Motor Starting Characteristics

[Marke]

Those of us in the 50 Hz world have seen a change in motor designs in recent years, particularly the rotor design with a push for higher efficiency motors. The high efficiency is often achieved by the use of lower impedance rotor bars which results in lower slip losses. The down side of this has been that the high slip operating conditions have been severely compromised. This shows up as an increase in the Locked Rotor Current of the motor.

I have just completed a design project whereby two pumps are to be run of a diesel generator set. One pump is 75KW and the other is 37KW. The 75KW pump always will be started first. In order to optimize the generator set sizing, I went on the hunt for a 75KW 2 pole motor that had a high "starting efficiency". To my dismay, I found that although years ago, it was possible to find motors that would enable me to start this pump at around 300% start current, the modern motors needed start currents of around 400 - 450% The best I found was 350%. I do not know if there has been a similar trend in the Nema range of motors.

To me, this may have been a gain in the full speed running efficiency of motors, but it is a definite backward step when it comes to starting the motors. On weak supplies and standby generators, the starting current is very important. Pump stations with only one pump running from a dedicated generator have to have an engine sized for the start conditions. This results in an oversized engine which runs lightly loaded and quickly suffers glazed bores.
Perhaps as engineers, we need to bring some pressure to bear to ensure that the starting characteristics of induction motors are taken seriously by the regulatory bodies as well as the running efficiency.
I am interested in the comments and experiences of others.
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[dpc]

What I have seen is a great increase in the transient asymmetrical current during starting for high efficiency motors. This is due to the increased X/R ratio for high efficiency motors. The NEC has been modified over the years to allow higher settings on motor short circuit protection to allow for this increased transient current.

I believe that the symmetrical locked-rotor current also increases as the efficiency is increased.

There is also a definite design tradeoff between efficiency and running power factor.

At one time there was a push to revive the old NEMA Design A category to allow for higher efficiency but with starting currents that are greater than the maximum allowed for NEMA Design B motor. I'm not sure how far that progressed.

 

[acmotorengineer]

Mark - We started this trend in the 1970's. Motors run more efficiently at less slip both in the 60 hertz AND 50 hertz world --- and subsequently higher starting current --- at NO INCREASE in cost. Designing motors for low starting current is a major reduction in running performance and the cost of the motor. The cost of energy is becoming a major expense. Why penalize the continuous run for a transitory start. If you need lower starting power - use a reduced power starter and you have the best of both conditions, the transitory starting and continuous run.

By the way, I would like to meet the engineering designer that could get 300% current, across the line start with Design B (100%) torque.

 

[Marke]

Hello acmotorengineer

I primarily use soft starters to start motors. That way we can minimise the start current to what is required to get the machine up to full speed.
The 300% was what we commonly achieved for pumping applications using soft starters. The drift in starting characteristics has made this a rare result now.

It is posible to design with contoured rotor bars to give a reasonable starting characteristic as well as a high efficiency. If the bar is relatively thin and penetrates well into the rotor, it will have the same DC resistance and a higher reactance. This type of profile will concetrate the current on the outer portion of the bar during start and yeild an affective increase in the bar under high slip conditions. A little like the old double cage technology.
Looking over old data, the range of LRCs of motors was typically 600% to 800%. From what I am seeing now, it has increased to 700% to 900%. This make a big difference if you reduce the voltage and try to achieve 300% start currents.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[aolalde]

In spite of the standardized motor designs that can be purchased from stock, custom made motors to fit specific load requirements can be ordered.
Obviously they have a premium cost and delivery time but this can be programmed in advance.

 

[DickDV]

Marke, if you have some control intelligence somewhere in your system, you can place 8% line reactors in the motor leads and bypass them with a running duty contactor when they are up to speed.

I offer this only because many softstarters don't take well to generator power whereas the line reactors hardly care about power quality.

 

[Marke]

Hi DickDV

My comment was on the change in rotor design to improve the running efficiency of induction motors and the consequential loss in the starting or high slip efficiencies. Unfortunately, the change was very much a case of robbing Peter to pay Paul. I believe that there could have been a better compromise in many cases where the efficiency was improved without compromising the start characteristics.
In many cases, the starting current is a major issue and the changes have made this issue much worse to the extent that if you need to limit the start current to a machine, it is much more difficult to find a motor with good starting characteristics than in the past. Using a reduced voltage starter, the minimumstart current is determined by the minimum start torque and the ability of the motor to convert amps into newton meters.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jraef]

Marke,
As ACmotorengineer stated, we have been seeing higher starting currents in NEMA design energy efficient motors for quite some time now, and the US NEC has had to deal with the repurcussions by making changes to acceptable trip levels in circuit breakers to allow higher settings as a result. But as you pointed out, an oft neglected aspect of this is what I have heard described as starting current efficiency, meaning that at an equal amount of current, the motor produces less torque now than they used to until after it is at full speed.

This was described well in a white paper put out some 10+ years ago by the "Motor Challenge" people, now part of the US Dept. of Energy. I have searched for that paper in recent years and have been unable to locate it unfortunately (but to be honest I didn't spend a lot of time). What it acknowledged was that although starting amps were higher, starting torque was actually lower. It warned users who were switching from old design to new design that if their application required high starting torque, they were either going to need to keep the old motor, or be prepared to pull a lot more current to get it. This paper did not address the issue of how that affects reduced voltage starting, but extrapolation would not be too difficult. Bottom line, it confirmed what you have observed.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[DickDV]

Marke, you are certainly rightly right about the starting currents going up while the starting torques, in many cases, have gone down. It makes setting up a softstart much more delicate because, as you know, starting torque drops off as the square of the voltage reduction.

As you pointed out, just getting the motor to start itself, much less any load, typically takes 300% nameplate amps or more.

I work in the inverter world mostly so all of this has worked in my favor but, whenever I do a softstart job, I see the same issues you do. And it doesn't seem to be much different with IEC or NEMA motors which makes me conclude that there is no easy way to somehow straddle the issue.

Of course, I'm a motor USER, not a motor DESIGNER, so, maybe there is a silver bullet out there somewhere.

Let's see now!!! Someone want to get rich quick?????