Variable Speed Drive FAQ

Variable Speed Drive FAQ

Q1. What’s the difference between a variable frequency drive (VFD) and a variable speed drive (VSD)?
A1. A variable frequency drive (VFD) refers to AC drives only and a variable speed drive (VSD) refers to either AC Drives or DC Drives.

VFD’s vary the speed of an AC motor by varying the frequency to the motor. VSD’s referring to DC motors vary the speed by varying the voltage to the motor.

Q2. What is the difference between a VSD and a Soft Start Starter?
A2. The difference is the speed that the motor can run, when at full speed. With a soft start starter, the motor is reduced voltage started, and then when the motor is at full speed, or a timing circuit has timed out, a “running by-pass contactor” pulls in and the motor continues to run at full base speed. With a VSD (Variable Speed Drive) or VFD (Variable Frequency Drive) (these are the same thing, just different names for the same unit), The motor will soft start, and you can vary the speed of the motor, by varying the output frequency from the VSD or VFD.

So, if you don’t need to vary the speed of the motor, once the motor is up to speed, then the correct solution is a soft start starter for that motor. If the process requires the motor speed to vary at anytime, the correct solution is a VSD / VFD.

The cost for a soft start starter is much less than a VSD or VFD.

Q3. Can a variable frequency drive replace a softstart?
A3. Yes, a variable frequency drive can replace a soft start starter, since most drives give you a current limiting ramp to start and stop your motor. BUT, if you do not need to vary the production speed of your motor, then the soft start starter is still the correct product for the application. This is because the cost of the soft start starter is less than the cost of a variable frequency drive.

Q4. Are the variable frequency drives phase sensitive?
A4. No, variable frequency drives are not phase sensitive. You can adjust the direction of rotation of the motor, through the software of the drive.

Q5. Can a variable frequency drive be used for a fan application, blower application or a pump application?
A5. This is one of the best applications for an AC drive. In most fan/pump applications, there is a need to vary the output of the fan/pump. This is very easy to achieve with an AC drive. Plus you get the benefit of reduced energy cost of running the motor!!!

Q6. How can a variable speed drive improve our production?
A6. The drive can be “tuned” to the application to achieve maximum output of the equipment. Sometimes this is slightly under the base speed of the motor, and sometimes, it is over the base speed of the motor. Since every application is different, contact us through an email or the phone to discuss your application in detail and discover how we can help improve your companies’ production output.

Q7. Can I convert my DC Drive application to an AC Drive?
A7. Yes, You can convert your DC drive system; to an AC drive system by replacing both the motor and the drive. Consideration must be given for the torque and speed range of the DC motor, when converting to AC. You will need to consider using a “Vector” AC drive for this conversion.

Q8. Will a variable frequency drive or variable speed drive save me money?
A8. Yes, the AC drive can save you money on operating cost, when the load and motor speed can be varied. By reducing the speed of the motor, you will consume less energy, which will reduce your electric bill.

Q9. Should I replace my old analog equipment with new digital equipment?
A9. DEFINITELY!!! For so many reasons. . . difficult and costly to get spare parts, Hard to find Tech. Support / serviceability, no communications, little flexibility, etc.

Q10. What is a line reactor and what will it do for my application?
A10. A line reactor is an impedance device. Its benefits are reduction of drive cross talk, and interference.
Input reactors will limit the waveform distortion and harmonic voltages. This will help to improve the power factor due to a reduction of the RMS currents in the system.
Output reactors are generally good up to 100 feet is distance (drive to motor). The benefits are the damping of peak voltage overshoots, which will reduce internal motor heating and lessen the “noise” that a motor can produce when it is run by a drive. On installations where the distance is greater than 100 feet, filters are recommended for motor protection.