Hi Guys, what is the norm for Starts per/hour for large LV motors 600kW 690Volt 50Hz either DOL or VSD's is there any literature available on this subject.
It's really up to the manufacturer.
Typical for that large a motor is a loooooOOOOOOOOOoong time between starts, likely no more than once maybe twice and hour with the requirement of x running minutes after a start required.
For the large motors, you should always specify the no. of starts per hour during (specifically during cold start and during hot start)during the ordering stage. Normally it shall be the part of motor data sheet.
Now, if the motor is already available at site, search for the motor data sheet. If not readily available, request the motor supplier.
For VFD driven motors, the permissible no. of starts may be relatively more compared to DOL started motor, due to the lower starting current or absence of starting inrush current.
Although a limit on starts per hour is common, I have encountered motors rated in starting seconds per hour. Rating this way addresses the issue of longer starting times for motors with greater inertia loads.
Bill
--------------------
"Why not the best?"
Jimmy Carter
Rough guide would be two starts an hour. For a motor this size, you need to contact the manufacturer. This would be for full-voltage or reduced-voltage. On an adjustable speed drive, these rules don't apply, since the motor is not being "started" in the traditional sense. It is being "run" from 0 rpm to desired running speed so it doesn't experience the heating associated with a normal full-voltage start.
If a motor load needs to be off as part of the operation of the machine cycle, I would suggest using a clutch. Best bet would be a drive depending on the opertion.
Dave
If overheating of the motor in question is a concern, there is always the possibility of incorporating some manner of extra motor cooling and/or winding temperature measurement devices, i.e. RTD's / thermistors, the additional cost having of course to be justified; the motor manufacturer should be able to advise in this regard.
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
It's a tricky one to implement though: a lot heat energy ends up in the rotor, and if the machine isn't spinning the rotor has difficulty dissipating heat. For obvious reasons the rotor temperature isn't the easiest thing to monitor. I would have guessed about four equally-spaced starts per hour, but dpc could be correct, or we could both be wrong. The OEM is the one to ask.
Note I used the phrase "equally-spaced starts per hour". Site operations usually interpret four starts per hour to mean "four consecutive start attempts, then report the dead motor to maintenance". Modern protection relays prevent that kind of stuff, but old electro-mechanical schemes don't. If you have the capability, a DCS historian is very much your ally when it comes to determining what happened to your motor. ;-)
Something to remember about starts per hour is that high inertia loads take longer to start and generate more internal heat per start than low inertia loads. What may be a safe number of starts for a low inertia load may destroy a motor driving a high inertia load.
The number of allowable starts per hour doesns't tell the whole story.
Also the motor design is important. Any-one who remembers the old "U" frame motors will remember the great reduction in the allowable locked rotor time and allowable starts per hour of the "T" frame motors.
Bill
--------------------
"Why not the best?"
Jimmy Carter
For each speed and horsepower rating (up to 250hp), it gives recommendations for:
A = Maximum number of starts per hour.
B = Maximum product of starts per hour times load Wk2 [in units lbm-ft^2]
C = Minimum rest or off time in seconds between starts.
We choose whichever is lower from column A or B depending on Wk2. For low-inertia loads the column A limit is followed. For high inertia loads the starting frequency is reduced below the column A value based on column B.
I've always been annoyed that they bothered to list a minimum rest time between starts (column C) but not a minimum run time between starts. It gives the [false] impression that running time after start is irrelevant. But actually for the first few minutes immediately after start, running is much more effective at removing the starting heat than resting.
=====================================
(2B)+(2B)' ?
Hi Pete. Your use of the phrase NeMA frame motors touches my nostalgia button. Nema "T" frame motors have been around for a long time. Before the "T" frame motors were the NEMA "U" frame motors. The "U" frames would take very much more abuse than the "T" frame motors.
Before the "U" frame motors were the NEMA frame motors.
If you find an old text that lists comparative frame sizes between NEMA frame, "U" frame and "T" frames you will see that between the NEMA frames and the "T" frames the HP has about doubled for a given physical size of frame.
Bill
--------------------
"Why not the best?"
Jimmy Carter
Hi Guys,
Thank you all for contribution I finally got confirmation from manufacturer on permissible DOL starts/per hour ( client specification calls for 6 x hour )
The temperature in Mozambique between September and April is around 45 deg C with 80% humidity. Its a coal conveyor drive, the clients 6 starts/per hour seem extreme to me. I have attached Manufacturers DATA sheet.
The motor is starting under virtually open shaft conditions if the fluid coupling is disengaged, so the starts are light and fast. That explains the relatively large number of permissible starts per hour.
On the other end of that spectrum, I was once asked to apply a soft starter to a 500HP 2300V motor being put into a Banbury Mixer, which for those unfamiliar with them is an absolutely brute force machine used to shred blocks of raw rubber. I was concerned for the "bargain basement" used motor they had purchased because the shaft was only 1.5" dia. And the millwright was fabricating an adaptor flange for it. I finally had to purchase the Westighouse design specs from Teco (who by then had bought them) and it turned out that it was an OEM fan motor designed for 1 start per DAY, in other words requiring a full 24 hour rest time between starts. Needless to say I told them the motor was not suited for that application, the shaft would not be likely to have survived the first start, with or without the soft starter.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
For ATL starting I tend to just use the locked rotor time divided by the time to start to figure out how many starts per hour would be allowed. In this case, the locked rotor time is 16 seconds so each start has to be under 2.6 seconds to achieve the required 6 starts per hour.
For reduced voltage starting, I use the I^2t to compare the start to the motor rating.
Don't forget there's another perspective on frequency of motor starts:
voltage sags / flicker on the rest of the system
Your facility may have issues with starts occurring too often, if it affects other equipment or even causes nuisance light flickers.
Your utility likely has quality standards that dictate a minimum interval of time for energy inrush events (large motor starts).
