Gary Braun
Brehob Corporation
If you’ve ever applied variable frequency drives (VFDs) to standard motors, I’m sure you know how critical it is to understand the specific application and to get all the required data so that you can apply the drive properly. It’s not the time to guess or make assumptions. Most failed applications are a result of not getting all the information.
When trying to apply VFDs to old or existing cranes and hoists, this holds doubly true. Several drive manufacturers sell their crane and hoist drives only through authorized “crane and hoist” dealers because the applications are fraught with potential problems. Before beginning one of these projects, make sure your insurance carrier covers you for the redesign and work on cranes and hoists.
Applying VFD To Drive
Let’s start by looking at applying a VFD to the bridge (also trolley) drive. The application is similar to a conveyor drive application. We need to answer the following questions:
- What is the applied voltage and phase? This is self explanatory; just don’t guess. Make sure that the voltage drop on the collector rails is within acceptable limits for the VFD you are using.
- What are the horsepower (hp) and the full load amp (FLA) rating of the bridge motor? Are there two bridge motors, one on each end of the bridge? Remember that VFDs are really rated by amperage capacity, not hp. Many cranes use special purpose motors with FLAs that are higher than standard motors and the VFD needs to be sized for the current required. If there are two bridge motors, both will have to be supplied by one VFD that is large enough to handle both motors. In addition, two thermal overload relays will have to be added —one for each motor in order to meet National Electrical Code (NEC) requirements. Using a separate VFD for each motor on a two-motor bridge system is normally not effective and will cause bridge tracking problems. The bridge may “crab” or bind on the rails if both sides do not run at exactly the same speeds and accelerations.
- Are the motors intermittent or continuous rated? If it’s a two-speed motor, pick the speed that is best for the application. Use its amperage rating to size the VFD; the other speed will be abandoned. If it’s a wound rotor motor, there is a significant potential that it may not be able to be run by a VFD. Simply shorting the rings of the wound rotor motor may work, but on some motors the starting torque will be reduced to levels that fall below the load requirements and the motor will stall. The only choice is to replace the motor with a standard motor with sufficient torque and hp for the load. Refer to Section 3 in the EASA Technical Manual for more information on wound rotor motors. In addition, all the other motor concerns for VFDs hold true, e.g., insulation stress, breakdown, increased heating, etc. If you run the motor for long periods of time at low speeds without sufficient cooling, you still take the risk of destroying the motor because of heat build up. It may be advantageous to add a blower to provide a consistent volume of cooling air.
- Is there a bridge brake and/or how does the bridge stop? Normally, if there is a brake on the bridge motor, it is connected in parallel to the motor winding. When the motor is energized, so is the brake. This energizes a solenoid and releases the brake. When power is removed from the motor the brake sets via a spring. When applying a VFD, the brake cannot be connected to the motor. The brake is not suitable to be connected to the variable voltage and frequency supplied by the VFD. If a motor brake is used, additional circuitry and/or VFD software may be required. This will depend on the make of the VFD being used. There may be other types of brakes on the bridge including hydraulic brakes. This again will require additional circuitry and/or VFD software. The VFD can be used, in lieu of a mechanical brake to dynamically stop the bridge, but remember this will not give you any holding power when the bridge is at rest. This may allow unwanted creeping of the bridge when a load is being picked up. For most applications this is not an issue, but it may be on your application. Also you must give careful consideration to the amount of regenerative energy that the moving bridge will generate and will have to be absorbed by the VFD. This may require additional dynamic braking capacity in the VFD.
- What speeds does the customer want and for what period of time? Assuming we have worked out all the potential motor problems related to running at variable speed, we must now consider how we are going to get the control commands to the VFD. To apply a standard VFD, normally the drive is programmed to respond to multiple preset speed commands. You program the VFD for the best speeds for the application along with the acceleration and deceleration rates desired. The number of speeds available to the operator are dependent on the number of contacts per button in the pendant station, e.g., 2 contacts and detents per button allows 2 speeds; 4 contacts and detents per button allows 4 speeds, etc. To make the standard VFD continuously variable either an additional 2 buttons or a potentiometer will have to be added to the pendant station.
- Does the customer want multiple presets speeds or continuously adjustable speeds? The special “crane VFD” has special software and makes continuous variable speed much simpler. This VFD only requires 2 contacts and detents per button to achieve continuously variable speed. The addition of the VFD will require some control rewiring. Motor brake circuitry and programming may still have to be added. Remember, success lies in the details.
- What pendant station and/or control scheme is to be used? The pendant buttons and contacts must be compatible to the circuit desired along with the number of wires in the pendant cable. Since most VFDs are slightly different in programming and set up, and cranes have endless variations in wiring diagrams, there is no “fits all” system. The “crane VFDs” do make the job easier.
Applying VFD To Hoist Motor
There are a few points to keep in mind about applying a VFD to the hoist motor. When dealing with the hoist motor the complexity increases and the risk is much greater.
Misapplication may put the operator in a dangerous and unsafe position where injury or death are possible. Again, success lies in the details, but using a VFD designed for hoist service is the way to go. When applying a VFD to the hoist, we go through all the same steps as for the bridge and trolley, plus we have the additional problem of the hoist brakes. The hoist will have a motor brake, plus it may or may not have a load brake. Again, when dealing with a wound rotor hoist motor simply shorting the rings may not work because you are changing the torque characteristics of the motor drastically. When we tackle the problem in our facility we try to get lots of data on the motor and application to do the engineering.
But in the end, sometimes it just comes down to trial and error. In these situations, we short the rings on the motor, load the hoist to its maximum capacity and throw the motor across line to see how the motor and load will react. If the motor handles the load across line in this manor it will probably work on the VFD and we proceed with the project. If the motor stalls or can not proceed to pick up a suspended full load, the motor will have to be replaced if possible. When trying to replace a hoist motor, remember that many have special flanges, output shafts with splines or gears, brakes, service factors, etc.
Need To “Prove The Load”
When a VFD is used on a hoist without a load brake, the VFD and motor must first hold the load. You will need to “prove the load” before releasing the motor brake. If not, gravity will pull the load down when you first start and want to go up. This is a big problem. Most standard drives do not have this feature built in and will require special programming which may or may not be available. Most crane drives use closed-loop Vector VFD technology along with special software. You will also have to consider the regenerative energy the VFD will see. A hoist with a load brake and motor brake will have a completely different set of parameters and the setup will be different. I am sure that someone has applied a standard VFD to a hoist with success. But again, misapplication may put the operator in a dangerous and unsafe position where injury or death are possible. Be sure to use a VFD designed for hoist service.
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