VFD Control of Hoist Motor and Break

This 40 Tons overhead crane has a standard general purpose VFD with break chopper and also controls the mechanical break. The problem is it does not use DC breaking to hold the weight before applying or releasing the mechanical break. This results in very high wear of break leathers. A few times weight fell and drive or break could not hold. Fortunately no one was hurt.

I need suggestions for a suitable VFD and guidelines regarding wiring and commissioning of this VFD.
 
I haven't done anything with cranes for years, before VFDs for that matter when all cranes were based on slipring motors.

I would imagine that a VFD should be able to raise and lower the load without using brakes at all and just use the brakes for emergency and parking. It should be possible to fully release the brakes before slowly accelerating away.

Of course on power fail the brakes have to be capable of arresting the load under brakes alone.

I will be interested to see what sort of comments you get.
 
J

James Ingraham

I don't want to sound too much like I'm shilling for Allen-Bradley here. They're a major supplier for us, just in the way of full disclosure. With that being said, I think the PowerFlex 755 drive is a good fit. I've never dealt with that big of a vertical load, but I do know that A-B makes drives plenty big enough for that, and recommends that drive for vertical loads. They even have specific documentation for vertical load applications.

Many times I've said that technical issues shouldn't be the driving force for picking a supplier. Given my company's relationship with A-B, they would be my first call for a project like that. However, some other vendor may be a better fit for you. Location is part of this; Rockwell / A-B is the preferred supplier for a lot of people in the United States. In Europe, Siemens is the top dog. Similarly, if you've got a plant full of Logix PLCs, then A-B is the obvious choice, whereas if everything in on Profibus, then Siemens make more sense. (Although every major drive these days plays nicely with just about everything, with a few corner cases.)

I think any of the major drives suppliers would be happy to help with an application like that. If I were doing it, I'd get bids from Rockwell / Allen-Bradley, Siemens, Yaskawa, ABB, Danfoss, Mitsubishi Electric, Toshiba, and Schneider Electric (not necessarily in that order.) If you already have a relationship with one of those, that's who I'd lean towards.

-James Ingraham
Sage Automation, Inc.
 
I have done lots of Hoist / Crane applications with VFDs, specially Siemens MM4 & Sinamics G120 & S120. There are some features that allow "Overlap" between VFD holding the motor using DC injection brake and enabling the mechanical brake.

if you need more info, feel free to contact me: [email protected]
 
MoSawy a few questions for the curious.

On a common VFD application the Voltage sort of follows the frequency in a somewhat linear fashion. I imagine this wouldn't be ideal for a crane, if you did that it would accelerate away as the field stopped rotating. Do they increase the Voltage to provide more braking? Would they run field backwards to overcome slip under a heavy load.
What is the ideal Voltage/frequency relationship.

What type of motor is used, regular squirrel cage or something else?
How is the brake operated, some sort of electro hydraulic?

It would be interesting to get a simple explanation of how a typical modern crane works. A long way from slip-ring motors and drum controllers I'm sure.

Thanks
Roy
 
I'm interested in the above question.
What kind of motor is used for vertical lifts with a VFD squirrel cage or wound?
Is the DC injection supplied by the VFD or is it only controlled by the VFD and supplied externally.
Thanks
Jono
 
First of all you have to know what types of operation is done by a lift?

The answer will be traction operation…

In older days for traction purpose DC series motor was the best because of its very high starting torque . But there are several disadvantages of DC SERIES MOTOR

  • Poor speed regulation
  • Poor voltage regulation
  • Can't be started at no load conditions
  • Speed control can't be done smoothly with out DC drives..
  • DC motor drives (chopper circuit with GTO switches are needed).. external protection circuit is needed like over current and shoot through protection which is not easy to implement practically…
Due to this disadvantages DC series motor is obsolete now a days…

Now a days we use 3-phase induction motor with the aid of. Variable voltage variable frequency drive (VVVFD)… This drives are used to control the speed of the induction motor very smoothly by controlling the fluxes it the motor..

The voltage/frequency ratio is varied by this drive so that the flux per pole also varied in such a way that the speed is confidently controlled but one thing have to be remember , the controlling of that ratio have to be done by avoiding flux saturation in the air gap..

The advantages of using be VVVFD are…

  1. The speed control can be done easily by varying the airgap fluxes
  2. No permanent magnets are used so there will be very less hysterisis losses
  3. Electronic speed control of induction motor makes the motor very efficient and effective..
  4. The braking operation can be done without any restriction…
 
The latest elevator designs use 3-phase induction motors with variable frequency drive electronic controls. Some may also use a similar system with permanent magnet (brushless) DC motors. Some current and prior designs may use commutator-type DC motors with electronic controls. Through practically the entire 20th century, elevators used an AC motor driving a DC Generator feeding power to a DC motor in a configuration called a Ward-Leonard system. For elevators going only 2 or 3 stories, a hydraulic system is used with a simple single-phase or 3-phase motor driving a hydraulic pump
 
It depends.

Older cabled lifts/elevators used DC motors, massive things up to 200HP. Smooth as silk due to the inertia. Now lifts use AC induction motors or AC permanent magnet servo motors with static inverters. Motors may be geared to the ropes so the motor may be a cheap high RPM-low torque standard motor, usually the induction motor type. PMSMs often are direct drive, low RPM-high torque.

Of interest is the large numbers of DC motors manufactured from the 1920s that are still in operation today. Those motors are commonly rebuilt in place. Often these are operated by SCR inverters. While not as efficient as the nweer ones the economics of removing a 20,000 pound motor located 500 feet above ground level means the old motors will be used for a while longer.

Hydraulic lifts often use a common AC induction motor and control speed with a valve in the oil supply.
 
There are 4 different methods of braking a motor that VFDs can use. Two of them, Dynamic Braking (DB) and DC Injection Braking (DCIB), are included in most standard VFDs.
Charles Cowie already described the Dynamic Braking wherein the kinetic energy in the rotating load is transmuted into electrical energy and pulled off of a motor by making it a generator, then is converted again into heat energy in a resistor (or distributed out to other inverters on the same DC bus), removing it safely from the motor.
In DCIB, the transistors are fired in a pattern that pumps DC into one set of windings in the stator, setting up a now stationary magnetic field. As the rotor bars pass through that field, they create a counter-rotating magnetic field that opposes the direction of rotation and brings the motor to a stop. In this method, the kinetic energy of the rotating mass is absorbed by the motor rotor and stator, so it has negative consequences if over used.
The third braking method is called Flux Braking (FB) but requires the use of a good quality Vector type VFD. This is because in Vector Control, the VFD can calculate and separate the flux producing current from the torque producing current, which is one reason why Vector control can be so much more accurate. The normal goal in doing this is to give the motor only JUST ENOUGH flux current to excite the windings without overheating them, reserving more of the available current for producing torque. But in Flux Braking, the VFD turns the motor into a generator just as in Dynamic Braking, but instead of pumping that energy into external resistors as heat, it purposely consumes that energy by increasing the flux current into the motor. So what it is doing is using the MOTOR as the braking resistor. FB therefor has similar possible negative consequences seen in DCIB, but does not require the external resistors like DB. Not all VFDs sold as "Vector Drives" are capable of this, it takes a true Flux Vector Control capability drive.
The fourth braking method, Line Regenerative Braking, requires another different type of VFD, called an "Active Front End" (AFE) topology. Instead of the simple passive diode bridge rectifier, the VFD has another set of active transistors on the front-end converter section, called a "Line Inverter" (as opposed to a "Load Inverter"). So again, taking the regenerated energy off of the motor, the Line Inverter then pumps that energy BACK into the line side to be used by other AC loads in your system. Like DB and FB it removes the energy from the motor as a generator, but instead of "wasting" it in a resistor or into the motor flux, it is recaptured for use elsewhere. The downside of this type is that the VFD is significantly more expensive because you are using two inverters back-to-back. It's not quite twice the price of a standard VFD, but it is a lot more money. However if your duty cycle is high and/or you need to provide continuous or near continuous braking, this method provides a major benefit.
 
VFDs do not allow plugging due to the stress on the switching devices. Most VFDs are probably sufficiently protected from damage is plugging occurs, but it is generally considered to be an abuse of the product.
As described by @Transistor, regenerative braking tends to occur whenever the operating frequency is reduced. As soon as the motor becomes a generator, it begins to supply its own losses and some of the losses in the VFD. That means that some level of braking occurs without switching in a braking resistor. Even though the mechanism of braking is called regeneration, dissipating the braking energy in a resistor is generally called dynamic braking. Regenerative braking generally means returning braking energy to a power supply that can accept the energy and use it elsewhere.
Many VFD products do not provide a built-in braking transistor. Those products protect the VFD from regenerative energy by limiting the rate of deceleration. Most VFDs have the ability to do that even if they are equipped with a braking circuit. As a backup, VFDs are generally equipped with a DC over-voltage trip circuit shut-down provisions that can safely de-energize the motor to a coasting condition.
There is a VFD braking mode in which DC is applied to the motor windings. That is sometimes provided for induction motors without a VFD. That is called DC Braking or sometimes (unwisely in my opinion) called DC Dynamic Braking. Many VFDs, probably most, have DC Braking built in as an optional configuration setting.
 
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