variable speed drive (VSD) 1

[autoguru]

Can you guys check this out?

http://www.wpcorp.com.au/html/business/smart_ways_to_save/sees_variable_

speed_drives.html

It talks about variable speed drive (VSD) which seems to be commonly
used in industrial motor application. How does that work?

Is that the same kind of system used on the Toyota Prius?

Any difference between the 2?

Thanks

 

[autoguru]

I did some searching myself. I found this website:

http://www.machinedesign.com/BDE/motion/bdemotion1/bdemotion1_1.html

It states:"In contrast to mechanical speed-control technology, which usually employs gearing or belts to change speed, electronic speed control manipulates applied electrical power to control velocity and torque. Electronic speed control in ac motors employ special amplifiers or drives. These generally vary ac motor speed with adjustable-frequency inverters. Though such electronic controls are more expensive than mechanical speed controls, they provide the advantage of reduced energy costs. Applications for such equipment include fans, blowers, pumps, and compressors.

Adjustable-frequency inverters of a different type are also used to control the speed of dc motors. Dc motors have been preferred over ac motors for adjustable-speed applications because dc motors tend to cost less. However, ac variable-speed drives are often preferred for equipment that is frequently started, stopped, and reversed, especially in hostile environments. Ac drives are also widely used for multimotor applications that require precise speed matching. Both ac and dc variable-speed motors are used in manufacturing applications such as spindle drives where appreciable amounts of torque are required at relatively high rotational speeds.
"

I wonder how electric speed control can be more efficient over a mechanical speed control since gears are 98%-99% efficient. And the inverter is not that efficient.

Thanks.

 

[autoguru]

Can anyone explain to me the difference between the VSD and mechanical variable speed belt?

It seems they performs the same task, ie, optimize the speed/load.

 

[Skogsgurra]

With all respect, autoguru.

If someone stepped up to you and asked "I saw a peculiar carriage this morning. It had no horse or donkey but moved at an amazing speed. What was it?"

Would you take that guy seriously? Would you try to explain about combustion engines, fuels, legislation, driver's license, environment and so on. Probably not.

I think that the reason you get no answer is that the question you ask is similar to the one about the "horseless carriage".

There are many fundamental presentations available. Have a look at the ABB, Allen-Bradley, Siemens web pages. There is also a number of FAQs available (look at the top of this page). One of them is faq237-1063.

Your claim that "gears are 98%-99% efficient" may be true for a one-stage gear, but seems to be on the high side. If you need more reduction or variable reduction, you will easily have efficiencies going down to 80 percent or less.

Weight and space requirements increase and maintenance issues are getting more and more important - making accessability another problem in compact machinery.

PWM VFDs have been with us for more than 25 years. Their use is steadily growing and from a humble start in paper mills and textiles they have gained wide acceptance in process plants, ships, HVAC, water and sewage treatment, food processing, electric vehicles, white goods and just about every application you can think of.

As you know, the market is always right. And the market has accepted PWM VFDs wholeheartedly. There is no known technology ready to take over (except for a variant; the NFO Sinus) and market share is growing faster than any other technology. So, market has decided, and I think that you too can benefit from learning more about VFDs.



Gunnar Englund

http://www.gke.org

 

[autoguru]

I'm a mechanical guy and know practically nothing about electric motors or electronics. Please excuse me.

I think one of the biggest advantage from my
understanding, is that VFD allows for energy saving by controlling
the "throttle" of the motor. This is because it can control both the
torque and the rpm, and therefore the horsepower.

But using a mechanical drive, you can control the horsepower delieverd,
only the gear ratio. This means for a lower load, all that extra power
is wasted.

For example, if you are driving a car and wants to slow down, the
efficient way to do this is to go easier on the throttle (VFD), rather
than keeping the throttle the same and then using the brake to slow
down.

That's what I think of how a VFD works, but not sure.

 

[jraef]

Simplistic, but sufficient.
The equivalent auto analogy to a full speed AC motor would be a deisel engine that runs best when operated continuously at the same speed. Braking it isn't really a valid way of changing speed, it just increases the load. Putting a hydraulic transmission on it can change the final output shaft speed, but changes the shaft torque as well. The same would hold true for changing shaft speed with a mechanical varidrive on an electric motor running across-the-line (full speed).

An AC motor can run at any speed as long as the voltage and frequency are controlled together. In your car analogy, that would be the same as your air/fuel mixture. Choking off the fuel only or the air only starves the engine of needed torque and you stall. Changing them both together provides for reduced power at lower speeds without stalling. Gear reduction can still play a role in both scenarios.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[autoguru]

If a conveyor system only has to fulfill two operating conditions: both conditions require the same torque, but one requires a speed that's double the other. So in one situation, the horsepower requirement is doubled.

There is no other variability of load or speed for the life of the conveyor.

Is a VFD still needed? or can a mechanical gearbox be cheaper? (taken into account of the operating cost of the conveyor into account too)

what type of motor is best suited for this?

Thanks

 

[Skogsgurra]

autoguru,

The way you put the question* there is no need for a VSD. Why should there be?

But as soon as you need to continuously adapt speed to need or need a better starting torque, you will need some sort of VSD. Probably a standard PWM and a standard asychronous induction motor. It will give the lowest cost and best efficiency (at least in stationary applications but it is also heavily used in mobile applications like cranes) - that's why the market for VFD+ASM is the fastest growing in the drive sector.



*You have even ruled out the need for a soft start or the possibility to have an increased starting torque by utilising the peak torque of the motor.

Gunnar Englund

http://www.gke.org

 

[autoguru]

Is there any way to reduce the power output of a motor?

\If a 5 HP motor is used to drive a load but when the load drops to a point where only 2 HP is needed. Is there a way to reduce the motor output to 2 HP? Or does the motor still runs at 5HP regardless of load and extra energy simply get wasted?(in resistor or other ways)

I have no knowledge of electric motors but common sense dictates that it is probable that if only 2hp is used then only slightly more than 2hp of input is consumed. Maybe I have got this wrong and will soon be told so if that is
the case.

but then someone told me that "alternating current motors generally will draw only sufficient current to match the load. An AC motor under no load draws very little current
regardless of its rated horsepower. It will increase its draw as the load increases. There is such a thing as over-motoring a piece of equipment with an AC motor. I'm not sure how it works, but there is an inefficiency
in say, attaching a ten HP motor where a 1 hp would be adequate. Direct current motors will draw a fixed amount of current regardless of the load."

Just want to get things straight once and for all.

Thanks.

 

[Skogsgurra]

Oh, I see. You have fallen in a trap, sort of.

A 100 HP internal combustion engine outputs 100 HP maximum. Agree? But it doesn't do that if the power isn't needed. Right?

An electric (asynchromous induction) motor is exactly the same. A 5 HP motor can output 5 HP, maximum. But if not loaded more than 3 HP, it doesn't output more than that. Speed is determined by the mains frequency. Consumed power is determined by the shaft load. That's it. Nothing more to it. You needn't do anything to reduce power.

I think that the trap is that sometimes the speed of a motor is used to vary the amount of air moved by a fan or a pump. It could also be varied by using a damper or a valve, but that would cause additional losses. Varying the speed is a "cleaner" way of controlling the air flow - with better efficiency.

Gunnar Englund

http://www.gke.org

 

[autoguru]

I think I've got it now. Here's my understanding so far.

On an internal combustion engine, if the throttle is constant but the load increases, the rpm of the engine goes down. Elementary physics say an increased load tend to slow down a moving object unless more work is to put into it. To keep it at the original speed (rpm), throttle needs to increase so more fuel is injected. So more work is done per unit time, therefore more horsepower. If the engine creates more horsepower than needed, than the rpm will go up, which makes sense from simple physics.

But on certain type of electric motors, the rpm is sort of fixed, and it depends on the frequency of the power source. I'm new to electric motors and this confused me at first. It seemed the rpm of the motor is "fixed" by the frequency of the power source, regardless of load. This confused me at first because an object will tend to accelerate/slow down depending on the net work done on it.

Yet the motor can stay at the same rpm even if the load increase. This made me think the motor is always delivering the maximum power (equal or more than the load). And the extra power just get wasted through some kind of resistor.

It was confusing because a motor didn't have a "throttle". Now I understand that a motor can draw more or less CURRENT depending on the load, while keeping the speed fixed. This makes a lot more sense now. Therefore, a motor's output power can be varied depending on the current it draws.

I think the reason a VFD can be beneficial to a application like the fan is that the power is proportional to the cube of the rpm. This means precise speed control is very important since even a small difference in speed translate to a very big difference in required energy.

But on my previous example regarding the conveyor, a VFD isn't as important since the torque is constant and there isn't as much variability to the speed required.

 

[Skogsgurra]

autoguru,

Your first first part is absolutely correct. It is nice to see insight growing.

The latter part (about conveyors) is less clear. There are many applications where it is desirerable to vary the speed of a conveyor. Running full speed when it isn't needed is a waste of energy and it also wears the belt out prematurely. So a VFD is often used with conveyors as well. Remember that power is torque times rad/second and even if torque is constant, you save power by reducing speed.

Good to see that you are getting this.

Gunnar Englund

http://www.gke.org