Categories

# Variable Speed Drives

#### 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 ...

#### 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 ...

#### 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 ...

#### 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 ...

#### Choose a VFD base on motor rated current

##### Sunday, November 2, 2014

When a motor power is stated, it is always the mechanical power, shaft power i.e. NOT the input electrical power.

Variable frequency drives (VFD) are also (mostly) rated in terms of kW albeit it's wrong. Because VFD output is electrical (not mechanical). Mechanical power (i.e. motor shaft power) is always "active" power, where in electricity we also have reactive (doing nothing but losses) power. The apparent (total) power (stated in terms of ...

#### Variable Frequency Drive (VFD) Selection Guide

##### Sunday, October 19, 2014

Variable frequency drive (VFD), also called frequency inverter, variable speed drive (VSD), adjustable frequency drive (AFD). This article is about the basic guide of how to select a VFD.

The correct selection of the variable frequency drive (VFD) is critical for the normal operation of the mechanical equipment electrical control system. To choose the best suitable VFD, we need to know the specified machinery type, the load torque characteristics, speed adjusting range, static spe ...