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Minimum acceleration time


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#1 Mark Lees

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Posted 10 May 2006 - 06:40 PM

Hi,

I wonder if anyone could provide me with some factual info on this question. I am working on a project where we intend to control a 275kW ac motor using a variable speed drive. What I need to know is what the minimum acceleration time would be from a speed of 11% to 100% - in other words how quickly could I accelerate from idle to full speed without exceeding motor design parameters. I know this is slightly hypothetical, but it is important for the application I am working on. Also, if I wanted to accelerate the motor to full speed in half the minimum time, what effect would this have on motor full load current & inrush current?

I hoped to get some help from motor manufacturers, but to date none of the manufacturers I have contacted has been able/willing to help, so I hope that someone here can.

Regards,

Mark


#2 marke

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Posted 10 May 2006 - 07:01 PM

Hello Mark

Welcome to the forum.

The minimum acceleration time is a funcion of the load and motor inertia as seen at the motor shaft, and the maximum torque that the motor and drive can produce.
If you increase the inertia, you will extend the start time and if you increase the torque, you will reduce the start time.

Basically, at full speed, your motor and load will have a kinetic energy. During ramp up, you must transfer this energy from the electric supply.

Some drives are rated at a maximum current of 110%. That results in a maximum torque of around 110%. Other drives are rated in the order of 150 - 160%. There are some that can be rated higher than this, but they are less common.

With modern drives, if you try to accelerate faster than the motor drive is capable, you will usually get torque or current limiting which will override the acceleration rate that you set. On older drives, you may get over current trips or even drive failure.

Best regards,

#3 mariomaggi

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Posted 10 May 2006 - 07:45 PM

Dear Mark Lees,
QUOTE
how quickly could I accelerate from idle to full speed without exceeding motor design parameters.

You can calculate the minimum time considering motor/load inertia, available accelerating torque and load torque.

You don't wrote if you have to start an high-inertia load or a low-inertia load. In case of high-inertia you have to look at rotor temperature (only one cold start or multiple starts?). Considering that with variable frequency drives the slip is usually low and that stator current is never so high like during DOL starting, probably you don't find problems. About stator currents, you can check the I*square(t) for expected currents for the acceleration time in seconds. All serious manufacturers could supply this number as maximum value acceptable for a certain motor size.

In case of low-inertia you don't have any motor problem regarding design parameters. Only if the acceleration is very very short, please check with the manufacturers if connection between rotor and shaft is enough strong. Using an inverter with a better peak current (i.e.: 200%) you will have better results. Don't forget to check the main power availability during starting, if main supply is poor you have to supply additional energy, i.e. with additional capacitors on DC bus (be careful, there are other problems involved).

Regards
Mario

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#4 kens

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Posted 10 May 2006 - 09:36 PM

Hi Mark, as Mark and Mario have already stated the limiting factor is the VSD current rating. The drive will only supply a limited amount of current to the motor during the ramp. You need to calculate the torque requirement starting from the load. Considering the cost of a drive that size the drive suppliers engineers should be able to help with this calculation. As it is a physical limitation that limits a VSDs current you can succesfully install an oversized drive to supply additional current to the motor.
Good luck
Ken
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#5 jraef

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Posted 11 May 2006 - 12:16 AM

The motor manufacturers could have given you the acceleration time formula for a motor based upon DOL starting, but they were probably intimidated by the issues of having a VFD as described above. However, if you have enough information on the motor and load, you can calculate that acceleration time at least as a starting point, and as mentioned, oversize the VFD to be able to provide the maximum values for the motor. Here is the formula, although unfortunately I only have it in SAE units, sorry.

Time for an AC motor to reach operating speed: (in seconds)

Seconds =
WK^2 (lb-ft^2) x speed change (in RPM)

308 x AAT (lb-ft)


WK^2 =
Rotor inertia + Load inertia x load RPM^2

Motor RMP^2


AAT = Average Accelerating Torque, =
{(FLT + BDT)/2} + BDT + LRT

3


Where :
BDT = Break Down Torque (ft-lb)
FLT = Full Load Torque (ft-lb)
LRT = Locked Rotor Torque (ft-lb)
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