Auxiliary Motor Speed Impact on Transient Undervoltage?

[ElectoTeknika]

I would like to tap into our motor experts.

I am working on an internal study to improve the ride-through capability of VFDs. This led me to deal with several low voltage induction motors that are part of larger motor’s auxiliary system interlocking system. Cut the story short, I would need to get some interlocked auxiliary process system motors’ contactor circuits not to drop-off on a transient undervoltage less than 85% (say 70% or less). The solution is to connect the contactor circuit from a UPS power. In this regard during a equal or less than 70% voltage dip caused by utility disturbances,

Now these low voltage motors (0.5 to 3 HP) are seal pump and cooler motors for the larger motor’s sealing system in which certain circulating pressure needs to be maintained and hence a certain amount of motor torque and consequent flow rate/pressure needs to be maintained so as not to contaminate the larger pump’s seal (the suction pressure needs to be lower than the seal pressure). WE are planning to consider an auto-restart in 2 seconds at the larger VFD driven main motor. This means, the smaller LV motors will be subjected to 2 seconds voltage dip of less than 85%. If the main motor auto-restart timer lapse and voltage does not recover to > 70%, the VFD protection will terminate the gating process, shutdown the VFD. The auxiliary system LV motors may still be running on a circulating mode.

My questions are universal to all TEFC induction motor between 0.5~3 HP:

1. If UPS is applied to the contactor circuit feeding these motors, the contactor can be held closed during a transient voltage dip of 70% or less. In this regard, the running motor will be subjected to the same amount of undervoltage. In this regard, the speed may not change but the motor torque will weaken, and the torque curve can shift downwards and/or to the left. With the amount of load torque requirement at the shaft, the speed may decrease. Is my assumption correct?
   
   
2. If item “a” is true. Is the amount of decrease in speed owing to the weakening of motor torque be large enough to cause the speed to significantly decrease and place the seal system in jeopardy? The motors are normally 80% loaded by design.
   
   
3. If the VFD auto restart delay is only 2 seconds and will shut down if the voltage does not recover after 2 seconds, will this be a big of a deal with the seal pump motor if we will create an interlock to trip the seal motors when the main motor is tripped or place an undervoltage voltage device with a 5 second time delay?

Thank you for any comments and guidance.

 

[ElectoTeknika]

By the way, the seal pump is a positive displacement pump

 

[waross]

You should be able to hold in the contactors with a VFD.
The smaller motors should be able to produce about 200% of rated torque before they stall.
Centrifugal pumps often shed load faster than they lose torque when slowed down.
Positive displacement pumps may stall.
You may have a two second stall and recovery with PD pumps.
If this is not acceptable then consider changing out the motors for larger motors so as to have a greater torque reserve to carry through low voltage events.
Another option is driving a DP pump with an inverter. This may be either a conventional inverter or a VFD with beefed up DC bus capacity.
jraef can give you the details on a VFD solution better than I can.
I suspect that the larger motor without an inverter may be the most cost effective and dependable.

With the amount of load torque requirement at the shaft, the speed may decrease. Is my assumption correct?

YES
With a constant voltage, and increasing torque demands, the motor slip speed speed will increase to about double and torque will increase to about 200% to 250% of rated torque before the motor stalls.
Motor slip speed: The difference between synchronous speed and rated speed.
eg:
1760 RPM motor, slip speed = 40 RPM. Double rated slip speed = 80 RPM. 1800 minus 80 RPM = 1720 RPM, = 200% torque.
1740 RPM motor, slip speed = 60 RPM. Double rated slip speed =120 RPM. 1800 minus 120 RPM = 1680 RPM, = 200% torque.
(This is a general rule of thumb that may vary from 200% up to 250% in individual motors.)
This gets complicated with low voltage as the available torque drops as the voltage drops.
But with 80% loading and a couple of rules of thumb, you may be able to accept a voltage dip to about 64% of rated voltage before stalling.
DON'T TAKE MY WORD FOR IT. VERIFY THIS WITH FIELD TESTING.

Coolers:
If these are centrifugal fans then these should ride through an under-voltage event.

 

[iop95]

May use a battery bank connected to VFD DCbus.
By contactor or diode and separate charging.
Total battery voltage need to be above max nominal DCbus, so keep them "isolated from VFD" and supply energy to VFD at grid undervoltage only.

 

[jraef]

I’m not sure about the torque increase part being 200%. The REASON the slip will increase is because there is already a LOSS of torque with the voltage sag, close to directly proportional to the magnitude of the voltage sag when the motor is already running (it’s worse if the motor has to start during the sag). So yes, the slip increase could drop the motor into the Breakdown Torque part of its torque-speed curve, which is typically up to 200%, but that will be 200% of a lower value due to the voltage sag, which affects the peak torque capability at the SQUARE of the voltage sag. So if, for example, the sag is 70%, the peak torque will drop to 200% of 49% of nominal torque, so in reality you will be closer to 100% of nominal torque. That still SHOULD allow it to keep running since it was ALREADY running, but the effects of that voltage sag are magnified so much that if the sag gets down to 60%, even briefly, you will not have enough torque to maintain that 80% loading and the motor will stall, especially on a PD pump. There are tables starting on page 10 of this paper that illustrate this issue very nicely (albeit derived from a Matlab simulation).

Bill has a good idea there with regard to using VFDs on the seal and cooler motors. You can buy “ride through” power accessories for the smaller VFDs that would easily allow a 2 second ride through capability, even longer if you add ultra-capacitors or batteries. It’s a form of UPS for the motor itself. With this, you would not need the UPS for the contactors, because you replace them with the VFDs, which will ride through from a control standpoint.