The most important factor in selecting the correct variable frequency drive (VFD) for your application is to know what type of load will be applied to your electric motor. The type of load will determine the size of the VFD needed and, in turn, the cost. Without the correct current capability from the VFD, the motor won’t be able to produce the torque needed to move the load.
Your VFD application will likely be one of the two most common types of loads, constant torque or variable torque.
Upwards of 98% of loads are considered constant torque. So, chances are you’ll be quoted a constant torque capable VFD at purchase unless you indicate otherwise.
What is Constant Torque?
Constant torque loads are those that require the same amount of torque at low speeds and at high speeds, where torque is at a consistent output throughout the entire speed range. As such, torque is not a function of speed; as speed changes, horsepower (HP) increases and decreases at a rate of 1:1 to speed while torque stays the same.
So, how does the VFD achieve this? As speed increases, the VFD increases the voltage to the motor to maintain torque. These VFDs draw a higher starting current at motor start-up and at low speeds to get the motor moving while under load. The VFD is required to provide a starting torque of 1.5x or more to overcome the weight of the load and static friction. They also have a higher overload rating to account for sudden shock loads, which are quite common in constant torque applications.
Constant Torque Applications
- Traction Drive
- Fixed Displacement Compressors
- Fixed Displacement Pumps
Variable torque loads are those that require lower torque output at low speeds and much higher torque at high speeds. As your motor spins faster, the load on the motor increases and more torque is needed to keep the load moving. The torque required by a variable torque load is proportional to speed2. As speed increases, torque increases twofold until it reaches 100%. As speed decreases, torque decreases at the square root of the speed. In variable torque applications, the HP required varies at the cube of the speed.
As a motor spins faster, it draws more current from the VFD to provide the required torque, which increases quickly at high speeds. As speed decreases, less energy is needed to move the load. Variable torque drives rarely experience overload, so they have much less overload protection than a constant torque drive.
Variable Torque Applications
Choosing a variable torque drive instead of a constant torque drive for the following applications is more cost-effective in the long run. The motor requires less power to maintain speed once operational speed is reached. This allows for energy savings and less budget spent on electricity.
- Centrifugal Pumps
- Centrifugal Compressors
Over speeding Variable Torque Application
Typically, variable torque loads cannot run faster than the motor’s nominal speed of 100% as they will require exponential torque that the motor won’t be able to produce unless engineered to be oversized.
- A 10% increase in speed, will require 21% more torque
- A 25% increase in speed, will require 57% more torque
Normal Duty vs Heavy Duty
Instead of using the terms constant torque or variable torque, European VFD manufacturers call their drives normal duty or heavy duty. Normal duty refers to variable torque applications. Most normal duty drives have an overload rating of 120% for one minute, depending on the model. While heavy duty refers to constant torque applications. These drives have an overload capacity of 150%.
Understanding the type of load your motor will be under will help you better select the correct VFD for your application. This will allow the drive and electric motor to operate optimally, save costs, and last longer.
Do you need help with selecting the correct drive for your application? Our technical experts are on hand to discuss your needs and help you on your electric motor projects.