VFD Influence On Induction Motors

An induction motor feels most comfortable when it is supplied from a pure sine voltage source which mostly is the case with a strong commercial supply grid. In a perfect motor there are no harmonics in the flux and the losses are kept low. When a motor is connected to a VFD it will be supplied with a non-sinusoidal voltage, this signal is more like a chopped square voltage. A square shaped signal contains all orders of harmonics.

As these harmonics will induce additional heat losses that may require the induction motor to be de-rated, a margin between maximum output power and nominal-rated output power is required. The required power margin depends upon the application and the supplied equipment. When in doubt contact the local Flygt engineering office for details.

The performance of the VFDs has improved over the years and is still improving, and the out put signal is looking more and more like an ideal sine wave. This implies that a modern VFD with high switching frequency can run with a low or no power margin whatsoever, while an old one might need a margin of 15%. Unfortunately the extensive work needed to develop VFDs' ability to reduce losses in the motor and in the VFD, tends to emphasize other problem areas. VFDs with high switching frequency tend to be more aggressive on the stator insulation. A high switching frequency implies short rise time for the pulses which leads to steep voltage transients in the windings. These transients stress the insulation material. Flygt recommends reinforced stator insulation for voltages 500 V and above.


Here Recommend You Delta VFD


The Delta VFD007B21A VFD-B series is a general purpose NEMA 1 drive and offers V/F, Sensorless Vector and Closed Loop Vector control. With its Constant Torque rating and 0-2000Hz output, the VFD-B is designed to handle most conventional drive applications found in the industrial manufacturing industry. The VFD-B series drives are used in many applications including: HVAC, Compressor, Crane Gantry, Elevator, Escalator, Material Handling, Water/Wastewater, and Woodworking to name a few.


Specifications:

Item Number: VFD007B21A
Manufacturer: Delta Products
Item Category: Drives
Subcategory: AC
Series: VFD-B
Nominal Input VAC: 208;240 Volts AC
Input Range VAC: 200 to 240 Volts AC
HP (CT): 1 Horsepower
Amps (CT): 5 Amps
Input Phase: 3
Operator Controls: Keypad Included
Max. Frequency: 400 Hertz
Braking Type: DC Injection;Dynamic Braking
Motor Control-Max Level: Open Loop Vector (Sensorless Vector)

Sizing Criteria
The data needed to determine the correct size of a
VFD are:
• Motor kVA rating.
• Nominal voltage
• Rated current
• Ratio max. torque/nom. torque

If the ratio between peak torque and nominal torque, Tp/Tn, is greater than 2.9 it might be necessary to choose a larger VFD. There are basically two reasons why a motor can have a ratio greater than 2.9:

1. The motor has a high magnetisation level
2. The motor has been de-rated.

Running Above Nominal Frequency

Sometimes there is a desire to run the pump at frequencies above the nominal commercial supply frequency in order to reach a duty point which would otherwise be impossible. Doing so calls for extra awareness. The shaft power of a pump will increase with the cube of speed according to the affinity laws. Ten percent over-speed will require 33 % more output power. Roughly speaking the temperature will increase by approx. 80%.

There is however, a limit to what we can squeeze out of the motor at over-speed. Maximum torque of the motor will drop as a function 1/F when running above nominal frequency. This is due to the fact that the VFD output voltage has reached its full value at nominal frequency and cannot be further increased. The area above nominal frequency is denoted as the field weakening range. The motor will be overloaded and drop out if the VFD can't support it with a voltage that corresponds to that needed by the torque. In reality the VFDs' over-current protection will trip after a short while if we try to run the pump too far into the field-weakening range. Running above nominal frequency is not recommended, but if required, use the following guidelines:

• Check rated power. Shaft power will increase to the power of three according to affinity laws.
• Check that the VFD is dimensioned for the load increase. Current is higher than nominal rated current (for nominal frequency) in this case.
• Change "Base frequency" of the VFD. Base frequency is the frequency where the VFD output voltage is the same as supplied nominal line voltage.

If possible, select a machine designed for a higher frequency. When running a pump designed for 50 Hz operation above nominal speed, select a 60 Hz motor.

NPSH-required increases, according to the affinity laws, when running above nominal frequency. Always check that NPSH-available is greater than NPSH-required in order to avoid cavitation.