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#1 fintanlawlor

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Posted 22 August 2005 - 08:58 AM

Can members advise me of their experiences (if any) with the "Magnastart" type Rotor Starter.

Thanks in advance

FINTAN1

#2 jraef

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Posted 22 August 2005 - 05:43 PM

I have no direct experience with these devices as a brand name, but have seen live demonstrations. There is a place for them in the industrial world to be sure, but their claims of being the end-all answer to every kind of starting and variable speed problem are exagerated. In general, magnetic drives are essentially slip control devices, so your motor runs at full speed and the magnetic coupling slips at various degrees to regulate how much of the load is coupled to the motor. This has advantages in particualr on high inertia loads, but a major disadvantage on other loads in that you have permanent losses built-in to the system. When used as variable speed controllers, these additional losses tend to negate some of the affinity law energy savings available on centrifugal loads such as pumps and fans. When used as a soft start device, these permanent losses add to the overall cost of ownership when compared to solid state soft starters that are bypassed at full speed.
"He's not dead, he's just pinin' for the fjords!"

#3 msdaif

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Posted 22 August 2005 - 07:55 PM

The conventional method to reduce the starting current of slip-ring or wound-rotor induction motor is using 3-phase power resistors. The
resistor may have 2 or 3 taps. Let us assume we have a 3-tap resistor.
At start-up, the highest resistance (Tap 1) is applied to the rotor, after some time the resistance is decreased by switching to Tap 2 and finally to Tap 3. When the motor reaches full speed, the resistor is shorted and all this is accomplished by timers and contactors. This method is not efficient because of the energy losses (I2R) in the resistor. Plus maintenance requirments.

MagnaStart replaaces the resistor with a 3-phase inductor or coil. The manufacturer of MagnaStart, www.adwel.com, says that MagnaStart
reduces the starting current considerbly. Moreover, it results in a smoother
acceleration than with a resistor. What I have seen is resistors or power electronic convertors but I have not come across a MagnaStart. I will test
with a 3-phase coil in my lab and I will see the results. I'll inform you in this forum when I get the results.

However, I believe the most efficient method is to employ power electronic
convertors which save energy and very useful for braking and starting of slip-ring motors.




http://www.adwel.com...g_Brochure2.pdf

#4 msdaif

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Posted 22 August 2005 - 07:56 PM

The conventional method to reduce the starting current of slip-ring or wound-rotor induction motor is using 3-phase power resistors. The
resistor may have 2 or 3 taps. Let us assume we have a 3-tap resistor.
At start-up, the highest resistance (Tap 1) is applied to the rotor, after some time the resistance is decreased by switching to Tap 2 and finally to Tap 3. When the motor reaches full speed, the resistor is shorted and all this is accomplished by timers and contactors. This method is not efficient because of the energy losses (I2R) in the resistor. Plus maintenance requirments.

MagnaStart replaaces the resistor with a 3-phase inductor or coil. The manufacturer of MagnaStart, www.adwel.com, says that MagnaStart
reduces the starting current considerbly. Moreover, it results in a smoother
acceleration than with a resistor. What I have seen is resistors or power electronic convertors but I have not come across a MagnaStart.
Please read one brochure of MagnaStart on:
http://www.adwel.com...g_Brochure2.pdf

I will test with a 3-phase coil in my lab and I will see the results. I'll inform you in this forum when I get the results.

However, I believe the most efficient method is to employ power electronic
convertors which save energy and very useful for braking and starting of slip-ring motors.

#5 jraef

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Posted 22 August 2005 - 08:17 PM

Oops, my error. I was thinking MagnaDrive, not MagnaStart, sorry.
"He's not dead, he's just pinin' for the fjords!"

#6 marke

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Posted 22 August 2005 - 09:16 PM

Hello msdaif

The secret here is that it is not just a coil. If you were to connect a pure inductance to the rotor circuit, you would definitely reduce the starting current, but you would also reduce the already low starting torque.
The inductors used exhibit a resistive component as well as an inductive component and it is the resistive component that produces the torque.
By altering the flux in the iron of the reactance, you can alter the eddy current losses. One of the effects of this type of impedance is that the actual impedance is going to alter as the rotor frequency reduces. It effectively automatically changes the resistance.
You will need to use an iron cored inductor with a reasonable level of saturation for it to work.

Best regards,

#7 msdaif

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Posted 26 August 2005 - 04:16 PM

Dear Mark

Thank you for the clarification.




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