Soft Start

The current drawn at starting (starting current, also called "locked rotor" or "inrush") is directly proportional to the voltage applied at the terminals. This is why soft starters (also called "reduced voltage" starters) are used in the first place.

A reasonable assumption for "standard" squirrel cage induction machines is to assume that the motor will draw somewhere between 7X and 10X the rated line current if started direct-on-line at full rated voltage. As the machine speeds up ...

The soft starter is actually quite a simple device and in many applications, you must use some form of reduced voltage starter to reduce the starting current of the induction motor. If you are compelled to use a reduced voltage starter to reduce electrical disturbances, or to reduce torque transients, then that is a good reason to use a soft starter which complies with these requirements and is stepless and ea ...

The prime application for the soft starter, and in many cases variable frequency drives on fixed speed applications, is to get the driven load up to full speed to enable it to do some real work. This focus on energy saving by soft starters and VFDs creates a lot of confusion and miss-information.

Soft starters do sometimes include an energy saving algorithm which is designed to reduce the voltage applied to the motor when the motor load ...

Question: I have a motor of 100HP with FLC of 123A. It's load is Centrifugal Fan. We are currently using a Star Delta Starter. It's current in star mode various from 276A to 220A for 35secs. Is it safe? While changeover in delta it takes a peak of 376A and then comes normal 60A phase current. In this operation we are using a microprocessor based relay which is bypassed initially for 25 seconds. I want to know how safe is this operation and life of motor in long run. Or should ...

Situation: Here is my current problem and was wondering if anyone had experience of the following:
Application - 260Kw submersible motors/pumps must be running up to full speed within 5-6 seconds of startup.

My proposal had always been to use a soft start until the pump manufacturer has stated that in order to provide sufficient cooling to the stator windings (using the sea water), it must be at full speed in 5-6 seconds.

I have been in touch with ...

I find that due to the fact that the variable frequency drive (VFD) rectifies the AC to a DC bus, the power factor drawn from the supply is near unity but has harmonics. Harmonic kill the power factor correction capacitors. You need to install harmonic filters to take out these unwanted inputs. GTOs used to power the electronics generally fail open circuit. If your VFD has a ramp down to stop you need to program in a fast stop. All safety devices like E-s ...

Overcurrent protects the wire, overload protects the motor.
Over load and over current protection is simple....and sometimes not. For me it is easy to think of it this way. The overload protects the motor, the overcurrent protects the wire. Because the time/current curves are very far apart from each other, don't ever think that one will adequately protect for the other.

Many electricians size the wire from the starter to the motor based on the ampacity of the overload protec ...

When a three phase induction motor has (1) delta connected stator windings for its normal running, and if (2) all the 6 terminals are brought out to the terminal box, than such stator windings can be connected temporarily in star formation also for the staring purpose. Such motors in star formation would draw 1/3 the staring current, compared to the starting current at delta formation. Once the motor reaches it’s near about the operating speed, the winding connection would be changed ba ...

Every motor driven application needs to be evaluated and the starting and control method properly chosen to meet those needs. If variable speed is not required for the process and the power and mechanical systems are robust the direct-on-line (DOL) starting may be the best choice. If the power system and / or the mechanical system are weak and only fixed speed is required for the process then a reduced voltage starter with bypass may be the best choice as long as the reduced starting torque t ...

I have involved with a soft starter study recently which was about a 750 kW motor had failed to start. In the investigation, we found that there were many flaweds in the system configuration as follows:

* The power supply was slightly weak to handle the full starting current this is why a soft starter was deployed - of course - but again with the soft starter there was some voltage dip during the operation.

* The cables were laid flat rather than in trefoil. This caused ...

Once a time one of our customers ask to install three numbers of 150HP capacity soft starters (with auxiliary I/O's) to build an intelligent motor control center. Their main goal is to limit the high starting current of the chillers compressors. They do not need an expensive variable frequency drive (VFD) as the system is designed to operate at full capacity most of the times. They need a AC motor soft starter with Modbus RTU communications ...

One of our clients asked is it possible to start a pump motor by directly connected power cable to diesel generator set without any means (breaker)? The diesel generator set is 1250kW and the motor is 630kW.

Initially I thought what an idiot and then stopped to pause. What are the design requirements and are you retro-fitting the pump to an existing diesel generator. The process of soft starting the pump using the DG should be able to be done if you are working from first principle ...

If you talk to someone in variable frequency drives (VFD) sales, they will often tell you that there is no place for a soft starter because the VFD reduces the current more than a soft starter and the VFD saves energy, but in reality there is definitely a place for both of them. If you need to vary the speed of the driven load, then a VFD is definitely the choice. If you are looking to reduce the starting current and the starting torque of an induction motor, then a soft starter can be select ...