve is the mirror image of the motoring torque curve. The same current limit should produce the same overhauling torque as motoring torque.

My understanding is that this is true for a motor connected directly to the line. There are a couple of issues that make the comparison less than perfact, but basically I accept the statement.
However, I understood that the torque limit mode controlled only the driving torque in a VFD. I understood that the torque control would have no effect with an overhauling load unless the VFD had regenerative capability. I understand that regenerative VFDs are not common. Usually the breaking torque is controlled by the dynamic breaking parameters and limited by the capacity of the dynamic breaking resistor. Comments?
yours

 

[DickDV]

waross, you are correct in saying that regenerative drives are not common. The key to getting the perspective right is in the fact that regen drives are usually defined as drives that regenerate the braking energy back into the AC power supply thus saving most of it. These drives are not common.

However, any AC PWM drive that has its DC bus terminals available for external connection can be converted into a drive with snubber brake capacity thru the addition of a brake chopper and brake resistor. These drives are not usually called "regenerative".

Or, stating it another way, a regen drive saves the braking energy while a drive with snubber braking wastes it as heat in a resistor.

As far as braking capacity is concerned, you can put an oversized snubber and resistor package on a drive but the current limit feature of the drive will cap the braking current and thus limit the system braking capacity. The drive has to do this to stay within its heatsink capacity and protect its solid state power circuit components.

You do raise an interesting question regarding drive torque limits working in both directions. The drives I deal with (ABB and Reliance) will limit torque in both directions but I have to wonder if they all do this or whether there are some exceptions.

If you are picking the pieces for a system with heavy braking requirements, if sure would be worth checking into that before buying.

Anybody have knowledge of drives with torque limits that don't work in braking mode?

 

[waross]

Apologies to LionelHutz and thanks to DickDV.
Yours

 

[Compositepro]

There is a very good chance of destroying the gearbox in this type of application. Putting reverse torque on the input of most 240:1 gear boxes can easily lock-up the gears and cause the load to come to an instant stop. The loads will not pass through to the motor. Worm gears in that ratio are considered self-locking the output cannot drive the input. Planetary gears would be more forgiving.

 

[Barry1961]

The gearbox is a right angle Brother hypoid which is about 85% efficient at 240:1 which can be back driven. The application is an outdoor rotating car display. The problem occurs when the wind speeds up the rotation cusing the current limit to decrease frequency. Sort of a double whammy. There is an additional 3:1 V-belt reduction.

Thanks for all the info!!! Big help!

Barry1961

 

[LionelHutz]

Waross;

How the braking torque is controlled depends on what you define as braking. If the drive is creating a 3-phase output and the motor is overspeeding then the system is operating in a generating mode. It is also braking the motor because it's not allowing it to accelerate. It's generating because the power is going from the motor to the drive. The drive can either dump this power into a "braking" resistor or it can put it back onto the power system.

If the drive is putting DC onto the motor then it will also be braking it. But, it will not be using the "braking" resistor.


DickDV

You are right in questioning the abilities of the drive while in a regenerative mode. I don't know of any but I would not be surprized if there are some cheap "shaft-turners" out there that don't provide this limiting feature.

When the system is operating in a generating mode the drive would have to increase the frequency to limit the current. If you have a max frequency setting, of say 60hz, then the drive would not increase the frequency over 60hz and it would quit current limiting once 60hz is reached. This is easy to overcome with the proper parameter settings in the drive.