motor controllers

jim123

Member
Location
new castle,de
just got in a square d motor controller for a 5hp two speed,3 ph motor.It has a high speed , a low speed , and off. [THE SWITCH IS ALL IN ONE , A THREE POSITION SWITCH }I thought when u went from High speed to low speed u needed a time delay to stop damage to the motor and a third contactor to short the low speed windings when running it in high speed. You can go from high to low instantly and U can hear the motor straining when that's done . Before I bring this to supervision just wondering what u guys think.Or because its a small motor I should not worry about it
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
just got in a square d motor controller for a 5hp two speed,3 ph motor.It has a high speed , a low speed , and off. [THE SWITCH IS ALL IN ONE , A THREE POSITION SWITCH }I thought when u went from High speed to low speed u needed a time delay to stop damage to the motor and a third contactor to short the low speed windings when running it in high speed. You can go from high to low instantly and U can hear the motor straining when that's done . Before I bring this to supervision just wondering what u guys think.Or because its a small motor I should not worry about it
This type of time delay is not a 'code' requirement, it is strictly a design issue.

Some systems require an 'accelerating' delay when going to a higher speed, while others actually require a 'deceleration' delay, still others may require forced starting in one speed before changing to the other.
 

GoldDigger

Moderator
Staff member
This type of time delay is not a 'code' requirement, it is strictly a design issue.

Some systems require an 'accelerating' delay when going to a higher speed, while others actually require a 'deceleration' delay, still others may require forced starting in one speed before changing to the other.
The blower motor controller in my evaporative cooler (3/4 horse two speed motor) stops the motor for ~3 seconds for both high to low and low to high transition).
 

kwired

Electron manager
Location
NE Nebraska
Electrically you need a transition time - but can be in the millisecond ranges. Mechanically - would depend on other factors - and primarily in the driven load.

I have seen several reversing applications where the only delay is the time it takes for contactor to open, clear any interlock contacts, and close the opposite direction contactor. If this is done frequently enough you may need to derate the motor if you want it to last longer. I really don't see changing speed as being as hard on a motor as changing direction with no delay.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
Electrically you need a transition time - but can be in the millisecond ranges. Mechanically - would depend on other factors - and primarily in the driven load.

I have seen several reversing applications where the only delay is the time it takes for contactor to open, clear any interlock contacts, and close the opposite direction contactor. If this is done frequently enough you may need to derate the motor if you want it to last longer. I really don't see changing speed as being as hard on a motor as changing direction with no delay.
Changing directions is pretty different than just changing speeds; primarily due to reversing power flow and magnetic field issues in a single set of windings (typically only 2 contactors are required). Of course, mechanical issues (e.g. load inertia) must be considered for any driven equipment. Time delays are fairly common.

Speed change can be as simple as two different sets of windings, effectively two motors, on a single shaft (typically two contactors required), or as complicated as actually reconfiguring the windings (typically 3 contactors are required). The mechanical issues of the driven load usually dictate the need for time delays. Time delays seem to be pretty much non-standard except for certain applications. One of the selling points of VFD's is their ability to change speeds quickly.
 
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