An AC induction motor is constructed with a rotor that has windings which intersect the rotating magnetic field generated by the stator windings.
At full load speed, the rotor turns slightly slower than the synchronous speed of the motor. This is because the magnetic field causes currents to flow in the rotor windings and produces a torque which turns the rotor; so if the rotor turns at the same speed as the magnetic field, there would be no relative motion between the rotor and the magnetic field, and no torque would be produced.
The amount of speed by which the rotor lags the rotating magnetic field is known as the slip of the motor. The higher the slip, the more torque is produced by the motor.
The speed at which the magnetic field rotates depends on the number of poles or coils distributed around the stator and the frequency of the supply current. This is called the synchronous speed.
Synchronous Speed = 120 x Frequency
Number of poles
Typical AC induction motor speeds are 3600, 1800, 1200, and 900 RPM.
Successful application and maintenance of VFD drives requires an understanding of their impact on the motor and electrical distribution system. The application of VFDs to induction motors can cause effects which must be considered for successful operation. Examples include:
• The ability of a motor to cool itself effectively is reduced as the motor is slowed down. Over-sizing the motor or providing external forced air ventilation may be required with extended operation at low speeds and high loads.
• Operation at different speeds can cause mechanical resonances in driven equipment. These speeds should be identified and programmed out of the motor’s operating range.
• VFDs generate harmonic voltages and currents which can, in some cases, cause undesirable effects on the electrical distribution system and affect equipment operation. If a power quality problem is suspected, the electrical system should be examined by a qualified person. Sometimes isolation transformers, line reactors or filtering devices will be required to minimize these effects.
Installation of filtering devices should be considered at the time of purchase of VFDs to minimize power quality issues in the electrical system. A practitioner trained in this area should be used to evaluate and determine this requirement. General Tech Services has experienced technicians for designing and determining the apt VFD drive base for your systems. We are capable of timely installation and panel designing and supply as well. General Tech Services is the most reliable VFD supplier and service providers in UAE.
Electrical Supply to Drives AC drives require an acceptable electrical supply for safe, successful and reliable operation. Single phase drives have standardized voltages of 120 and 240 volts. Three phase motors have standardized voltages of 200, 230, 460 and 575 volts. The nominal supply voltage of the distribution system is normally higher than the drive nameplate voltage to allow for voltage drops from the distribution transformer to the point of utilization.
How to choose a VFD?
It may be tempting to size a variable frequency drive (VFD) based on horsepower alone. Did you know there are six other factors you should take into consideration to ensure that you specify the correct AC drive for your application? Read below to learn more about the six factors you should be considering when choosing a VFD.
Full Load Amperage
The first step in this process is making sure the drive can handle the motors current demands. Check the motor nameplate for the Full Load Current requirement, then find a drive that’s rated for at least that much current. If you are feeding the drive with single-phase power, be sure to use the drive ratings for single-phase. Variable frequency drives are significantly derated for single-phase operation.
Be sure the drive can handle any overload conditions you may expect during startup or intermittent extra loading. You may need to upsize the drive until you find one that can handle it. Many applications experience temporary overload conditions due to starting requirements or impact loading. Most AC drives are designed to operate at 150% overload for 60 seconds. If the application requires an overload greater than 150% or longer than 60 seconds, the AC drive must be oversized.
There are two application types: variable torque (VT) and constant torque (CT) and separate ratings for each. Use VT ratings for fans and pumps or consult the CT ratings for conveyors and general machine control. It is important to know the application type because the drive specifications are organized accordingly. If you aren’t sure which one to use it’s recommended to go with CT.
The altitude at which you’re using your VFD also has an effect on cooling. As the altitude increases, the air becomes less dense. This decrease in air density decreases the cooling properties of the air. Most VFDs are designed to operate at 100% capacity at altitudes of up to 1000m. If you’re at a higher altitude, the drive must be oversized to compensate for the decrease in cooling.
AC drives generate a significant amount of heat and can cause the internal temperature of an enclosure to exceed the temperature rating of the drive. Enclosure ventilation and/or cooling may be required. Make measurements/calculations for the maximum expected ambient temperature.
Generally, you want to look for the lowest carrier frequency your motor can handle. Most of the time the default carrier frequency will work fine, but if you need to reduce the audible noise, the heat dissipation or the power consumption, then make sure you are able to modify the carrier frequency for the drive.
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