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# Blog

#### How do I start a motor in star delta connection?

##### Tuesday, November 18, 2014

If you reverse direction while running, your contactors must be AC4 rated. Not only will the mechanical load take a hammering but the coupling and the rotor too. When you buy your motor you need to state this requirement under duty cycle and chose 7, 8, 9, or 10. There would be a second timer to allow the motor to stop and then restart in the other direction. The timer on the star delta should not be changed once set. As the torque reaches maximum in star, you change to delta. Modern Motor Pr ...

#### Problem of VFD controlling positive displacement pump

##### Tuesday, November 18, 2014

Problem:
Pump type is a piston dosing pump with manual struck length adjustment with motor 3 phase 220 AC volt delta connection variable speed. We control it with a GK3000-2S0007 1 hp variable frequency drive (VFD) single phase 220 VAC to three phase output. We control the dose of the pump through the VFD with analog reference signal 4-20 mA which effect the frequency (V/F) which change the motor speed rpm. The problem is at low required dosing ...

#### Motor pump starting method from diesel generator

##### Monday, November 17, 2014

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 ...

#### How to determine the pole number of an induction motor?

##### Saturday, November 15, 2014

There are many much more interesting questions related to the pole number of induction motors, e.g.:
1. Does the induction motor supplied by the main grid (say, 50 Hz) increases its torque capability in "p" times with growing pole number "p" since its speed decreases in "p" time (like in a gearbox)?
2. Let's we have an induction motor with p=2 and feed it from 50 Hz grid. Then we re-connect the winding coils to arrange p=4 and feed if grow 100 Hz grid. Are performances of these tw ...

#### Efficiency of a 2 pole motor running at high frequency

##### Saturday, November 15, 2014

While modeling a 2 pole motor for 250Hz, we found out that at frequencies above 80Hz there is little to no difference in efficiency between a magnetizable or non-magnetizable shaft. We found this odd, because the field lines go through the shaft in either configuration. So we put a motor with and without a magnetizable shaft on the test bench and tested it between 0 and 135Hz. The results were compliant with the model, but not able to explain the phenomenon.

There is a relationship ...

#### Variable frequency drive for high inertia loads

##### Tuesday, November 11, 2014

High inertia loads present some challenges in both mechanically and from a control standpoint. Suppose the load is a "cement block". We could use a better description as to how this "cement block" is being put into rotation, what the inertia value is, what the speed of rotation is, and mechanically how the proposed motor would be connected to this load. Will it be directly connected? If so, what type of coupling is being considered? If there is a gear reducer, what is the type and gear ratio? ...

#### Variable Frequency drive or Adjustable Speed Drive

##### Monday, November 10, 2014

I don't even own a copy of IEEE 1566, but I note that its abstract defines it as a "standard for performance of adjustable speed drives rated 375 kW and larger." What do you call a 200kW drive? I have, however, worked in this field for 28 years. I'll concur with the complaint that words are used imprecisely, but that's the way life is. Had you said "adjustable speed AC drive"? I would say they and VFDs were the same thing, but simply "adjustable speed drive"? No, sorry.

As for your ...

#### Different VFD manufacturers have different designs

##### Monday, November 10, 2014

I think there are a few more differences between different variable frequency drive (VFD) designs. For example: I've used VFDs that never use output load reactors, and can go, depending on the VFD model, 300 to 1000 Ft between the VFD and motor unaided. To exceed that distance, they use an RFI filter. Not tooting a horn, but pointing out one of the manufacturer's capabilities versus others, with respect to wanting to define the application as thoroughly as possible before taking on system des ...

#### What we can do about VFD Operating Temperature?

##### Sunday, November 9, 2014

Generally the variable frequency drive (VFD) operating temperature range is -10 degrees C to 40 degrees C. I would not be too concerned about the upper operating temperature limit (40 degrees C) as it is rather simple to derate the VFD for high thermal limits. The same goes for the high elevation application as the VFD will have some additional derating due to the lower thermal mass of air at high altitude. One might check with the VFD manufacturer to determine if that variable speed drive ha ...

#### What is pulse number (6, 12, 18, 24) in drives?

##### Sunday, November 9, 2014

In variable speed drive (VSD) systems, both AC and DC, the first stage of power conversion is from AC to DC. In a DC drive, it's the only stage - from fixed AC to variable DC. In an AC drive there are two additional stages: filtration and inversion back to AC.

The simplest converter is a full-wave diode bridge, which converts the incoming AC to a fixed DC voltage. This isn't useful in a DC drive, but works fine for AC variable frequency drive (VFD). Standard industrial drives - bot ...

#### Soft Starter vs Variable Frequency Drive (VFD)

##### Saturday, November 8, 2014

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 ...

#### Can a Variable Frequency Drive control motor at Zero Speed?

##### Friday, November 7, 2014

In simplest terms, the variable frequency drive (VFD) starts at zero hertz (Hz) and can run there indefinitely. I can make such a bold statement because at that frequency, and with the motor stationary, there is only magnetizing current being taken through the semiconductors. The variable frequency drive normal cooling system can disperse the heat that would be generated in this situation.

The motor in this case is likely one of two types; Induction or Permanent Magnet (PM). We al ...

#### How much energy & money does a VFD save?

##### Thursday, November 6, 2014

The point is, if the motor is running at nominal load, the energy saved when using a variable frequency drive (VFD) is about 3%, for 75% load is about 25%, for 50% load is about 75%. So to calculate how much energy saved by the VFD, you have to check how long the pump/motor will run on under nominal load conditions and how big is the difference between nominal and operating load. Now, this x% has to be translated in kWh and then in money. This money you have to compare it with VFD price and s ...

#### 5 questions regarding specifying a VFD solution

##### Thursday, November 6, 2014

In my experience, after the hundreds that I have had to design & work with, over 27 years, there are a lot of physics & understanding that is associated with different applications that are possible. What it will ultimately come down to is motor construction and variable frequency drive (VFD) capabilities and size, based on specific application. There are also a few questions that must be answered before a solution will be specified. We have to deduct that some equipment is already in ...

#### What is a static (solid state) frequency converter?

##### Wednesday, November 5, 2014

Static (solid state) means no moving parts inside the frequency converter, the definition is relative to rotary frequency converter which using an induction motor to achieve variable frequency.

Static frequency converter is converting fixed frequency & voltage AC power to DC to AC by inner electronic parts and components, the multifunctional inverter converts ...

#### Can I run a 50Hz motor on 60Hz power supply?

##### Wednesday, November 5, 2014

You see the number of revolutions that an electrical motor turns is dependent upon the cycles per second (Hertz) that the alternating current (AC) is changing. A motor that is designed to run on 60 Hz and is plugged into a 50 Hz power supply system will turn 17% slower, the internal current will go up 17%, the amount of power (watts) will go down 17%, and the appliance mechanical cooling will be 17% less. The end result will be that the motor will be using a higher current then what the motor ...

#### Variable Frequency Drive (VFD) Harmonics and Solutions

##### Tuesday, November 4, 2014

A 12 pulse variable frequency drive (VFD) will develop a harmonic signature with high magnitudes on the 11th and 13th order. The good thing is the typical harmonic signature exponentially falls off as the order goes up. This is the reason that an 18 pulse VFD will have less harmonics than a 12 pulse VFD and so on and so forth. Another advantage is the DC buss will have less distortion when a multi-pulse front end is used. Most medium voltage VFD comes standard with 24 or 36 pulse front ends. ...

#### Why does a PM AC motor need a VFD?

##### Tuesday, November 4, 2014

The brief observations that follow will assume that you have a 3 phase permanent magnet (PM) AC motor. In a PM AC motor, max continuous torque is achieved when the stator MMF (or current) vector is 90deg displaced from the rotor's flux vector. Recall the Torque = Rotor_Flux x Stator_MMF ... i.e. the torque is the vector cross product of the vector Rotor_Flux with the vector Stator_MMF, or Torque = |Rotor_Flux| * |Stator_MMF| * sin(theta), where theta is the angle between the two vectors. Torq ...

#### What is vector control of induction motor?

##### Monday, November 3, 2014

The stator voltage and current to an induction motor produces a magnetizing flux to transfer the stator energy to the rotor. In Scalar or V/Hz control, this control does not provide very accurate speed regulation typically in the 2-4% range. This accuracy is good enough for controlling fans and pumps, since the control parameter is not speed accuracy, but temperature or pressure control over an average time period. Also using scalar or V/Hz control, you cannot control zero speed, ie no holdin ...

#### Why a VFD fail after powering back up

##### Monday, November 3, 2014

There has been a couple good thoughts as to why a variable frequency drive (VFD) would fail after powering back up, moisture will be a big player in the operation of a VFD but I will caution you about blowing air across a VFD. The components are a little picky when it comes to static and that can be all it takes to kill it. I have a couple MCC rooms that the VFDs are covered with powder and that's a concern but the greater concern is getting the powder off without incident. We have been succe ...