General Drives Questions Frequently Asked Questions
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Question:
What is sensorless vector control?
Answer
Sensorless vector control is a technique used in variable frequency drives to rotate the force vector in the motor without the use of a shaft position sensor. The goal of ac sensorless vector technology is to give the user "dc" like control while making traditional speed or shaft position feedback from the motor unnecessary. The SV control removes a major source of complexity and potential for failure (the encoder feedback), while simplifying many ac drive installations.
The sensorless vector algorithm must be tuned to match the characteristics of the particular motor being controlled by the drive, and improves torque performance at very low speeds as compared to typical Volts per Hertz control.
FAQ ID: 271
Question:
What is the duty cycle?
Answer
The duty cycle is the percent of time a device is on vs. off. It can be the ratio of operating time of a motor to its resting time. This parameter usually is specified in association with the allowable thermal rise for the device.
FAQ ID: 275
Question:
What is the main advantage in using an inverter to drive a motor, compared to alternative solutions?
Answer
An inverter can vary the motor speed with very little loss of efficiency, unlike mechanical or hydraulic speed control solutions. The resulting energy savings usually pays for the inverter in a relatively short time.
FAQ ID: 283
Question:
The term "inverter" is a little confusing, since we also use "drive" and "amplifier" to describe the electronic unit that controls a motor. What does "inverter" mean?
Answer
The terms inverter, drive, and amplifier are used somewhat interchangeably in industry. But there are subtle differences. A drive can refer to the motor, the control electronics, or both. This term is used particularly when the motor and electronics are integrated in the same housing. The term variable speed drive can include many types of devices - anything that has a variable speed output, which includes the GS series of inverters. Amplifier more commonly refers to a linear amplifier for servo motor control, or a stepper motor driver IC. Finally, we use inverter to describe the GS Drive motor controllers because of the way the switching electronics alternately inverts or directly couples its internal DC voltage bus to generate a variable AC output.
The terms AC drive, AFC (adjustable frequency controller), AFD (adjustable frequency drive), ASD (adjustable speed drive), and VFD (variable frequency drive) are also used to describe speed controllers for three phase AC induction motors.
FAQ ID: 284
Question:
Does the motor need a chassis ground connection?
Answer
Yes, for several reasons. This is for protection in the event of a short in the motor that puts a live voltage on its housing. Motors and other components exhibit leakage currents that increase with aging. And, a grounded chassis generally emits less electrical noise than an ungrounded one.
FAQ ID: 294
Question:
What type of motor is compatible with inverters?
Answer
Any 3-phase AC motor will work with invertors, but an Inverter Duty Motors is designed to handle much lower speeds without overheating, and can withstand the voltage spikes typically associated with inverter usage.
In practice, it's better to find the right size motor for your application; then look for the inverter to match the motor.
FAQ ID: 295
Question:
How many poles should the motor have, and how does this relate to the motor's speed?
Answer
Using a four-pole motor will work for most applications. Hitachi inverters can be configured to operate motors with 2, 4, 6, or 8 poles. The greater the number of poles, the slower the base motor speed will be, but it will have higher torque at the slower speed.
The number of poles that a motor has determines it's speed at a particular frequency. The synchronous speeds for three phase 60 Hz motors are as follows: 2 pole = 3600 rpm; 4 pole = 1800 rpm; 6 pole = 1200 rpm; 8 pole = 900 rpm.
FAQ ID: 296
Question:
How will I know if my application will require resistive (dynamic) braking?
Answer
For new applications, it may be difficult to tell before you actually test a motor/drive solution. In general, some applications can rely on system losses such as friction to serve as the decelerating force, or otherwise can tolerate a long decel time. These applications will not need dynamic braking. However, applications with a combination of a high-inertia load and a required short decel time will need dynamic braking. This is a physics question that may be answered either empirically or through extensive calculations.
FAQ ID: 298
Question:
What is ringing and what happens to the motor when I place it more than 50 ft away from the drive with no line reactor?
Answer
High voltage ringing (reflective voltage) occurs on all VFD's. With the advent of IGBT's replacing SCR's on smaller more in-expensive drives, it has become more pronounced. It is capacitive coupling effect caused by high speed switching (commutation.) Distance greatly enhances the effect.
Cables on the output side of the drive act like capacitors. When the IGBT's switch or produce the PWM output, the higher frequency part of the current will find a path through that capacitance. Think of the cable being a capacitor that increases in size as the cable length increases.
Ringing is very detrimental to motors; it weakens the first stage rotor windings and is detrimental to the life expectancy of a motor. This is especially true for motors under 10 hp, they are machine wound typically, thinner coats of varnish, and have neither phase nor end paper (I.E. aggregate insulation properties.) That's why we always recommend a drive rated motor. Quality is everything concerning motors. Vintage and non-drive rated motors that are used with VFD's have a lifespan of unknown proportions. You simply cannot know what to expect from them.
Long cable runs with its ringing can also add 10 to 15% of the drive current rating. Therefore, the drive may start to trip out on excessive current. The capacitance will also cause a voltage drop that may cause speed performance problems on the lower end of the speed scale (increased current demand for demanded torque.)
A few general guidelines:
(1) Try to reduce distances as much as possible
(2) Distances of greater than 50 ft (cabled distance not physical distance,) use an output line reactor
(3) Always use a drive rated motor.
FAQ ID: 747
Question:
Is there a standard for Inverter rated motors?
Answer
NEMA MG-1, Section IV, Parts 30 and 31. Part 30 pertains to fixed supply motors; part 31 pertains to inverter supply motors. Always talk with your motor manufacturer for additional details.
Note: The international standard is IEC 60034-17
FAQ ID: 748
Question:
What drive power cable do you recommend and why?
Answer
Recommend using VFD Cable
VFD cable is used to organize and protect the input and motor power wiring for variable frequency drives. It is specially designed to reduce VFD-related interference (EMI) on nearby equipment. These foil and braid shielded multi-conductor power cables have a black thermoplastic elastomer (TPE) outer jacket, and offer 4 conductors in up to 2AWG wire size.
FAQ ID: 771
Question:
Is there a standard for insulation testing of motors?
Answer
The standard is outlined in ANSI/IEEE STD 43 “IEEE Recommended Practice for Testing Insulation Resistance.” The standard (using a Megohmmeter) prescribes the following: a minimum acceptable reading is 1 megohm plus 1 megohm/1000 volts of the motor's rated voltage. Minimum acceptable resistance for a 460-V motor, for example, is 1.46 megohms. Prudence, however, dictates that the motor be removed from service for winding refurbishment while winding-to-ground resistance is still well above the minimum acceptable value.
FAQ ID: 772
Question:
Can a drive replace a softstart?
Answer
Yes, softstarts are used for reduced torque starting and stopping of standard 3 phase induction motors (E.G. centrifugal pumps, compressors, ball mills, jar mills, fans/blowers and saws.) If the motor is not adequately protected from sudden changes in rotational torque associated with starting and stopping, the current will be excessive to the system and the motor will surge, causing detrimental effects to all the equipment linked to it. Over the long-term this leads to increased Over-current tripping and increased mechanical wear of gearboxes, clutches, transmission, and conveyor systems.
A drive has all of the features inherent to it as a drive and can be programmed to handle reduced toque loads. The amazing thing is that they are about the same cost.
ANSI/IEEE Std. 141-1993 (Red Book) provides a comparison of different reduced voltage starting methods. An electronic softstart or VFD, although not listed, would have similar characteristics to the autotransformer starter.
Note: It is preferred that the motor is of the inverter duty type or have an inverter duty insulation rating (F or higher); The applied voltage at the motor's base speed will still be of the PWM signature, not a true sinusoidal waveform. Heating issue may occur if the motor's insulation rating is not suitable.
FAQ ID: 773
Question:
Can my mechanical motor coupling affect the operation of the drive?
Answer
Yes, but more as a problem to the entire system including the drive. Here is a list of "Do's" and "Don'ts"
DOs
- Determine power requirements using both hp and torque to choose the proper diameter core for the application.
- Use the highest possible rpm to transmit power, thus lowering the torque load on the shaft.
- Operate the shaft in the largest possible radius, avoiding sharp bends or kinks.
- Install the shaft properly, letting the casing be straight for several inches beyond the ferrule, clamping it into position whenever possible and avoiding excessive lengths.
- Always use a means of shaft alignment; mis-alignment causes heat and mechanical breakdown of the coupling.
DON'Ts
- Bend a core into a radius smaller than recommended, even during shipment (if you are an OEM.)
- Overlook direction of rotation in choosing a core.
- Use the flexible shaft to support anything - it was not designed to be a handhold or a footrest or a support!
- Ignore routine maintenance.
FAQ ID: 774
Question:
Can I run my application at extremely low speeds?
Answer
Three major problems exist with both conventional methods of motor control and VFD control.
- When a motor is directly coupled and run at low speed it becomes very inefficient. One could go with a high torque motor that is significantly larger than the standard (including higher cost), and would have the same results most of the time.
- Typical motors do not cool themselves well at extremely low speed. The added heat buildup in the windings can cause premature motor failure. Check with the motor manufacturer for more details.
- The other issue is load inertia, which is larger than that of the motor. These effects increase greatly with a transmission system that is sloppy. A speed reducer or gearbox should be employed to solve this problem. It will match the inertial changes and increase the efficiency. ADC does offer various Mechanical Power Transmission options.
FAQ ID: 775
Question:
What is the difference between dual voltage and dual wound motor?
Answer
Dual voltage is a reference to a motor that can use either of two different voltage levels, usually 230V or 460V. The motor windings have multiple taps, and the connections vary depending on which voltage is used.
Dual wound motor refers to two windings in one motor frame, and can be any combination of poles. With the increased amount of copper, the frames are usually twice the size of a standard motor. A dual wound motor can be used in a drive application with one of the windings not hooked up, but we do not recommend this. It is similar to a round rotor motor with the secondary shorted. The insulation properties are not sufficient to handle PWM.
FAQ ID: 816
Question:
What is the equation for converting Horse Power (HP) to kilowatts (kW)?
Answer
1 HP = 0.746 kW and 1 kW = 1.34 HP.
FAQ ID: 818
Question:
Are the variable frequency drives phase sensitive?
Answer
The input wiring is not sensitive to phase. If one would change one set of input leads, the rotation would not change. The output of the drive is sensitive to phase. Changing one set of leads to the motor changes the direction of rotation.
FAQ ID: 831
Question:
Can variable frequency drives be mounted above one another?
Answer
It is not recommended to do this unless the space is large enough to maintain minimum spacing requirements and have the correct amount of environmental controls
FAQ ID: 832
Question:
What is the difference between 208, 220, 230, and 240 VAC systems?
Answer
In North America, the terms 220V, 230V, and 240V all refer to the same system voltage level. However, 208V refers to a different system voltage level.
In North America, the utility companies are required to deliver split phase 240VAC for residential use. That is two 120VAC (+/- 5 %) legs. With electrical loads, the voltage will drop, hence the common reference to voltages below 120 and 240, such as 110, 115, 220, and 230.
- Very Important: When the drive is hooked up for single phase operation (it is a slight misnomer) L1 has a 120VAC hot and L3 has a 120VAC hot (of opposite polarity), for a total of 240VAC. The manual states that a neutral is used. However, the neutral is only used with European single phase 220 on a single leg.
The 208VAC system is not the same as 240VAC. It uses a "Y" style transformer with a neutral, and will yield 208VAC between two phases and 120VAC from hot to neutral.
We do not recommend using a corner grounded or midpoint tap grounded "delta" secondary style transformer. The phases are not all balanced; one phase is always tied to ground. The voltage mismatch can negatively influence the drive's performance.
FAQ ID: 834
Question:
Can I use wire-nuts on motor leads?
Answer
As long as the FLA of the motor is under the spec of the wire-nut, yes. Understand that wire-nuts are susceptible to vibration. And most institutions frown on using wire-nuts on anything other than residential projects. Always check with your local NEC code regulations.
FAQ ID: 835
Question:
What is a line reactor and what will it do for my application?
Answer
Line Reactors are a special type of inductor used on the line side or the motor side of a drive application depending on the specific circumstances.
They are used on the line side of the drive to smooth inrush current, reduce harmonic noise, and to act as a buffer and protect the drive system. The line reactors have a 3% voltage drop based on impedance. This drop can be beneficial for drives on systems exceeding the rated input voltage of the drive. In this regard, the line reactor has been used to replace the much larger and expensive drive isolation transformer.
They are used on the motor side of the drive to protect the motor by smoothing the drive output waveforms, and by reducing ringing and capacitive coupling, especially with long cable runs between the drive and the motor.
FAQ ID: 844
Question:
Can a line reactor convert voltage from 460 to 230?
Answer
No, a line reactor is not a voltage-converting device. The line reactor will output almost the same range as the input voltage. If this is your need, then a step down transformer is required. ADC Sells step down transformers.
FAQ ID: 846
Question:
What is grounding and what reference material can I use?
Answer
An equipment or conductor-enclosure ground refers to connecting the non-current-carrying metal parts of the wiring system or equipment to ground. This is done so that the metal parts with which a person might come to contact is at or near ground potential.
The grounding of motors is referenced in NEC article 430 part M and methods are described in article NEC 250.
FAQ ID: 854
Question:
My TEFC (totally enclosed fan cooled) motor had an insulation failure? I changed direction and ran it at lower speeds on some occasions.
Answer
The two basic concerns are direction and speed.
There are 3 main types of cooling fans used with TEFC motors.
- Radial type (some two and four pole motors; most six pole and larger motors)
- Sirocco type (most four pole motors)
- Propeller type (most two pole and some smaller motors)
Radial type fans are bi-directional and do not have a problem changing speeds. The sirocco and propeller types are uni-directional which means one direction only.
The general rule of thumb is half the rated rpm or 30Hz on a 60Hz drive. The best bet is to speak to the motor manufacturer.
If a motor cannot get adequate cooling, it will fail.
FAQ ID: 855
Question:
Can rewound motors maintain the same efficiency?
Answer
There are no studies that support either position of yes or no. The largest variable being the quality of workmanship of the rewinder. Many rewinders offer services beyond the basic repair/rewind. They can increase the insulation class, epoxy treatments, better bearings, advanced testing, and etc.
The biggest factor of repair versus replacement is cost. Other factors can be lead-time, application, and specialty shaft/frame.
FAQ ID: 869
Question:
Do I have to use thermistor protection with my motor/drive? Why does the thermistor resistance change from sensor to sensor?
Answer
Here in the US, there are no stipulations requiring the use of thermistors (or other temperature sensing devices). People use them as a common sense method of protecting an investment. NEC article 430-32 lists it as one of 4 ways to be compliant. Other mfg's offer it in many forms; thermistor, pyrometer, thermocouple, and RTD. All of these can be used as a temperature measuring/sensing device.
The reason that the resistance differs from sensor to sensor is because of the different type of material or different dimensions of the same material being used.
FAQ ID: 870
Question:
What are the common reasons why motors fail?
Answer
The EPRI Power Industry Study by G.E. in 1985 gave the following distribution of 6,000 utility motor failures:
- Bearing related 41%
- Stator related 37%
- Rotor related 10%
- Other 12%
Other institutions like the DOD and the IEEE came up with similar results.
FAQ ID: 871
Question:
I keep losing my bearings on my motor, can the drive cause bearing failure?
Answer
70% of all bearing failures happen because of two reasons. The primary reason is lack of lubrication. The secondary reason is improper installation.
The easiest way to detect a problem bearing is by sound and touch. A potentially bad bearing will be louder and more prone to yield an increased amount of vibration. The most accurate way to detect this phenomena is with vibration analysis.
If a motor has a bad bearing, and it is not known how long the symptoms have existed, make sure to check the entire transmission system. Look at any gearboxes/reducers, clutches, brakes and couplings. These all could have been affected by the bad element.
The motor, if unchecked, will start to affect the drive by changing the load characteristics. The drive will eventually start to present current related trips. An overload condition will manifest itself. The way to verify the load difference is to de-couple the motor and run it unloaded. If the drive does not trip out, check the entire transmission system.
FAQ ID: 877
Question:
My pump is losing speed; does the drive have a problem?
Answer
Pumps are divided into three basic categories
- centrifugal
- rotary
- reciprocating.
- the rotating element
- the casing
- the motor
- the drive
Here is a basic remediation list:
- Belts are slipping (if applicable);
- Impeller worn
- Impeller loose on shaft or shaft sheared
- Casing worn
- Original motor was replaced with a slower model or with greater amount of motor slip
- Requirements have changed beyond the design specifications of the system.
Verify that you have a speed problem with a manual tachometer. If all of the above have been investigated, then a fix could be to increase the maximum frequency of the drive. Understand that anything beyond 60 Hz will result in a shift from a constant torque to a variable torque situation. There is a low probability that the drive will affect the intrinsic speed of the system detrimentally.
FAQ ID: 878
Question:
Can you give some references for lightning protection for electrical equipment?
Answer
1.) NFPA- 780 Standard for the installation of Lightning Protection Devices.
2.) IEEE Std 142-1991
3.) IEEE Std 1100-1999
4.) www.lightningsafety.com has very good resources on this topic.
FAQ ID: 884
Question:
Is there a reference for application help?
Answer
NEMA standards publication "Application Guide For AC Adjustable Speed Drive Systems."
FAQ ID: 886
Question:
When I use a relay card for drive control, it seems that my relays stick or do not operate correctly. How can I correct this problem?
Answer
Review the minimum load requirements of the PLC card or relays in question. The intelligent terminals of VFDs do not represent a significant load. If the drive's input terminal load does not meet or exceed the minimum requirements of the controller, then additional load will need to be added. A large resistor in parallel is usually the fix to this occurrence.
The smaller the relay, the more it is likely to have a minimum load requirement because of contact material and surface area contact. Each relay has a wiping current it must exceed to work properly. The requirements of gold contacts versus silver-tin-oxide contacts are enough to cause a problems in some cases.
FAQ ID: 890
Question:
What is the difference between torque control and torque limiting? Can any of your drives do this?
Answer
Torque control can be done with any of our PID capable drives. Torque control would be a closed loop system where the process variable would be torque. You would have both a torque reference and torque feedback signal. The user would establish a torque signal and allow the drive to control to that set point. Many customers have used torque sensors, load cells, and current transducers to establish an analog input to the drive. Ideally a vector drive would work better than a volts/hertz drive. A vector drive would allow tighter speed regulation and better control in the lower speed range.
Torque limiting can be done with our Hitachi SJ300 series drive. Instead of using an external torque sensor, torque limiting uses the drive's internal current sensor. The operator would set the torque limit, and when that level is exceeded, the drive would act like a governor on a generator. The speed would be restricted until the appropriate level is again maintained. Mechanical devices, in addition to drives, are also employed for torque limiting. Items such as clutches, shear pins, gearboxes, and etc.
FAQ ID: 943
Question:
Can any of your drives be used as an internal torque regulator?
Answer
Currently only the GS20 Series Drives can regulate torque with a torque command input.
FAQ ID: 1043
Question:
How does a variable speed pump drive system save energy, compared to a system that runs across-the-line?
Answer
A pump drive system ran across-the-line will operate the motor at its base speed as the speed is determined by the fixed 60 Hz frequency. The flow will be controlled by mechanical restriction (valves, vanes, etc.); yet the motor will continue to consume the maximum required power. A variable speed pump drive system gives you the ability to adjust the flow by varying the speed of the motor. The torque load decreases as a function of the square of the speed (T = n^2) and likewise the power requirement decreases as a function of the cube of the speed (HP = n^3). This is known as affinity laws and it defines the relationship of power consumption to flow or speed. It only applies to variable torque load such as centrifugal loads: pumps (no static head), fans, etc. By lowering the speed, you will also be lowering the pump's flow as well as the motor's power requirement; thus, reducing the energy demand.
FAQ ID: 1044
Question:
Are there any special considerations for motor grounding when using VFDs?
Answer
Yes. The motor should have an appropriately size grounding conductor that is terminated at the dedicated VFD terminal. The motor ground conductor of a VFD has an induced current level that must be filtered through its source of origin. Thus a grounding connection needs to be made from the motor to the provided ground terminal on the VFD. An additional grounding terminal on the VFD connects to the common terminal board (comm. TB), which ultimately goes to the building grid ground. The above grounding procedures are necessary in order to reduce interference to the control circuitry as a result of the ground current induced from the drive.
FAQ ID: 1045
Question:
Can I use these on electric vehicles?
Answer
AutomationDirect is a direct seller of automation and industrial control products such as PLCs, sensors, pushbuttons, AC drives/motors, enclosures, operator interfaces, and much more.
AutomationDirect is aware that many engineers and hobbyists are currently designing or converting vehicles from internal combustion engines to electric engines. Our current drive and motor products are intended to be used in industrial applications. As a result, AutomationDirect does not currently support or encourage the use of our products in electric vehicles. We apologize for this, and may revise this policy at a later date if more suitable equipment becomes available.
FAQ ID: 1074
Question:
I have installed an ADC software product on an XP Virtual Machine running on a Windows 7 machine and cannot see any devices on a Ethernet network, what do I need to do to configure networking for a virtual machine?
Answer
Please go to the following link:
http://technet.microsoft.com/en-us/library/ee449441(WS.10).aspx
FAQ ID: 1323
Question:
How can I monitor the status of the digital IO on a GS20 drive?
Answer
By using Modbus and reading the correct registers, you will see what the Input and Output status of the drive's digital IO is.
Refer to the manual Page 4-99 for more details on parameter P02.50
Refer to the manual Page 4-100 for more details on parameter P02.51
FAQ ID: 1642
Question:
Why doesn't my ACN drive run the motor when I hit the 'RUN' key?
Answer
The drive's default settings is to obeying the digital inputs for run, not the keypad.
To change this:
- Press the up arrow until you the letters "Drv"
- Hit "Enter"
- Change the default value of 1 to a 0
- Hit "Enter" to accept
- Hit "Enter" again to confirm.
FAQ ID: 1647