In order to understand better the starting requirements using an AC drive and the selection of a VFD for your motors, a number of motor starting simulation studies (ETAP or other software) can be conducted for the load types and variable frequency drive control mechanism of the motor, in order to determine the drives size, control method and settings. These studies can verify that the drive and motor system meet the operational requirements. There are usually a set of speed-torques curves for each application. During the acceleration, frequency and voltage are generally controlled such that full voltage is reached at the design rated RPM of the motor following a ramp. Motor voltage and current are regulated to provide speed and torque control to the load. The voltage to frequency ratio is held roughly proportional. In practice torque developed at very low frequencies falls off and extra voltage above the value of the volts / HZ ratio (or other ctl) is automatically applied to regain the lost torque. The variable frequency AC drive modify the speed but also the input voltage in direct proportion.

We are having agitators with soft starters. There is MCCB than Fuses and then soft starter. The problem is that at the time of starting, the fuses got blown. The motor Kw is 160. Earlier the fuses were of 450Amps semiconductor type. We increased the fuses rating to 550Amps. But now the frequency of fuse blown is reduced but has not gone. Should we go for further fuse rating? Or could we bypass the fuses and be on the MCCB?

I am using this starter in an LV panel for a 5.5 kW load as per client requirement.
I am new to this type. During the testing of this feeder, I have connected a 5 hp motor which runs well on startup. But after the soft start (10 seconds ramp up time) it suddenly trips and red LED comes up which indicates STATE/ BYPASS FAILURE. I have also done every troubleshoot as per manual. Please suggest me some measures so that my motor runs continuously even after the soft start.

We see that the market is changing and clients are looking to optimize their process rather than just fit a VFD for all applications. Fitting a VFD on a fix speed motor will not give any benefits and using a soft starter will be more economical for the client and the process.

However, if a motor is lightly loaded and depending on the type of application we could expect to see savings of between 8 and 32% saving on the energy consumed by the motor. While this seems obvious to those of us in the soft starter industry, it seems many end users still use VFDs for fixed speed applications when a soft starter would be a more cost effective, efficient and reliable choice.

Start the unit, the soft starter will regulate the output voltage based on the current feedback read from current transformers located on the output. The thyristor will be commanded to let enough volts through in order to maintain the maximum current set (maybe 300%).

Once the motor start turning and accelerating the volts will increase and the current will remain 300% until full volts are reached and, at that point, the current will decrease down to what is required by the load. For example:

• If the compressor is fully loaded and the motor is designed with no margin (just for example) the current will be 100%;
• If the compressor is uncoupled the motor current will go down to magnetizing current (30% ?).

In the end, if you are under commissioning check if the maximum current parameter is set in your case to 400%.

Whether or not to use the switchgear relay will ultimately be your choice, what I would take into consideration are the following:

• Does the soft starter protection cause the switchgear to trip on fault, if not then I would incorporate the switchgear relay into the protection scheme.
• Is the switchgear protection connected to the soft start controls to prevent the soft starter trying to start when the trip has not been reset. If not affirmative then further system integration will be required.

To ensure that any fault can be quickly identified are both the soft starter and switchgear connected to either a SCADA system that monitors all trips or an indicator panel that shows all the alarms and trips.

Question: I have a soft starter that is rated for 7.5KW, and need to soft start a 3KW motor can the starters programming and / or parameters be changed in such that the starter can see the 3KW motor correctly and operate it as it should? I have found only 2 parameters where I can change the values for the motor size, but cannot set it for 3 KW, will only allow me to go down to a 5.5 KW motor size, must I plug in a PC and try through the software?

There are two factors here:
1. electronic soft starters are current chopping thyristor devices that work at up to 400% motor rated current. The chopping of such large current in this way generally sends large harmonic currents and voltages into your power system that can disturb sensitive communications devices by voltage disturbance, currents in the earthing systems and harmonic neutral currents. Soft starters are relatively crude and noisy devices compared to other methods of soft starting motors such as VVVF, autotransformer, etc.

You should firstly verify that the new pump has been installed electrically correctly. After that you should record both process parameters and electrical parameters. Flow and pressure increase across the pump, preferably also absolute pressures and temperatures, so you can eliminate pump cavitation etcetera as part of your problem, actual pump speed (RPM) and all relevant electrical parameters, such as voltage and current both during start up and during steady state run. You should then check that the soft starter actually closes its bypass contactor as soon as the pump is up to speed.

If I were you, I would also do a couple of DOL starts of the pump, just to see if the soft starter is really required. If you do not experience too much of a flicker when starting this little pump, I would then simply remove the Soft Starter.

Soft starters have no Hz regulation capabilities. They can boost current and voltage at start, but it's a voltage boost to initiate movement if the motor bogs at start, after which the boost voltage drops to the accelerated low end ramp settings. My intent was, and I think I accomplished getting the point across, to illustrate simple physics regarding voltage/Hz regulation as variable frequency drives can do and the torque losses incurred when the motor rpm/Hz/FLA apex is varied from, as happens by virtue of how soft starter works. In general, asynchronous motors are the choice in overhead crane, and many if not most conveyor controls we build as the slip helps with generating higher starting torque. The torque curves, and herein may be a point of difference in our viewpoints, directly correlate to the rpm/Hz/FLA efficiency design point of the motor in torque creation at start, as well as in running torque. If the initiating frequency is at 60 Hz and the voltage is reduced from the peak motor design efficiency point, the resulting motor 'starvation' will cause the motor to draw excessive current. That is inescapable and as such current boost is barely a help.

We need to know the type of machine (motor, electrical heater, etc.), rated voltage, estimated distance between them and power source, connection diagram (star or delta), voltage level on the power source, type of start (for motors it is important to know if is DOL, with variable frequency drive, with soft starter etc and with type of machine is motorized with this motor - oil pump, water pump, fan etc.) and also we need to know the the service type (continuous, intermittent with which periodicity, for evaluate the cooling possibility of the cable).

There is no standard answer. So much depends on ambient temperature, load inertia, non-inertia load and time between starts. Things the motor manufacturer needs to know, to determine (with reasonable accuracy) the effect of multiple starts include: