Understanding Variable Speed Drives (Part 2)

Sizing VFDs for the Load

How do you size a VFD drive for an application and feel confident it's going to work? First, you must understand the requirements of the load. It helps also if you understand the difference between horsepower and torque. As electrical people, we tend to think of loads in horsepower ratings instead of torque ratings. When was the last time you sized something based on torque? Thus, both torque and horsepower must be carefully examined.

Torque. Torque is an applied force that tends to produce rotation and is measured in lb-ft or lb-in. All loads have a torque requirement that must be met by the motor. The purpose of the motor is to develop enough torque to meet the requirements of the load.

Actually, torque can be thought of as "OOUMPH". The motor has to develop enough "OOUMPH" to get the load moving and keep it moving under all the conditions that may apply.

Horsepower. Horsepower (hp) is the time rate at which work is being done. One hp is the force required to lift 33,000 lbs 1 ft in 1 min. If you want to get the work done in less time, get yourself more horses!

Here are some basic equations that will help you understand the relationship between hp, torque, and speed.

hp = (Torque x Speed)/5250 (eq. 1)

Torque = (hp x 5250)/Speed (eq. 2)

As an example, a 1-hp motor operating at 1800 rpm will develop 2.92 lb-ft of torque.

Know your load torque requirements Every load has distinct torque requirements that vary with the load's operation; these torques must be supplied by the motor via the VFD. You should have a clear understanding of these torques.

• Break-away torque: torque required to start a load in motion (typically greater than the torque required to maintain motion).
• Accelerating torque: torque required to bring the load to operating speed within a given time.
• Running torque: torque required to keep the load moving at all speeds.
• Peak torque: occasional peak torque required by the load, such as a load being dropped on a conveyor.
• Holding torque: torque required by the motor when operating as a brake, such as down hill loads and high inertia machines.

Practical Knowhow Guidelines

The following guidelines will help ensure a correct match of VFD and motor.

1. Define the operating profile of the load to which the VFD is to be applied. Include any or all of the "torques" discussed above. Using a recording true rms ammeter to record the motor's current draw under all operating conditions will help in doing this. Obtain the highest "peak" current readings under the worst conditions. Also, see if the motor has been working in an overloaded condition by checking the motor full-load amps (FLA). An overloaded motor operating at reduced speeds may not survive the increased temperatures as a result of the reduced cooling effects of the motor at these lower speeds.
2. Determine why the load operation needs to be changed. Very often VFDs have been applied to applications where all that was required was a "soft start" reduced voltage controller. The need for the VFD should be based on the ability to change the load's speed as required. In those applications where only one speed change is required, a VFD may not be necessary or practical.
3. Size the VFD to the motor based on the maximum current requirements under peak torque demands. Do not size the VFD based on horsepower ratings. Many applications have failed because of this. Remember, the maximum demands placed on the motor by the load must also be met by the VFD.
4. Evaluate the possibility of required oversizing of the VFD. Be aware that motor performance (break-away torque, for example) is based upon the capability of the VFD used and the amount of current it can produce. Depending on the type of load and duty cycle expected, oversizing of the VFD may be required.