Locate Service Centers | Site Map
Login | Browse and Buy | Promotions | Catalog | Line Card PDF | Supplier Diversity | Premium Brands
  Applied Industrial Technologies The website for Applied Industrial Technologies
   
   Products > ... Motors & Controls > ProTalk - Motors & Controls

 ProTalk - Motors & Controls


Q. Only single-phase power is available and my application requirements call for variable frequency. How do I properly size an inverter for a single-phase power supply with a three-phase motor?

Q. What is a vector VFD (variable frequency drive)?

Q. What is the difference between a C-face motor and a D-flange motor?

Q. Why does the VFD manual require such a large fuse?

Q. Can I run multiple motors from one variable frequency drive?

Q. My motor is making a whining noise. Is it damaging my motor?

Q. What are regenerative DC drives?

Q. What is a “Fly Catcher” feature on a VFD?

Q. How does an AC variable frequency drive work?

Q. Can I use the existing control voltage from my starter to operate the VFD?

Q. My VFD supplier asked whether my application was constant torque or variable torque. What is the difference?

Q. There is a lot of water and dust in the area and my controller keeps going out. What can I do to prevent that?

Q. How do I convert a metric motor to a NEMA rated motor?

Q. Only single-phase power is available and application requirements call for variable frequency. How do I properly size an inverter for a single-phase power supply with a three-phase motor?
A. Determine motor horsepower required for a particular application. Divide the motor’s full load amperage by 0.7. The result of that calculation is the basis of sizing the inverter. Select an inverter with continuous amperage rating that is equal to or greater than the result.
EXAMPLE: 230Vac, 3HP motor = 9.6A
9.6A/0.7 = 13.7A
230Vac, 3HP inverter = 10A continuous
230Vac, 5HP inverter = 16.5A continuous
You would select the 5HP inverter for this application.

Q. What is a vector VFD (variable frequency drive)?
A. A vector VFD will always require that the motor have an encoder. If it does not, and the VFD is a vector, the VFD is running in sensor-less mode of operation. Vector VFDs are only necessary for cases where it is desired to have very tightly controlled speed, position or holding torque as zero speed.

Q. What is the difference between a C-face motor and a D-flange motor?
A. The difference between a C-face motor and a D-flange motor has to do with the attachment of the driven equipment to the motor (via bolts). With a C-face motor, the bolting is from the driven side. In other words, the pump or other driven equipment is bolted to the motor. With the D-flange, bolting is from the motor side. In other words, the motor is bolted to the driven equipment, such as a gear box.

Q. Why does the VFD manual require such a large fuse?
A. The VFD has a large capacitor that tries to charge up when power is applied. This inrush of current requires a large fuse or circuit breaker to avoid nuisance trips. The actual amp draw for the VFD will only be slightly larger (103%) or the rated output.

Q. Can I run multiple motors from one variable frequency drive?
A. Operating motors from a single adjustable frequency drive (VFD) is possible as long as specific steps are followed to ensure proper sizing of the drive to the connected motors. The first step is to sum total connected motor horse power or FLA. Of the two, FLA (full load amps) is the better parameter to use, but sometimes not available. Once this figure is calculated, selection of the drive based on total horse power or FLA can be made. The drive should always be sized equal to or greater than this figure. Drive operation will enable all motors to accelerate / decelerate and maintain a constant relationship of speed to one another.

Some additional points should be noted:
1. The relative speed of one motor to another cannot be changed when they are connected to a common drive.
2. The drive will require separate thermal protection for each motor.
3. A motor cannot be picked up (started) while other motors are already on the drive, unless the selected drive is sufficiently oversized.
EXAMPLE : 3 motors @ 460 volts AC
5 HP @ FLA
5 HP @ FLA
10 HP @ FLA

If all motors are accelerated, decelerated and run in unison, the sum of the connected motor FLA allows use of a 20 HP drive. If it were necessary to accelerate and run the 5 HP motors and then start the 10 HP, the sum of the FLA must be recalculated. The FLA figures of 5 HP motors would be used, but the Locked Rotor Amps (LRA) for the 10 HP must be used. As this motor would not be accelerated from zero frequency and voltage to its running condition, it would look at the drive as a fixed voltage / frequency line starter and would require its full LRA rating to accelerate to drive’s output frequency. This effect on sizing is shown below:
5 HP 6.2 FLA @ 460 volts AC
5 HP 6.2 FLA @ 460 volts AC
10 HP 86.5 LRA @ 460 volts AC
98.9 AMPS
In this example, a 75 HP drive with 112 Amp continuous rating would be required.

Q. My motor is making a whining noise. Is it damaging my motor?
A. It’s called “PWM Whine” and it's caused by transistor switching at low frequencies. Inverter fed motors always make more audible noise than line fed motors. To reduce the effect, fast switching IGBT (integrated gate bipolar transistor) inverters raises the frequency of the noise into a range above human hearing. This unfortunately contributes to early insulation failure. Ironically, the noisier the motor, the better! That’s OK in a rock plant, but not OK in your air conditioning system. It’s a balance of one vs. another. The optimum switching frequency is the lowest possible setting that minimizes the noise. This should be in the 2kHz to 5kHz range. Frequencies above 8 or 10kHz should be avoided. If possible, apply reactors and low pass filters.

Q. What are Regenerative DC Drives?
A. Non-regenerative drives will control the output current of the drive in only one direction. Therefore, the motor torque and rotation will be in one direction. Power is delivered from the AC source to the DC motor. The power applied can only be controlled in one direction. This configuration is used in applications such as extruders where only one direction of rotation is required.

A regenerative drive, also known as a 4 quadrant controller, is capable of operating a DC motor in any of four torque-speed quadrants. In quadrant 1, both motor voltage and current are in the positive direction. Therefore, speed and torque are positive. In quadrant 3, both speed and torque are negative. Therefore, motor rotation is reversed. At this point the motor is still driving the load using power from the mains in one direction. In quadrants 2 and 4, the speed and torque are mutually opposing each other. In other words, the motor torque is in the opposite direction of the rotation thereby providing a controlled braking effect. In essence, the motor is behaving like a generator and delivering power into the AC supply. This whole control concept is known as “Regeneration.” A major advantage of four quadrant drives is the ease in which a motor can be reversed in direction. A non-regenerative controller can only be reversed by means of mechanical contactors in series with the motor armature or shunt field. The contactor must be interlocked so as not to cause a short circuit when switching motor rotation. This method is usually more costly in larger horsepower drives. A regenerative drive can simply and easily change direction of rotation to provide easy speed and torque control in all four quadrants.

Regenerative drives are used in machines that have large rotating masses or overhauling loads that need to have controlled braking to zero speed. Applications that require this type of controllers are payoff drives, uncoiler drives, nip roll drives, and machine tool spindle drives.

Q. What is a “Fly Catcher” feature on a VFD?
A. A fan or pump which is “wind milling” or turning without power to the motor will cause the motor to generate EMF (Electric-Magnetic Force) voltage back to the VFD. This phenomenon can occur during a momentary power outage or by starting and stopping a VFD quickly. When power is removed from the drive, the output voltage and frequency drop to zero. The “wind milling” fan or pump continues to turn the motor, which generates a voltage to the VFD. When power is restored or the VFD is restarted, the VFD will attempt to output voltage to the motor at a different frequency than that being generated by the motor. These voltages collide, sometimes causing damage to the VFD. To overcome this problem, some VFD manufacturers provide a “Flying Start” or “Fly Catcher” feature. Simply put, the circuitry of the VFD will recognize the motors frequency and match that frequency on start or restart.

Q. How does an AC variable frequency drive work?
A. An AC variable frequency drive must simultaneously control output frequency and voltage to efficiently control the speed of a three phase induction motor. Frequency controls the motor’s speed. Common 60 Hz induction motors are typically offered at no load speeds of 3600rpm (2 Pole), 1800rpm (4 pole) and 1200rpm (6 pole). Applying 60 Hz to a 4 pole motor will produce a motor speed of 1800rpm at no load. Actual speed at any applied frequency is influenced by motor load requirements. If frequency is cut in half (30 Hz), then motor speed is cut in half. Voltage is applied in proportion to frequency to achieve rated motor torque. If the motor is running half speed (30 Hz), the voltage applied is also cut in half. Failure to reduce applied voltage with reduced speed will result in excessive current draw and motor overheating.

Pulse Width Modulation (PWM) is the present state of the art method used to control frequency and voltage. An AC power source is connected to the drive rectifier, converted to DC, and then “inverted” in a logic controlled output of DC pulses of varying width (voltage) and polarity (frequency). A motor is an inductive device constructed of coils of wire embedded in iron. The motor’s inductance resists the rapid voltage changes, averaging (smoothing) the pulses and making them appear to the motor as a 3 phase sine wave.

There are three major elements in the PWM process:
Rectifier – converts AC power source to DC.
DC Bus – Pulsating DC is smoothed by large capacitors. A measurement at the output of this section indicates a DC voltage equal to the AC peak value of approximately 1.4 times the AC input.
Inverter – Receives instructions from control logic, then converts DC to variable frequency, variable voltage 3 phase output.

Q. Can I use the existing control voltage from my starter to operate the VFD?
A. No. VFD inputs are “dry contact” (no voltage). You can get option cards from the manufacturer or just simply use an ice cube relay (a stand alone panel mounted relay).

Q. My VFD supplier asked whether my application was constant torque or variable torque. What is the difference?
A. Variable torque loads require torque values that vary as the square of the operating speed. Starting torque is usually very low and increases rapidly as the speed of the driven equipment increases. Examples of this type of application are fans, centrifugal pumps and centrifugal compressors.

Constant torque loads, as the name implies, require a constant amount of torque at any speed to do the work. Examples of this type of application are conveyors, extruders, and positive displacement pumps.

Q. There is a lot of water and dust in the area and my controller keeps going out. What can I do to prevent that?
A. It sounds like you have a standard enclosure on your controller. Replace it with a NEMA12 enclosure that is completely sealed to keep out water and dust.

Q. How do I convert a metric motor to a NEMA rated motor?
A. With the influx of foreign equipment into the U.S., we are seeing more and more metric motors. The following will help you to convert the metric nomenclature to U.S. standards. KW / HP Rating Motor ratings for metric motors are given in KW (kilowatts). To convert KW into HP, multiply KW by 1.34. For example, a 2KW motor would be equal to 2.7HP. Since 2.7HP ratings do not exist, the next larger size, 3HP would be the right choice.
If you are called upon to replace a metric motor, and the nameplate only shows motor speed based on 50 Hz current (European Standard), the following chart can help you determine a replacement motor’s speed based on 60 Hz (U.S. Standard).

50 Hz Frequency Speed (RPM) 60 Hz Frequency Speed (RPM)
# Poles Synchronous Full Load Typ. Synchronous Full Load Typ.
2 3000 2850 3600 3450
4 1500 1425 1800 1725
6 1000 950 1200 1150
8 750 700 900 850




Locations | Terms of Access | Terms of Sale | Privacy Policy | Supplier Terms | Site Map | Home | Return Policy
Copyright © 1999-2014 Applied Industrial Technologies. All Rights Reserved.
Get Acrobat® Reader®