Running an Induction Motor in Delta Connection Instead of Star

It could be that the motor is working too hard. The bearings might have bad lubrication or the load is a bit too much for the motor. Is it a two-pole wound rotor with 3-phase stator?

GA

AC motor speed is related to the applied frequency and the number of poles in the design. It is then further delineated by design characteristics that determine the "slip speed" of the motor, i.e. how much the actual running speed differs from the synchronous speed. Factors that determine slip are rotor resistance and load. As you increase load, slip increases and the motor pulls more current as it tries to "catch up". But if you are observing different slip speeds on identical loads, then the difference is probably rotor resistance, in other words the motor was made that way. This may have been on purpose, it may also just be a quirk of that particular motor. Most likely it was on purpose, you may have ben observing a "high slip" motor design, what we in North America would call a "Design D" motor. They have their purpose but are not common.

This article explains it in more detail.

PS: The issue of suing a different motor connection has to do with a false notion that changing it will save energy and people do it with surprising regularity. It's not true, but that doesn't stop them. What it does is change the torque capacity of the motor, which if the load is the same, allows for higher slip. But if the load is the same, it also risks stalling.

This may come as a silly question, but did you look at the manufactures rating plate. You may be chasing a problem that doesn't exist.

Hi All,

Thanks for all the responses.

From vibration point of view bearing lubrication is OK. This can be gleaned from the spectra. Load could be an issue.

I haven't looked at the nameplate and will do so next I am on site. I don't expect it to show a speed this low though. In my vibration work I constantly check the exact speed motors run and never found any this far below the speed they should do given poles and LF.

So from JRaef's reply it would seem possible that if it were connected opposite the intended it changes torque capacity allowing for higher slip for a given load. Then it would seem it's a matter of how much extra slip.

I will run this by the site's electrical people when I am there in a months time. Thanks again to all responses. rgds

Factors that will hinder ur attaining synchronious speed are as follows:

1) System voltage as compaired to ur motor rated voltage.

2) System frequency as compaired to ur motor rated frequency.

3) If the PF of ur motor is low, the slip will be more.

If the above are in place with ur system requirement, it could be a design spesification if the motor is not runing hot and have not been rewound in error. Bearing faults and unballanced winding could led to severe vibration that will further increase the slip speed.

Motors that are consiquently connected have very high slip than motors with normal pole connections. Since no consequent pole connection will produce a two pole connection, ur answer will not be far from the previous and present answers.

Dickson.

Let us calculate, ASSUMING the motor you are talking about is a 3-phase INDUCTION MOTOR with squirrel cage construction.

N (RPM) = 120 x f / p ; substituting f = 50 Hz and p = 2

N (RPM) = 120 x 50 / 2 = 3000 RPM

On no-load condition, for INDUCTION MOTOR the slip is 3% and hence ROTOR speed reduces by that much.

Therefore, Shaft RPM = 3000 * (1 - 0.03) = 2910 RPM. The slip could be slightly less on latest EFF-1 and EFF-2 type or special motors.

Having said that, speed is directly proportional frequency. So, check the motor RPM in NO-LOAD condition, if possible without any connection to its shaft. Measure FREQUENCY and compare.

If all are fine, the motor could be mechanically restricted from rotating or is being overloaded. But then if this condition exists, motor's OVERLOAD protection must trip.

Also, speed reduces if the gap between ROTOR and STATOR is more. This might happen if motor is rewound too many times.

Trust this starts some thinking process !!!

www.maintenancecircle.com

Also, speed reduces if the gap between ROTOR and STATOR is more. This might happen if motor is rewound too many times.

would you explain this statement please?

Thanks

If it gets to the stage of replacing laminations it's usually more cost effective to replace the motor.

I've seen laminations replaced just the once in 39 years, but that was a special case. Your not going to be able to phone your supplier for a 100MW combined motor generator and get one off the shelf. The stator was rebuilt in situ from a bare shell.

This is a slip of just over 3%, perfectly normal if the motor is operating at full load.

Cheers

Martin

Correct.

I have seen some motors used in process plants, where designed with 6 wire for Star/Delta. The applied voltage and current consumption is varied in the connection but not the Speed. As said earlier connection does not relate to speed of the motor.

Hello, thank you very much for the support.

Is the induction motor is also called synchronous motor ?

No. Synchronous motors, as the name implies, run in synchrony with the AC line frequency. A two-pole synchronous motor will run at 3000 rpm at 50 Hz. or 3600 rpm at 60 Hz. What happens under load is that the armature drops a bit behind the position it holds in the rotating magnetic field under no-load conditions. If it is unable to keep up with the field, it generally stalls. Synchronous motors are preferred for motor-generator sets with AC in and out, and for VFD systems where precise speed control is important.

An induction motor, on the other hand, slips constantly when it is loaded, so its speed can be and generally is less than the synchronous speed. Induction motors are also generally cheaper to build than synchronous motors except for very low power applications such as electromechanical clocks and timers.

Please read this from other thread:

18 power thyristor module are used for controlling 6.6 Kv Synchronous motor's Rotor with Exciting Voltage of 60 Vdc, and 375 Exciting Current

Is that mean the speed of synchronous motor can be changed by using thyristors to control the motor's exciting ?

No, the thyristor module is used to implement a variable frequency drive, the motor remains "synchronous" to this variable frequency; slip in a synchronous motor is usually a very bad thing.

No.

No. An induction motor is an ASYNCHRONOUS type of motor wherein the ROTOR is always trying to CATCH UP with Stator but will not reach and the difference is called SLIP which is typically 3% under no-load conditions.

Is that means, the Induction motor can run with slip but synchronous motor not ?

Thanks for help.

Yes. If a Synchronous motor "slips", it slips an entire pole, usually with dire consequences.

Thanks for your answer.

I have another question, now i understand that both synchronous and induction motors are supposed to run with fixed speed,is that mean we can't make variable speed control for those motors ?

Not without varying the voltage AND the frequency proportionally.

That's the purpose of a VFD (Variable Frequency Drive).

Thanks for your reply,but i can see many commercial AC motors with SCR (phase control)to control the speed ?

Only a couple of specific types of single phase AC motors can be speed-controlled with SCR voltage controllers.

1. Shaded Pole motors, which are usually small light duty applications like ceiling fans and aquarium pumps. The "speed" is actually not directly controlled, but the lower voltage reduces the torque and the lower torque allows the load to increase the slip severely, but a Shaded Pole motor can run fine at high slip without damage.
2. "Universal" motors which are actually an AC/DC hybrid like motor, usually found in portable power tools and small appliances.

Sometimes people use SCR voltage control on what are called "vibrator motors" which are 3 phase high slip designs, i.e. 10% or more, for vibratory feeders because the application is narrowly specific and at lower 'speeds" (really lower torque again) the idea is that the feeder will be doing less work, similar to a centrifugal load.

Speed of the motor is dependent on supply frequency, no. of poles and rotor resistance also. If the bearings and lubrications are O.K.,the speed of motor may decrease with overload condition whereas motor current will increase w.r.t. F.L.A..If the motor is rewinded, with replacement of rotor copper bars, and if the used copper bars have different resistance ( due use of different sizes of copper bars/different quality of copper conductors etc.), the rotor resistance will differ w.r.t. to original design value and hence speed can drop ( or slip can increase)

Hi

Got another answer for you to consider

Have you checked for Harmonics?

Negitive sequence harmonics, 5th, 11th, 17th etc, will cause heating (additional) in the motor because they try to run the motor slower than its designed speed, because negitive Harmonics create a reverse rotating magentic field within the motor.

Positive harmonics, 7th, 13th, 19th etc still cause heating but have the opposite effect.

This information I obtained from Fluke, makers of Power Quality Analysers