Mike Dolt Enterprises
Mike Dolt Enterprises
User: W@ttson, Location: NJ-USA
Asked on: Tuesday, April 1, 2025 at 9:43 am EDT
Answered on: Tuesday, April 1, 2025 at 9:43 am EDT
Question: Hello,If I have a motor that is connected to a drive and it is for a crane application, I can have an idea of what the currents on the output of the drive would be as well as input of the drive.If I require 100% torque for the load to pick it up and go rated speed, I would see roughly the rated current on the output of the drive and slightly less current on the input of the drive due to PF difference.Now if I need 120% torque to get the load going to rated speed, I would again expect to see something of a 1.20x relationship on the output of the drive and similarly on the input of the drive from the rated values (ignoring that its not exact since the torque producing current would be the one being multiplied by 1.2 and through vectoral arithmetic the resultant current would be slightly less).If I decide now to run the motor at 50% speed but at rated load, I can expect the same current on the output but roughly 50% current on the input. Due to the voltage remaining the same on the input but different voltage on the output and the application being a constant torque application.Now, what happens on the lowering and the load becomes overhauling?Say I am lowering the load at rated speed but the load becomes overhauling and the load begins to move faster than what the motor is telling it to go? (like car engine going down hill). The drive will have its DC bus voltage begin to raise. The chopper circuit will turn on an begin to dump energy into the dynamic braking resistor.What is the input current to the drive? What is the output current to the drive?How does that change when at 150% overhauling torque?How does that change when at 50% speed?
You've got a good grasp of the basics of drive and motor operation in crane applications. Let's break down the overhauling scenario in more detail
Understanding Overhauling Loads
An overhauling load (also called a regenerative load) in a crane application means the load is trying to drive the motor faster than the commanded speed. Think of lowering a heavy weight: gravity wants to accelerate it downward. The motor, instead of actively powering the load down, now has to resist the load's downward motion to control its descent. This resistance generates energy, which flows back from the motor towards the drive.
Drive Behavior During Overhauling (with Dynamic Braking) Output Current (Motor Side)
The motor is acting as a generator. The output current direction is reversed compared to motoring (lifting). The magnitude of the output current is directly related to the torque being exerted to control the load. If you need rated torque to hold back the load (rated speed), you'll have rated output current, but in the opposite direction.
DC Bus Voltage
The energy being generated by the motor tries to push current into the DC bus capacitors within the drive. This causes the DC bus voltage to rise.
Dynamic Braking Resistor (DBR) and Chopper
To prevent the DC bus voltage from exceeding safe limits, the drive's chopper circuit activates and dumps excess energy into the dynamic braking resistor (DBR). The DBR converts this energy into heat.
Input Current (Line Side)
This is the crucial point. Ideally, the input current from the power line should be close to zero or even slightly negative (in phase opposition). Here's why: The drive is primarily absorbing energy from the motor (regenerative energy). Ideally, all that energy is dissipated as heat through the dynamic braking resistor, and the drive is drawing minimal power from the line. The actual input current can deviate from zero depending on drive losses, efficiency and compensation for power factor by the AC Line chokes
Specific Scenarios and Considerations1. Rated Speed, Overhauling at Rated Torque (100%) Output Current
Rated current, but reversed direction (relative to lifting). Magnitude reflects the need to control rated torque.
Input Current
Should be near zero (or slightly negative), primarily supplying drive losses, and PF correction if any. The regenerative energy is being handled by the DBR.2.
Rated Speed, Overhauling at 150% Torque Output Current
150% of rated current (reversed direction). The motor is generating more power to resist the increased downward force.
Input Current
Should still be near zero (or slightly negative). The dynamic braking resistor is working harder to dissipate the larger amount of regenerative energy.3.
50% Speed, Overhauling at Rated Torque Output Current
Close to Rated current (reversed direction). You still need to apply rated torque to control the load.
Input Current
Should be near zero (or slightly negative). Still only supplying the losses of the drive.
Important Considerations and Caveats Drive Efficiency
Drives aren't perfectly efficient. There will always be some losses (heat) within the drive itself. This means the input current will likely never be exactly zero during overhauling. It will be very small, but not perfectly zero.
Drive Design
Some drives have "active front ends" or regenerative capabilities. These drives can actually push power back onto the power grid during overhauling. In this case, the input current would be significantly negative, meaning the drive is acting as a generator and feeding power back into the grid. This is more efficient than using a dynamic braking resistor, but the drive is more complex and costly.
Power Factor Correction
Drives often have power factor correction (PFC) circuitry. This can affect the phase of the input current, even when the magnitude is small. A high PF will mean the current is nearly in phase with the voltage, while a low PF means there's a significant phase difference. The drive might draw a small current to maintain a good power factor.
Overspeed Protection
Drives usually have overspeed protection. If the overhauling force is too great, and the motor speed exceeds a preset limit, the drive will trip to protect the motor and load.
Application is King
The exact behavior will depend heavily on the specifics of the crane application, the drive model, and its configuration.
In summary:
During overhauling with dynamic braking, the goal is to minimize the power drawn from the line by dissipating the regenerative energy through the dynamic braking resistor. The output current reflects the torque being applied to control the load, while the input current should be close to zero, only supplying the losses of the drive. Be aware of regenerative drives that can push power back onto the grid, which will have significant negative input current. Flag for review