Date: 12/21/2020
Previous Blog: How to Choose a Hook for Lifting Equipment
What Are Contactor Controls and Variable Frequency Drive Controls?
Both contactor controls and variable frequency drive (VFD) controls are used to control the motors that drive the motions of cranes and hoists. Contactor controls are like a light switch, and the VFD controls are a dimmer switch. While the contactor controls are either “switched on” or “switched off”, VFDs ramp speed up and down and can hold any speed the operator chooses.
At R&M, the controls we select for equipment depend on two important factors: the equipment’s application and the motion being controlled. In standard cases, we use 2-speed contactor control for the hoisting motion (vertical movement of the load) and VFD controls for trolley and bridge motion (horizontally across the bridge girder and down the runway).
You can see this system at work in many of the applications in our recent case studies. By using VFD controls for the trolley and bridge motion, we can slowly ramp up the hoist’s speed to naturally reduce load swing.
In application, this variable motion and reduced load swing increases safety for operators who are working around a moving load. The other important consideration is that using VFD controls for trolley and bridge motion helps protect equipment in the long run by reducing wear to motor and brake components.
How Can VFD Controls Protect Equipment?
Let’s consider an application where you have 2-speed contactor controls and need to move a load to precisely set it as part of an assembly line. Often, this leads to jogging the crane – meaning that an operator is pressing the push button repeatedly to get the load into the right position. This recurring jogging motion wears out the contactor more quickly because the contact is energized each time the push button is pressed.
Jogging is also hard on the hoist’s brakes. To demonstrate the effect this has on the hoist’s brakes, imagine you were parking your car. Rather than using your pedals smoothly to get your car into the parking spot, you had one foot on the gas and one on the brake, and you had to slam either back and forth to park the car.
Not only is that less safe and more difficult to do well, it’s also harder on the brake components. Because they’re being applied so often, the thickness of the friction material is worn down more quickly. It can also glaze the surface, which means that the friction disc is still thick enough, but it’s essentially sanding down the grit of the brake.
When you have a VFD, it controls the speed of the motor down to 0 feet per minute and then the brake is set.
To follow the car parking analogy, with the VFD motor you have a smooth acceleration and deceleration while parking the car, similar to a gas pedal. Then, when the car is in position, you set the parking brake as an additional safety measure. In your car, you almost never have to replace the parking brake, because it isn’t used for stopping motion. With proper use, that component isn’t used frequently enough to produce wear.
With VFD controls, the same applies. And because you don’t typically need to replace the brake, it turns a wear part into a long-lasting, safe piece of equipment with a lower lifetime cost of ownership.
What are the Applications for Contactor and VFD Controls?
So far, we’ve illustrated why VFD controls are a safe and long-lasting option for trolley and bridge motion. You may ask: Why not also use VFD controls for the hoisting motion?
While you certainly can use VFD controls for all motions, one consideration is the size and cost of the components for VFD controls. The inverter for the hoist motor will be bigger than that for the trolley and bridge motors. This means the inverter component is more costly and additional electrical panels are needed to house it.
Additionally, in most applications, contactor control hoisting is sufficient and safe. As we mentioned earlier about jogging horizontally, if you have an application where you need precision and speed for your lifting motion, you could consider VFD for that motion as well.
For example, the calculated number of contacts is approximately 640,000 in the life of the contactor. Imagine you’re using 2-speed contactor controls and you have to jog a load down every time you place it, and you’re lifting a load every 10 minutes. You could easily end up with 30 contacts every 10 minutes, which compounds quickly when you consider the lifetime of your components.
If that’s the case, and you need that level of positioning with your hoisting motion, you will likely replace your contactor nearly every year. However, if you choose VFD controls for that application, it will generally require less service and downtime.
Open Loop Vs. Closed Loop Inverter Controls
VFD controls can communicate with the motor in two different ways: with open loop feedback and closed loop feedback.
Open loop controls are most common for trolley and bridge motions but are also a good option for hoisting applications with high levels of jogging and a need for faster speeds. The open loop control means that the inverter tells the motor how fast to go, but it does not receive any feedback from the motor. R&M’s open loop hoisting control is more advanced than other open loop options in the market because it does include a bearing sensor which provides feedback to the open loop inverter.
An advantage of open loop controls is that they have extended speed range (ESR) after a load has been removed from the hook. This means that a hoist rated for 20 ft. per minute could go 120-150% of that speed without a load attached. This allows operators to move more loads over the course of their shift by reducing the time between lifts. R&M’s open loop control is also unique because it features an adjustable speed range (ASR), which gives the operator a slower speed at higher capacity loads to increase safety by enabling the right speed at the right time.
Also, in general, open loop controls are the most cost-effective inverter hoisting solution. For example, open loop controls might be used to load and unload trucks at a dock. The truck backs in and the load is removed. However, because the position of each truck will change slightly when it’s backed into position, this process can’t be automated easily. The extended speed range would help to increase throughput of this process, and the open loop inverter controls would help to protect the motor and brake components from the heavy usage.
For the hoisting motion, closed loop controls are more common. With a closed loop, the inverter controls how fast the hoist is going, and then it gives that feedback to the inverter via an encoder at 600 pulses per second. Because closed loop is more often used when precision and speed are key for the application, closed loop controls have more robust software and hardware options. This precision is augmented by the continuous communication in the closed loop system and its more advanced data-processing.
An example of a closed loop control application could be setting dies in a metal fabrication facility. The super slow speeds, smart control options and extreme precision would help to precisely set large dies onto their positioning pins. This helps keep employees safe and prevent damaging costly equipment.
Given the different applications and potential savings of contactor vs. VFD controls, which should you choose? Contact us online to speak to an expert about your next project.