Top 7 Benefits of Using Variable Frequency Drives (VFDs) to Control Your Crane or Hoist


by  on February 25, 2016

Variable Frequency DrivesVariable frequency drives, or VFDs, are a type of AC motor controller that drive an electric motor by varying the frequency and voltage supplied to it. VFDs are also commonly known as variable speed or adjustable speed drives, AC drives, micro-drives or inverters – depending on the industry or application.

In the material handling industry, variable frequency drive controls are often used in electric hoists as well as in overhead cranes and lifting systems. Drives are designed around a microprocessor, which allows for the creation of programmable functions for specific applications.

So, what are the benefits of using a VFD to control your hoist or crane?

1. Greater Speed Adjustment: VFD controls let you choose from multiple speeds, allowing you to customize your hoist controls to your application.

2. Improved Load Control: Positioning a load using a single-speed hoist may cause shock loading or load swings. A VFD allows for smooth operation by gradually slowing down a load. Variable frequency drives also allow for smooth acceleration to prevent load swings in the traverse motions.

3. Duty Cycles: CMAA Class A-F. A VFD is concisely matched with a braking resistor, depending on the duty cycle of the motion. A regenerative drive may also be used, which is not dependent on duty cycle.

4. Efficiency: A VFD will only consume the power that’s needed, thus saving energy compared to contactors and soft-starters.

5. Increased Hoist Life: Some variable frequency drives provide thermal overload and overcurrent protection for the hoist motor, prolonging its life. VFDs also utilize a ramp-down-to-stop method of braking rather than using brake shoes to slow down. The brake is only used for parking and emergency situations, which prolongs brake life.

6. Regenerative Energy: Deceleration and lowering of a hoist creates regenerative energy produced by the motor. This energy can be transformed into heat using a braking resistor or conditioned and send back to the source, thus netting energy savings. An AC regenerative drive will redistribute that energy, which would otherwise be wasted when converted to heat with a braking resistor.

7. Digital Diagnostics: Aid in maintenance and troubleshooting on the machine or remotely.

Magnetek VFDs are designed for crane and hoist applications with various performance and safety features. A few of these features include:

1. Safe Torque Off: Provides a redundant hardware safety circuit that guarantees motor and brake power are removed when an E-STOP switch or safety controller opens the drive input, eliminating the need for external disconnects.

2. Torque Proving: The motor is pre-torqued to guarantee that the load can be held before opening the brake.

3. Load Check: Continuously checks for hoist overloads and prevents the hoist from lifting when an overload condition is detected.

4. Brake Checks: Monitors the opening and closing of a brake to ensure that it is safe and healthy.

5. Micro-Speed: Allows the operator to make slow, precise movements.

6. Electronic Programmable Limit Switches: Allows slow down and stop limits without physically geared limit switches.

To learn more about variable frequency drives, watch our safety webinar.

{ 4 comments… read them below or add one }

Casey Cummins Casey Cummins June 7, 2016 at 5:06 pm

Hello Harm and Ravindranath,

Thanks for your question and comments as they bring up a great topic for discussion.

As you mentioned, when a hoist is braking or lowering a load, the motor is acting like a generator and feeding power back into the hoist controls.
That regenerated power has to go somewhere, so there are various options for this. I’ll start off by saying that some hoists have load brakes and some don’t. Without going too deep into the theory of a load brake, if the hoist has one, it will dissipate the regenerated power via the mechanics without the need for a braking resistor. If the hoist does not have a load brake, the most common option is to pass the regenerated power through a resistor and convert it into heat.

There are other options such as adding a regeneration unit to recondition the power and send it back to the line. This eliminates the need for a braking resistor and also lowers energy costs due to that power being recycled. This is a new trend we’re seeing in the material handling industry as “green energy” becomes more favored around the world. A third option is to have a VFD algorithm that dumps regenerated power into the motor. This also eliminates the need for a braking resistor, but it comes with the side effects of potentially more wear and tear on the motor due to added heat. This option requires special consideration to the construction of the hoist, so it’s not very common.

To circle back to the original question, if your hoist doesn’t have a load brake and doesn’t have a braking resistor connected, it is very possible that it will have problems that appear during braking and during a lowering operation. I’d feel confident in saying that if you added a braking resistor, the problems you’re experiencing will go away.


Ravindranath Andurlekar June 7, 2016 at 10:34 am

Dear HarmDu Plooy,
Yes you are correct . in lowering direction if you do not use braking resistor the motor goes in generation mode and sometimes voltage generated is so high and generation frequency is so low/ high the motor characteristic disturbed causing failure.

Harm du Plooy May 29, 2016 at 2:28 am

Hi. I use a vsd for my hoist application without a braking resistor. The hoist is a chain hoist applied with a electromagnetic brake. After a few times the motor fails on the winding insulation. Can it be by not using a braking resistor?

Ashmita Rai March 29, 2016 at 1:44 am

I would like to say that your blog is well-written and it contains lots of useful and up-to-date information.

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