When to select a slow start motor 1

[Lovison]

Most of the motor's I've seen are across the line.

What are some of the rules that one should consider when opting to go to a slow start? Wayne E. Lovison
service-parts@naglepumps.com

 

[electricuwe]

There are lots of aspects which have to be considered, I'd like to post just the most important ones :

Size of the motor:
The p.u. starting current for bigger motors is higher than for smaller ones

Available short circuit current at the PCC:
On a strong grid (e.g. in a power station) a high starting current can be tolerated whereas at the end of a long line in a rural area thats not possible.

Type of load:
For loads having an high inertia (e.g. a conveyor) slow starting will be usually neccessary

Number of startings per hour:
If a motor is started very frequently, slow starting might be necessary to limit the dissipated power in the rotor.

 

[dpc]

electricuwe is right on the mark. There are really two basic reasons to consider reduced-voltage starting:

Protecting the driven equipment from high starting torques

and

Limiting the inrush current to reduce the system voltage drop caused by motor starting.

As electricuwe mentions, there are also cases where it is desirable to limit starting current to protect the motor - this is generally for motors that are started and stopped very frequently - many times per hour.

 

[Lovison]

electricuwe & dpc

Is this a general rule that comes in specifications or can we stipulate that this type of starting is required because of:

1. The size and torque of the motor.
2. The long length of the driven shaft.
3. The possibility of frequent start/stop cycles

Across the line starting of a long extended shaft will have a momentary delay before the rotation begins at the bottom of the shaft this is my concern as well as the electrics.
Any thoughts on this? Wayne E. Lovison
service-parts@naglepumps.com

 

[buzzp]

How long is this? I think your concern is a valid one. I have seen across the line starts on shafts as long as 90' with monel shafts. This doesn't include the 45' of shaft within the motor (submersible). Certainly, a soft start configuration would reduce the twisting affect at the end of the shaft since the start-up torque will be reduced with a soft start.

 

[dpc]

I don't think there is a standard criteria that you can apply that will always give a correct answer. It is application specific.

For cases where the concern is the torque on the driven equipment, the pump manufacturer should provide data on the maximum allowable torque.

 

[electricuwe]

I agree to dpc. It wouldn't be a good argument to sell a pump if a pump manufacturee states that a slow start is requiredgenerally since this means additional cost for the installation. But a pump manufacturer should supply the information to check if slow start is necessary from the mechanical issues.

 

[Lovison]

You're all right. I agree we do normally provide torque requirements for all our units. And the weakest point is always the coupling end or the stud end on the shaft.

It's a design issue of which we do address. And yes some of the settngs will be 55' or longer. I only ask these questions because should an event in the field occur I want to have the right answers. Yes they are applications specific but I was looking for rule of thumb guidelines.

I believe vari-speeed packages have allowances for such slow start and ramp provisions. The cost have come down dramatically for such these electrical provisions.

I appreciate the feedback and will check with engineering on the issues we addressed. Thanks Again. Wayne E. Lovison
service-parts@naglepumps.com

 

[jbartos]

Suggestion:
1. In some applications, e.g. the auxiliary system in power generating stations, the induction motor started with the full voltage across its terminals is widely used. Apparently, there is a plenty of electricity and good power supplies; therefore, the soft starts would just be an extra inconvenience, e.g. higher cost, lower reliability and availability, higher maintenance and operation costs, bigger real estate requirements, etc.
2. If one is constrained by the power distribution system, and one needs a higher HP motor, still within the power distribution capability, then the soft motor start (reduced voltage across the motor terminals) is a reasonable solution.