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Inverter Duty vs Conventional AC Induction Motors

12/06/2010 1:23 AM

Dear all,

What is the difference between an Inverter Duty Motor, that can be used with a VFD and a Conventional Motor - in terms of their construction, operation, insulation, efficiency etc?

Why cant we use a normal AC Induction motor with a VFD, what are the dangers, disadvantages?

What is the cost difference between the two?

Your suggestions would be highly appreciated.

Thanks in advance,

Rooney

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#1

Re: Inverter Duty vs Conventional AC Induction Motors

12/06/2010 5:23 AM

dear friend,

the basic difference in inverter duty and normal asyncronous motor is the cooling area and insulation class.

in inverter duty motors the cooling area of the core is increased and in addition to that a separately powered cooling fan is placed instead of ingral cooling fan due to reduce the high heat generation during low speed.

the winding insulation is incresed (aprox. 2 times) to atempt the high heat generation of the motor during low speed operation.

efficiency of the motor is higher than the normal induction motor as the cooling effect has been increased.

we cant use normal induction motor in the inverter duty as the cooling effect will be lower in low speed operations and in normal speed harmonics efect to the motor temp. rise may resulted to failure of the motor.

i think these are the main reasons and a slightly higher costing effect for the purchase of inverter duty motors.

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#2

Re: Inverter Duty vs Conventional AC Induction Motors

12/06/2010 10:18 AM

Only your textbook knows for sure.

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#3

Re: Inverter Duty vs Conventional AC Induction Motors

12/06/2010 3:12 PM

Here's part of an article you might use in your report;

How to choose a general purpose motor vs. an inverter-duty motor
General purpose motors have been around for many years. They are the workhorse of almost every industry. An inverter-duty motor is a much newer concept that became necessary as motors began to be driven by VFDs (inverters or AC drives). An inverter duty motor can withstand the higher voltage spikes produced by all VFDs (amplified at longer cable lengths) and can run at very slow speeds without overheating. This performance comes at a cost: inverter-duty motors can be much more expensive than general purpose motors. Guidelines for choosing an Ironhorse general purpose motor vs. an inverter-duty motor are given below. If your application falls within the guidelines below, there is no need to apply an inverter-duty motor. NOTE: Marathon inverter-duty motors have limitations as well. Please see the Marathon section for more details.

Background: AC motors can be driven by across-the-line contactors and starters. The electricity sent to the motor is a very clean (true) sine wave at 60Hz. Noise and voltage peaks are relatively small. However, there are drawbacks: the motors can only run electrically at one speed (speed reduction is usually handled by gearboxes or some other, usually inefficient, mechanical means) and the inrush of electrical current (when the motor is first turned on) is usually 5 to 6 times the normal current that the motor consumes. The speed reduction apparatus is expensive and bulky, and the inrush can wreak havoc with power systems and loading (imagine an air conditioning system in an old house - when the compressor kicks on, the lights dim; now imagine the same circumstances with a motor the size of a small car). Note: The following discussion applies only to 3-phase motors.

Enter the VFDs (variable frequency drives): Drives were introduced to allow the speed of these motors to be changed while running and to lessen the inrush current when the motor first starts up. To do this, the drive takes the incoming 60Hz AC power and rectifies it to a DC voltage. Every drive has a DC bus that is around 1.414 (sqrt of 2) * incoming AC Line Voltage.
This DC voltage is then "chopped" by power transistors at very high frequencies to simulate a sine wave that is sent to the motor. By converting the incoming power to DC and then reconverting it to AC, the drive can vary its output voltage and output frequency, thus varying the speed of a motor. Everything sounds great, right? We get to control the frequency and voltage going out to the motor, thus controlling its speed.
Some things to watch out for: A VFD-driven general purpose motor can overheat if it is run too slowly. (Motors can get hot if they're run slower than their rated speed.) Since most general purpose motors cool themselves with shaft-mounted fans, slow speeds mean less cooling. If the motor overheats, bearing and insulation life will be reduced. Therefore there are minimum speed requirements for all motors.
The voltage "chopping" that occurs in the drive actually sends high-voltage spikes (at the DC bus level) down the wire to the motor. If the system contains long cabling, there are actually instances where a reflected wave occurs at the motor. The reflected wave can effectively double the voltage on the wire. This can lead to premature failure of the motor insulation. Long cable lengths between the motor and drive increase the harmful effects of the reflected wave, as do high chopping frequencies (listed in drive manuals as carrier frequencies). Line reactors, 1:1 transformers placed at the output of the drive, can help reduce the voltage spikes going from the drive to the motor. Line reactors are used in many instances when the motor is located far from the drive.

In summary, general purpose motors can be run with drives in many applications; however inverter-duty motors are designed to handle much lower speeds without overheating and they are capable of withstanding higher voltage spikes without their insulation failing. With the increased performance comes an increase in cost. This additional cost can be worth it if you need greater performance.

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#6
In reply to #3

Re: Inverter Duty vs Conventional AC Induction Motors

12/08/2010 11:31 PM

Thank you all gentlemen for your invaluable inputs.

But i have few more questions for you all -

How much should be the appropriate length of the wire between the motor and the VFD so that the reflected waves are not generated?

I have heard about the TURN DOWN RATIOS also for eg 10:1.

What do we mean by the TURN DOWN RATIO and could you explain the above ratio?

Thanks

Rooney

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#8
In reply to #6

Re: Inverter Duty vs Conventional AC Induction Motors

10/02/2011 8:06 AM

Dear Mr. Rooney,

Pl. refer Comment No.7, - the safe and max distance is 20 Metres for the Cable. If higher -chokes/filters are to be used.

Turn Down Ratio is the ratio between Max speed to Minimum speed. Let us say Max .speed of the Motor in a particular system is 1200 R.P.M., , we can operate the motor at 120 R.P.M at Minimum speed and the ratio is 10:1. Therefore the Operational Speed Range is 120 R.P.M. to 1200 R.P.M.

If the System is properly selected and instaleed - the best efficiency and operational results can be achieved.

Thanks,

DHAYANANDHAN.S

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#4

Re: Inverter Duty vs Conventional AC Induction Motors

12/07/2010 12:02 AM

I'm not an electrical engineer but in my readings of the issues one of the bigger problems lies with the air gap in the windings of the motor. round wire has a larger air gap between the windings, to overcome this square copper wire is used. WEG, as one manufacture, does not charge a premium for this and rate all their motors as "inverter duty". Low speeds do not allow the fans to cool the motor so dedicated fan motors attached to the rear of the motor have been used. I would not try to use a motor at less than 50% rated speed just to stay away from the slow speed high torque (high load) problems.

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#5

Re: Inverter Duty vs Conventional AC Induction Motors

12/07/2010 12:50 AM

There are a couple of other things to think about as well. Because an invertor rated motor does not need to start D.O.L. it does not need some of the compromises that have to go in to it on a standard motor design. This means two things ......an invertor rated motor may not start well D.O.L. , but if it is run via a VFD unit it will be more efficent than a standard motor. The running at slow speeds can be a problem with over heating, but that depends on if the motor is designed to run only on a VFD.

There are several "grades" (sic) of invertor rated motor.

The first just has its insulation beefed up a bit to handle the dt/dt voltage transients better.

Another level will prehaps have a fan pack on the end of the motor to give improved cooling at slow speeds. The motor magnetics will also look like a "high efficency" motor with smaller air gaps and not be so good at D.O.L. starting.

Above this the motor becomes more expensive and is not designed to D.O.L. at all, but is designed to make the best use of the VSD drive capabilities. It will also probably have a build in rotor circuit fan and be rated for full torque at stall. One trade discription of this type of motor is a "vector" speed motor. This kind of motor can behave as a "servo" with very good performance at a much more reasonable price than the true servo drives, at up to large (several hundreds of kW)sizes.

All in all, as normal, you get what you pay for. It is better to put money into output filters for the VSD drive first, before up-specing the motor, if all you want is a VSD for starting and running at a high % of full speed (say above half speed).

Regards Bob

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#7

Re: Inverter Duty vs Conventional AC Induction Motors

10/02/2011 7:56 AM

Dear Mr. Rooney,

Supplementing to the posting by Mr. onlymeher ( see comment no.1) the other differences are as follows.

1. The inverter duty motor SHOULD have BEARING INSULATION or INSULATED BEARING. This is VERY ESSENTIAL to avoid the circulation of shaft current. There is a difference between BEARING INSULATION and INSULATED BEARING and later being very costly.

2. The EARTHING for the INVERTER system should be perfect and double the required cross section of the Normal Earthing.

3. The Cable Length between the motor and the Inverter Panel should not exceed 20 Metres. If exceeds special chokes/filters are to be installed.

Thanks,

DHAYANANDHAN.S

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#9

Re: Inverter Duty vs Conventional AC Induction Motors

10/02/2011 10:46 PM

I think someone is unclear on the use of the "Rate" button and has erroneously given "Off Topic" (negative) votes to some excellent responses in this thread. I'm giving GA votes to help counteract them.

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