Iacdrive|Automation Solution

Do I need select high rating AC drive for motor?

Actually, it’s no need to use high rating of AC drive than the rating of Induction Motor. AC drives of same rating can protect the motor even better as anybody cannot select high set point for current than the motor rated current. If you will select higher rated AC drive than that of Motor and anybody select current and over load range higher, than the result shall be damage of motor in case of overload. Further you will pay extra money and keep an inherited danger in your circuit. The best design is inherently safe design rather than it shall be made safe by some adjustments.

Always better use a VFD drive of the same motor power. Maybe some derating is needed for temperature above 45°C and for applications installed higher 1.000 m.a.s.l. Usually the derating is 1% every 100mt.

Why 4-20mA is preferred over 0-10V signal?

My experience has been that most process automation sensors use 4-20mA to stave off voltage drop and interference in long wiring runs (where your wiring runs are in the hundreds or thousands of feet through a process plant). It also gives you the ability to determine if a sensor is dead.

In industrial automation (primarily the automated equipment / motion control world), the 0-10VDC (or 0-5 or 0-12) seems to be more of a standard. Most servo motor drives take a +/-10VDC signal as a torque/velocity demand, and most position/distance sensors provide an analog voltage output. In these cases, the distances are generally in feet or tens of feet, not hundreds or thousands.

Difference between DCS and PLC

The differences between DCS and PLC are: DCS (Distributed Control System) is a CONTROL SYSTEM that works using several controllers and coordinates the work of all these controllers. Each controller is handling a separate plant. This controller is referred to the PLC.

The PLC (Programmable Logic Controller) is a CONTROLLER which can be re-program back. If the PLC is only a stand-alone and not combined with other PLCs, it is called as DDC. It means PLC is a sub system of a large system called DCS.

What’s difference between PLC and DCS by its definition:
PLC is a controller or processor that can be programmed (programmable) whose function is to run (execute) logic functions. Logic means the discrete / sequence function is usually handled by the relay. From the beginning of the vendors who carry the name of the PLC is engaged in the business discrete / sequence control.

Regardless of the distributed control system (as opposed to the DDC = Direct Digital Control) is categorized as a DCS. In DDC all system controls are done in the central processor so that if it failed, the entire control plant will also failed. DDC is mostly used as a Regulatory Control. And from the beginning vendors that carry the name of DCS is to use the product as a regulatory control.

Different in functionality means that the DCS and PLC cannot be implemented on the same application.
1. DCS is not a large PLC. Because system architecture of DCS and PLC are different.
2. DCS is not PLCs that integrated into one large system. “Controller” in the PLC is more intended as a “Logic Controller”, while “Controller” in the DCS is more intended as a “Process Controller”.
3. Both DCS and PLC is a configurable and reconfigurable.

AC reactor vs. DC choke in AC drives?

The Harmonic Distortion caused by an AC drive is related to its size & load, choke size, and the supply network parameters. With no AC Reactor or DC Choke, the harmonic distortion will be greater.

Another consideration should be a properly sized source transformer that provides enough impedance. The sized source transformer used as an isolation transformer (although a bit more of an investment) should provide 3 to 5% impedance yet also provides Voltage Transient mitigation with ten to one reduction in impulse peaks, as well as noise reduction through the use of a Delta primary to Wye secondary with center tap ground. It provides additional protection for the inverter front (Converter) end while proper ground of the Source to AC drive, AC drive to Motor and Motor to Voltage Source assists in mitigating high frequency noise, especially when flat braid is used as the grounding straps. This protects your investment and assists in keeping the variable frequency drive from generating noise into the supply that can compromise your nearby instrumentation, and PLC power supplies, etc. As well you can tap up the transformer giving you a higher input voltage mitigating the voltage drop issues resulting from the higher impedance.

The DC link assists in mitigating DC Bus Ripple and increasing the input impedance enabling a slower inrush for power on and sudden demand current requirements furthering the life of your capacitors, while a sized supply transformer protects the front end of the AC drive by providing voltage noise protection and adding input impedance for smoother current and adding a capability to change taps to prevent a voltage drop, while input reactors slow inrush current furthering the life of your input components and capacitors but add no protection from Voltage impulses or noise to the drive converter components, and add voltage drop increasing stress on those components. The important thing to remember is that “Proper” systemic design protects your AC drives and system components investment.

What is Carrier Frequency in AC drives?

Carrier frequency in fact causes the audible noise. You can diminish the noise with high frequency ripple filters which will add cost to the installation. This may be necessary in places where the public or employees are in close proximity to the SMPS or AC drive. In most cases of a AC drive you may increase the carrier frequency beyond the audible level of human hearing level, however, not without drawback. The drawback is simply that the AC drive now has higher losses in the Power Stack due to higher switching frequencies and may have the need to be derated in terms of highest continuous current available.

Higher carrier frequency will reduce audible noise but at a cost in losses in the ac drive. Lower carrier means high motor noise and higher losses in the motor. Some AC drives are rated full output at only 2.5kHZ max, others at 4kHz and higher. So, if you push the variable frequency drive carrier higher than its standard output, it has to be derated (unloaded a bit). Take the carrier down, the AC drive likes it and the motor hates it. Motors like pure sinusoidal waves coming from generators – any reconstructed wave form coming off a DC bus (all AC drive outputs) has harmonics that heat up the motor and the lower the carrier, the more the harmonics and motor losses. Finally, be aware these losses are significant. Derate curves in the AC drive instruction books show 5 – 7% losses just to go from 2.5 to 4kHz or 10 to 14HP degradation on a 200HP motor.

What is AC drive?

Speed / Flow Reducers

From a design standpoint, it is most desirable to operate all motors within a plant at full rated speed. Due to product considerations, however, there are many applications requiring variable speed operation. When a process requires the quality of speed variability, variable speed control is necessary.

The most common types of variable speed controls in the process industry today are: eddy current clutches, hydraulic couplings, variable pitch sheaves, DC solid state controls, AC variable frequency drives, and control valves. The type of variable speed control finally selected depends on the initial cost, application parameters, and environmental operating characteristics.

Does motor frequent start affect its lifetime?

Starting a motor puts the highest level of stress on a direct on line motor. Very high currents cause quick heating in the copper which causes expansion. At the same time there is high torque trying to distort the motor. If a second start is attempted before the copper has time to transfer the heat into the steel then local hot spots can exceed the insulation break down temperature which starts to degrade the insulation. This erodes some of the built in safety factor. Since it is impossible to know exactly where the hottest spot is and what peak temperature is reached there is no way to know how much a set of start attempts actually damaged the motor. If that particular spot has a bubble (or void) in the insulation one set of 3 or 4 start attempts could reduce motor life by 50%. If the insulation is strong at that point then there may only be a 5% impact.

Can I control 2 motors with one VFD?

It’s depends, if only you want a Speed Control, you can connect 2 motors with a VFD; You have to consider, you are limited in current, and have to look at the max. current which is provided by the VFD. For the rest, as well as it’s said by the mates, you have to take care with the thermal protections. Don’t forget that the motor’s velocity has to be the same.

Make sure that the two motors’ total power rating does not more than the VFD power capacity. For Example if u want to connect 2 motors of 30 kW each then use the 60 kW VFD. Its good practice to use slightly more power of VFD then the total power of 2 motors.

What is the electrical engineering roles in pumping system

The good news: centrifugal pumps tend to have no break-away torque, and torque goes up with speed. This is a desirable characteristic.

There are many types of positive displacement pumps that can be the opposite, and can be “sticky”. You need to know all of this to make sure drive equipment is properly selected. With some knowledge of pump curves and hydraulics, it’s even possible to calculate (estimate) meaningful process variables from information harvested from an AC drive. You can create a model in a PLC to calculate flow rate and pressure from speed and power factor corrected power, if a mechanical engineer can furnish a pump curve and a system curve. There are a lot of things like this that are unique to pumps (and compressors) that are things that typical electrical engineers don’t know.