Gordon Fenton is the engineering manager at SEEPEX Inc., specializing in the optimization of electrical and mechanical processes to increase manufacturing throughput. Fenton has more than 20 years of experience with PC and various other pumps.
There are numerous factors to consider to ensure the best performance with this technology.
Table 1. An example of the potential effects of different gearbox ratios for the same pump
Consider the example in Table 1. Both Case A and Case B will work for the hypothetical application; however, Case B will have no issues starting using a motor two-thirds the size of Case A (see Figure 2). Case A will require more motor horsepower and starting torque to start the pump. The initial capital cost savings and power saving over ownership can be large.
Figure 2. Examining starting torque capacity
VFD Parameters of Interest
Most VFDs have multiple parameters that can assist in situations where the starting torque is marginal. Some motors respond better to these changes than others; however, they should be checked as soon as there is an issue with starting the pump. Below is a list of these parameters, along with descriptions and advice:
Acceleration/deceleration time: This is the time period the VFD will take to increase or decrease speed from current speed to a new speed. It is a filter that will help limit the maximum amount of current generated during starting. This value is usually listed in seconds. Appropriate values range from 1 to 15 seconds. Shorter time allows more current to be produced, plus more torque and heat. Values from 3 to 7 seconds usually meet most applications without creating unnecessary mechanical stress.
Starting overload capability: In some drives, you can adjust this parameter to purposely limit the maximum current the drive will use to start the motor. In most cases, this should be left at factory settings, which are usually drive maximums.
Carrier frequency: Carrier frequency controls how often the voltage is modulated in the waveform. The most noticeable effect of raising the value is lessening motor/VFD whine as it passes outside of the range of human capabilities.
Increasing carrier frequencies increases motor thermal stress and sometimes reduces the maximum current capability. The rule of thumb is to leave this at the lowest value and only raise it if you have the extra capacity and auditory restrictions that you cannot meet otherwise.
Motor overload: This parameter is usually in the form of a percentage. Taking the full load amperage from the motor’s data-plate and multiplying by 100 and dividing the result by the VFD’s maximum continuous current rating gives the correct percentage. In most cases, the possible values range from 30 to 100 percent.
Motor overload type: Motor overload type usually has two options. One will be speed compensated and the other will be non-speed compensated. Speed compensation should be used whenever the motor fan speed is proportional to motor speed (non-blower-cooled). Non-speed compensated should be used in instances where the motor is cooled by a fan that does not change speed according to motor speed (blower-cooled).
Fixed boost: Fixed-boost parameter is rarely needed and almost always overused. This parameter sets the drive power’s minimum voltage. As we stated before, the voltage in a constant torque application should be in a linear relationship with speed. This parameter will override it at low speeds and not let the voltage fall below the preset value percentage. For example, if the parameter was set to 40 percent, a 460 VAC drive could not lower the voltage below 0.40 times 460 VAC or 184 VAC. The problem with the increased voltage is it will only temporarily help in starting, but always contribute to the motor overheating. Always keep this parameter at 0 percent. If the need arises to change it, check, check and recheck other parameters. Otherwise, this may be creating another problem.
Drive mode: The available parameters for this selection will vary according to the drive manufacturer and drive capability. In all cases, it should only be set to one of two selections. The first selection would be “constant volts/Hz,” which would be for drives that do not incorporate vector control capability. The other option will be “vector speed CT” control for drives that incorporate vector control capability.
Motor rated voltage: This sounds trivial, but this cannot be stressed too much in the world of dual-voltage motors. The drive voltage must be the same as the motor voltage, and the motor must be appropriately wired for the same voltage. Additionally, this parameter should be set for that same voltage.
Motor rated current: This parameter must be set to match the motor nameplate full load current in amperes.