Lower locked rotor current 1

[davva]

After preforming a voltage drop study I have asked a manufacturer if they can reduce the locked rotor current from 600% to 400%. They can achieve this.

I was wondering what are the pratical implications of lowering the locked rotor current. What changes are made in the construction of the motor. More iron, smaller air gap??

 

[RRaghunath]

The efficiency takes major hit. Size and cost ofcourse.

 

[electricpete]

Three thoughts:

1 - Motor design parameters are interrelated. Rarely is only one thing changed at a time.

2 - You could simply reduce the flux density. But this also reduces the torque curve (starting torque, breakdown torque).

3 - Assuming you want to keep the starting torque the same, a fairly simple way to decrease starting current is to increase rotor resistance. But this has an adverse effect on running efficiency as mentioned above above. A double cage design or some other design heavily exploting deep bar effect might be selected to provide the high rotor starting resistance at starting (for high ratio starting torque per starting current) while keeping moderately low resistance while running (for good efficiency)

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[Marke]

Altering the rotor design to a higher resistance design, will reduce the start current and increase the start torque, but this will also increase the full load slip.

There are ways that the rotor design can be altered to exhibit an effective higher resistance at start and low resistance at run and this can give good starting characteristics and reduced slip losses.

If the rotor bar is a thin bar arranged radially within the rotor, the outer edge of the bar will have a much lower inductance than the inner edge. When the slip is high, (during start) the frequency of the current in the rotor is high. At 100% slip, the rotor current is at line frequency. With high slip frequencies, the reactance of the bar near the surface is much lower than the reactance deeper in the rotor. The result is that the current is concentrated on the outer edge of the bar. As the frequency in the rotor reduces, (reducing slip), the current becomes more evenly distributed across the bar, reducing the effective resistance of the bar at low slip.

Even with clever rotor design, there will be a reduction in full load efficiency and an increase in full load slip, otherwise the design would be used all the time!!

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[waross]

I enjoyed reading your very well written description of the principle of the double squirrel cage design.
Thanks Marke
Respectfully

 

[electricpete]

I think my #3 above should have more properly been combined with #2. Increasing rotor resistance alone will only give a modest decrease in starting current (which is primarily controlled by reactance). But after you have decreased the current using other means (such as reducing flux) with an undesired side effect of reducing starting torque, you can bring starting torque back with increased rotor resistance.

Regarding the deep bar effect, another way to look at it is as an extension of the familiar skin effect. For the skin effect of conductors in air, we remember that as frequency increases, the current flows more on the outer portions of the conductor (where less flux is linked) which increases the effective resistance. That is the same behavior that we want for motor rotors: higher resistance at high frequency corresponding to starting.

What may be a little counter-intuitive is that for a given cross sectional area of rectangular bar in air, the geometry that maximizes skin effect would be a square. But for a given cross sectional area of rectangular bar within a slot in iron, the geometry that maximizes deep bar effect is a deep thin bar. (the opposite of a square). Also we can achieve the effect for much smaller bars when in a slot than when in air. These differences can be understood by examining the current and flux distributions.


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[electricpete]

Just to fill out the analogy between skin effect and deep bar effect a little further... The top (airgap side) of the bar in a slot plays the same role in deep bar effect as the outside of the bar in air: that's where the current crowds as frequency decreases. Makes it sort of easy to remember to remember the deep bar effect if you already know the skin effect.

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