Two speed control for NEMA Size 1, 2, 3 480VAC Motor Starters 1

[1bobn29]

I am trying to determine the proper way to control a 2 speed, 2 winding, 3 phase 480VAC motor using a 4 position maintained selector switch. The switch is a "break before make" between positions. Left to right positions are "Fast, Slow, Off, Auto". Is it advisable to provide a time delay (adj. 0 to 10 seconds) between positions to allow the motor to completely stop before selecting the new speed? What affect will switching positions have (i.e. fast to slow or slow to fast) without any delay? The motors are being used for an HVAC project.
Thanks,
Bob

 

[dpc]

You should incorporate a time delay when going from fast to slow speed, but it is generally not required going from slow to fast. The time delay should allow the motor to coast to something close to the slower speed, or slower.

But this can typically be provided in the starter, along with appropriate interlocks, mechanical and electrical, to prevent energizing both contactors at the same time.

 

[1bobn29]

DPC,
Thanks for the advice. I'm using GE CR309 series 2 spd starters which will be mechanically and electrically interlocked.
Bob

 

[jbartos]

Suggestion. Alternately, the motor can be controlled by a manual switch, if sufficiently small. Visit

http://www.sea.siemens.com/contrlbu/upld/files/p41-46.pdf

for more info
Also, there are GEH-XXXX instructions for the GE CR309 series two-speed controllers.

 

[jOmega]

dpc:

Curious as to your rationale for only needing the time delay when switching from high to low speed.

The time delay serves what purpose ?

Please explain.

Thanks

 

[jbartos]

Hint: Saves fuses.

 

[jOmega]

jbartos:
That I agree... but my question was directed at dpc's comment: "You should incorporate a time delay when going from fast to slow speed, but it is generally not required going from slow to fast." ...

Why not both.... and for the same reason....

 

[jbartos]

Hint: The time delay should be implemented in both: the motor acceleration mode and the motor deceleration mode to save fuses, unless there is some reason not to.

 

[jOmega]

jbartos:
Hmmmm. That's an interesting statement. I would offer that acceleration/decleration is not the issue; but rather, the transition from one set of windings to the other.

Hint: stored energy in inductive field(s)

which is why I have asked dpc to illucidate us with why he stated that the time delay is only required when going from high speed to slow speed and not vice-versa as well.

 

[jbartos]

uggestion to the previous posting: I often tie the motor starting with the motor acceleration curve and motor stopping into some load, e.g. decelerating, with the motor decelerating curve. Different motor winding connections will have different acceleration and deceleration curves. I do not think that the stored energy in motor inductive coils is as in SMES. I see the rotary motor as kinetic energy storage in the rotor and attached load dynamics. The motor dynamics needs energy or current input when increasing/accelerating and will release the energy or current when decreasing/decelerating. See Reference:
1. IEEE Std 399-1997 IEEE Recommended Practice for Industrial and Commercial Power Systems Analysis,
Chapter 9 Motor Starting Studies. Figure 9-19 Typical motor speed-current characteristic

 

[jOmega]

jbartos:

I think you took a left turn.

The purpose of timing the transition from high speed winding to low speed winding and vice-versa has nothing to do with acceleration or deceleration.

Consider:

At the moment that one set of windings are disconnected from their source of power, what is happeing at the winding terminations?

What would happen if there was .1 seconds between the opening of one set of windings and the closing of the other set of windings....

Hint: what is the transient condition.....

Hint: do starter contacts open cleanly on all three poles
at the same time ?

Hint: what is transformer action between windings ?

Hint: what is consequence of closing line power to alternate winding before field has collapsed ?

Hint: Big Bang Theory ....