Iacdrive|AC Drives Solution

Advantages of using AC drive in injection machine

Motor soft starts
In the line frequency state, the motor adopt star triangle step-down delay startup, and the current is 4～7times of rating current. If there are several high-power motor startup at the same time, it will make great impact to electric network. If it adopt the frequency control, the motor can soft starts at rating current, the current smooth and have no impact, so to reduce the impact of startup current to motor and electric network and prolong the useful-life of motor.

Avoid the personal safety accident
When the injection molding machine is append an AC drive, the speed analog variable of plastic machine have no output, and the motor stop, the hydraulic system have no pressure, so avoid the misoperation of mechanism, and put an end to the personal safety accident when the operators take piece from the mold, clean and repair the mold.

Reduce the reactive power
The reactive power not only add the line loss and the heater of equipment, but also induce the reducing of the active power of electric network because of the reducing of power factor. When use the frequency control saving energy system, the power factor approach 1 because the function of filter capacity of AC drives, so as to increase the active power of electric network and save the power wasted by reactive power.

Energy saving
The injection molding machine usually use triple asynchronous AC motor which can’t vary its speed, the constant vane pump which drags and the flux of output hydraulic oil can’t be varied. The plastic machine which operation at the low speed, excess fluid is returned via overflow valves to the fluid supply ,and the energy is wasted. The AC drive can adjust the speed of motor according to the function of control system, and can vary the flux of hydraulic oil output from vane pump according to the speed of plastic machine action, and reduce the energy waste of hydraulic oil from overflow valve to oil supply, so to save more power. According to the products of injection, the AC drive saves energy can be 20～70%.

Disadvantages of using AC drive in injection machine

1. When the injection molding machine is append an AC drive, the rotate of the host motor vary with the speed of the action of plastic machine, the injection produce cycle is short, the frequent rotate of action will lead the actor in the process of deceleration, and the response time of reducing and increasing the speed of motor is long, it will lead the increasing of the time of injection cycle and the reducing of productivity. The AC drive and plastic machine computer can carry online control, through the control of computer to variable AC drive, it can realize the decoupling adjustment of the output frequency and output torque of AC drive, and reach the optimal match of the AC drive and the action of the injection process, so it can resolve the choke point of efficiency reducing which is result from the application of frequency saving energy technology in plastic machine.

2.With the decrease of the motor rotate and the fan blade rotate which is jointed with the motor rotor, so it lead the badness of motor rejection of heat. Additional, the insulation effect will decrease of the insulating layer impacted in conversion voltage of AC drives insulate. Now, the AC drive produced by motor manufacturer aim at the two point and make the improvement, the motor fan are independent with the rotor, and the rotation cannot be affected by the decrease of the motor speed ,the insulation layer adopt the excellent material, so as to resolve the matter that AC drive impact the motor lift.

3. It mainly depend on the rotor of high speed kicking the blade out, so to reach the aim of oil suction. If the rotate of rotor decrease, the blade will not have enough centrifugal force and cannot effective compress the stator surface, so the leakage of bump will increase, and the efficiency will decrease, when the rotate decrease to the critical value (e. g 400r/min), the oil suction capacity will become bad, even it cannot yield oil, when appear the question, it must increase the efficiency set signal gain. Besides, the development of crescent gear pump will replace the vane pump and become the developed mainstream of the plastic machine oil pump, accordingly to resolve the question of the badness of the oil suction of the low speed of the vane pump.

4.When LP mold close and slow speed mold open, the AC drives operate at the low speed and great torque, the current easily exceed the rating value and make the overcurrent protect, the input end of the AC drive are set the enter point of mold close and open, which is linked with the output point of the mold close and open of the plastic machine computer, through the specifically variable AC drive software let the motor keeping the motor running at the high speed when mold open and close, avoiding the emergence of the low speed and great torque.

IGBT Applications

IGBTs major application drives from their ability to switch at control demand at reasonable high frequency 1k-2k in power circuits for AC drives and few 100 KHz in UPS type of application. IGBTs are virtually dominated Ac drive field in almost all area of medium voltage application eg Transportation (Locomotives, EMUs Metros), cement, paper industry to common motion application. And has almost replaced DC motor drives due to robust design of motor less maintenance, better control, efficiency and regenerative capability of drive.

How AC drives save energy?

Energy saving using AC drive is always vary application to application. Following points you need to consider before using AC drives for energy saving.

1. How much process flow and pressure requirement through AC motors
2. Existing control methodology like control valve in pumps, Damper or guide vane for Fans & Blowers etc. and position of the valve or dampers
3. If you have process flow and pressure data and pump or fan design data, you can calculate energy saving using affinity law
4. Loading and Unloading cycle for compressor application. If the unloading time is higher for compressor application, you will get better energy saving
5. Using affinity law, you can calculate the energy saving with consideration of AC drive losses. With this you can calculate the Pay back of AC drives.

Simply put, an AC drive can save energy COMPARED to OTHER means of VARYING flow and/or pressure in centrifugal machines, like pumps and fans. If you are not VARYING the speed, you will not save energy, regardless of whether is full speed or not. There are more cost effective means of permanently CHANGING the output if it will run continuously at that output. But if VARYING the speed is a part of your process (the work that the machine does), then compared to OTHER methods of accomplishing that, a AC drive will save significant energy. This is a very commonly misunderstood and overstated part of the picture.

Supply 240V 60Hz power to 240V 50Hz motor

If you want to supply your motor by using power source which has different frequency than it is written at the nameplate of your motor, you need to take care about value of magnetic induction in ferromagnetic core of your motor, because of possibility of appearing of unallowed warming of ferromagnetic core which can contribute to damaging of ferromagnetic core of your motor. This fact is a consequence of entering magnetising curve into area of saturation in H-B characteristic which describes behavior of magnetic induction in depends of intensity of magnetic field. In the area of saturation, as long as you would like to increase voltage, magnetic induction will stay at the same value which is the equal with value of magnetic induction under nominal voltage. So, by increasing voltage you will have only unallowed warming of ferromagnetic core, so you need to take care about ratio of voltage and frequency:

U1/f1=U2/f2=const.

This means that if you have motor designed for 240V and 50Hz and you want to connect it into power supply with 60Hz, your maximum allowed voltage will be:

240/50=U2/60 => U2=60*240/50=288V which is higher than 240V

So, as you can see after calculation, you can supply your motor with 240V and 60Hz.

How much voltage drop allowed to the motor terminals during starting time

DOL is the simplest and most cost-efficient method of starting a motor but actually involve the biggest increment in the starting current, it can be many times higher than the rated currents of motor and switchgear and correspondingly places the latter under thermal loading.

If we consider that most of the supply networks may have variations based on the standard regulations up to (+) or (-) 10%, most of the equipments are made to handle these voltage variations and the operation performances not to be affected too much. In this hypothesis come the electric motors where another 10% voltage drop during startup should allow a successful start. It is well known that once the starting torque has been reached, the current drops sharply to the value of nominal current and the terminals voltage increase. But just as well should kept in mind that the torque varies with the square of the voltage being possible the mechanical torque developed at the shaft to be less than resistant having irreversible effects at startup.
A very small voltage drop (roughly 3-5%) means over sizing the power supply which is not economical since the start is successful and the other loads in the system are not affected by voltage dips.

Calculate AC drive energy savings

I’ve helped a few clients in the past determine energy savings on an AC drive based on the changing in speed to combat disturbances in the plant. It’s mostly geared towards tuning the PID loop associated with the drive (if it’s controlled by a PLC) and better tuning typically yields less speed changes. Typically, the client will know it’s a critical loop and know the energy cost before/after we optimize it, however, it can be a bit of a pain to separate your impact trying to improve the process and the impact of other systems interacting with the variable frequency drive – just something to think about.

If really want to calculate the drive’s energy saving, you should take the energy consumption before changing the system or adding anything to it. More accurate you do this work out more you will be satisfying. Most of the old system (5 years or more in running) including Motor, Best, gear consume more energy than currently available energy saving motor, gear or AC drive. But you must evaluate how much you can save by replacing all or part or adding AC drives to the system. Specially for the AC drive on a constant torque load does not give satisfactory saving.

AC Drive vs. Soft starter

A soft starter is only good if you run at full speed. If the speed is variable you can only use an AC drive and the saving is higher the lower you run the motor. By law you have to use a IE2 motor or better from June 2011 when the motor power is over 0.75 kW anyway, but regard the load the best saving is always obtained by correct sizing and choosing of the right components, so you run the close to nominal speed as possible.

Soft starters also have a limitation on the number of starts per hour. In this application, there would typically be 20-30/hr.

Regarding AC drives, three steps are needed to protect the motor:
1) High enough winding voltage withstands voltage (dielectric strength),
2) Adequate thermal capability to counter the extra (5% or so) winding heading due to the harmonics, and
3) protecting the bearings from developed stray voltage (grounding, bypassing or insulating).

Why motor trip at 20% speed running with AC drive

Install a blower with a sail switch to interlock on and moving the required quantity of air. Use the AC drive and motor for what they are designed and rated for. Remember the motor and AC drive are current devices and this is their limit.

As mentioned you could also put in a reduction drive and if necessary run the motor above base speed, but I doubt that this is necessary at 20% of base line speed.
As to the spikes generated by the AC drive the inverter duty motors have extra insulation at the input to the winding to prevent chronic discharge and the damaging effects and punch through of the insulation. After a few turns the inductive reactance reduces these spikes and they are no longer a problem.

Also when using a constant speed you should use a motor with an insulated ODE bearing or a shaft grounding brush to eliminate arcing from induced currents in the motor shaft. Most large motors and all synchronous motors will have this insulated opposite AC drive end bearing for this reason, even before variable frequency drive were invented.

The constant speed applications tended to have the discharge at the same point on the bearing and accelerated the pitting of the bearing when used in clean room fan applications.

How to select Variable Frequency Drive?

Variable Frequency Drives (Named AC drive, variable speed drive or frequency inverter, shorted for VFD) can alternatively be chosen based on Torque (Nm) requirements. The basic thinking is that Torque is related to current (amps). Furthermore you need to take into account, for this approach is Nominal/Maximal Torque to ensure that especially starting Torques which are typical to each application can be generated. Applications are very quite distinctively, so a variable frequency drive controlled fan application is quite different than a hoist operation but the logics is applicable in the same way.

Using Power (kW) is in fact less logical if you look at the definition of Power (is equivalent to delivered Torque at a fixed speed). When this is applied to a variable frequency driven system, the power calculation actually varies quite dramatically over the complete speed range although you would be applying this to the same components.

There combination current versus frequency is a bit more tricky since the current is the main component for the magnetic field in the motor. In fact if you dive into details you will find out that the Torque being delivered to the driveshaft is in fact caused by the difference in speeds (slippage) between the frequency of the electrical field rotation and the actual rotation speed of the shaft.

This means that at VFD drive speed shaft rotation 0, Torque can be generated while the speed of the rotation of the magnetic field in the motor is already running. What is also true is that the Torque can be quite high (ie max torque) at driveshaft speed is 0. This is why AC VFD systems can be used in motion and traction applications.