Magnetic Motor Starter

kody916

Member
Location
Illinois
The function of motor starter is to limit the early inrush of the current when the motor starts in order to prevent motor from any damage. How is this thing accompanied in Magnetic motor starter? The reason I am asking this question is because Magnetic Motor Starter is a combination of a Coil ( for contactor) and Thermal Overloads. So when the coil is energized the contacts close and the motor is subjected to direct online voltage. Isn?t this gona cause damage to motor? How does magnetic motor starter avoid this damage?
Also there is a reset button in magnetic motor starter. Is it used for resetting the coil after fault has been cleared or does it serve any other purpose? I read somewhere that it is used for re setting thermal overload relay? How does this reset button work?

Thanks
 

RichB

Senior Member
Location
Tacoma, Wa
Ok let's take these one at a time

1. A magnetic starter starts the motor by connecting it across the line.
this causes no damage to the motor, however in cases where there is a large starting torque then some sort of soft start is preferable to reduce the initial current draw to an acceptable level.

2. The thermal overloads are there to protect things in the event of an overcurrent situation, like a conveyor being overloaded. These generally have a small drop of solder that holds a spring loaded set of contacts closed. When the solder melts the contacts come open thus de-energizing the starter--the reset button is pushed once the fault is corrected and the solder has cooled thus reclosing the contacts and allowing a restart.

I have been out of that part of the trade for a long time now and if there are any inaccuracies I hope some one else will hop in and correct them or make it clearer.

Hope this helps..

Rich
 

Besoeker

Senior Member
Location
UK
Ok let's take these one at a time

1. A magnetic starter starts the motor by connecting it across the line.
this causes no damage to the motor, however in cases where there is a large starting torque then some sort of soft start is preferable to reduce the initial current draw to an acceptable level.

2. The thermal overloads are there to protect things in the event of an overcurrent situation, like a conveyor being overloaded. These generally have a small drop of solder that holds a spring loaded set of contacts closed. When the solder melts the contacts come open thus de-energizing the starter--the reset button is pushed once the fault is corrected and the solder has cooled thus reclosing the contacts and allowing a restart.

I have been out of that part of the trade for a long time now and if there are any inaccuracies I hope some one else will hop in and correct them or make it clearer.

Hope this helps..

Rich
Not a lot to add.
Sometimes a soft-start is used for mechanical reasons to reduce shock on the driven equipment.
 

GoldDigger

Moderator
Staff member
It seems to me that a solder based thermal trip would be very hard to reset. I would instead expect it to be a thermal mechanical trip using a bimetal element similar to what is used in a breaker.

Tapatalk!
 

electric_cal

Member
Location
California
The function of motor starter is to limit the early inrush of the current when the motor starts in order to prevent motor from any damage. How is this thing accompanied in Magnetic motor starter? The reason I am asking this question is because Magnetic Motor Starter is a combination of a Coil ( for contactor) and Thermal Overloads. So when the coil is energized the contacts close and the motor is subjected to direct online voltage. Isn?t this gona cause damage to motor? How does magnetic motor starter avoid this damage?
Also there is a reset button in magnetic motor starter. Is it used for resetting the coil after fault has been cleared or does it serve any other purpose? I read somewhere that it is used for re setting thermal overload relay? How does this reset button work?

Thanks
The function of a motor starter is to control the motor, not to control the early inrush of current to the motor during start. The purpose of the overload relay is as the name describes, to monitor the current flowing through the motor feeders and motor. If the overload relay is sized correctly, the momentary inrush during start is not enough to cause the overload relay to trip open. The reset you ask about is to reset the contacts in the overload relay if it has tripped open. The contacts are generally used as the holding circuit for the motor starter coil.

The operation of the overload relay is based upon the amount of heat being generated by the amount of current flowing through the motor feeders. Current = heat. The internal mechanism in the overload relay reacts to the heat very much the same as a set of contacts do in a thermostat. Excessive heat causes the operating plunger to trip a paddle which will cause the internal contacts to open. This is a simplified description, but I hope it helps to explain the function of the overload relay. You can get a much more detailed description at any of the manufacturers websites.

As far as controlling the motor, what else would you use? A fused or non-fused disconnect switch operated by hand? A direct connection to a circuit breaker? A means of safely starting, operating, and stopping the motor is necessary to protect both the equipment being operated, and the worker using it.

As far as current inrush to the motor during start, the motors are designed for this momentary inrush of high current. The overload relays purpose is to shut down the system if a problem developes during start, and or run.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
It seems to me that a solder based thermal trip would be very hard to reset. I would instead expect it to be a thermal mechanical trip using a bimetal element similar to what is used in a breaker.

Tapatalk!
Solder or eutectic (sp) are not hard to reset. The trick is not to reset them until they cool. Repeated resets while not quite cooled ruins them. AB & CH used them for years.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Solder or eutectic (sp) are not hard to reset. The trick is not to reset them until they cool. Repeated resets while not quite cooled ruins them. AB & CH used them for years.
SquareD did too. Also you just can't install the heater upside down or us a wire jumper to bypass this type of heater...at least you couldn't do that with the AB or SquareD not sure about CH.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Seems a semantics lesson is in order.

Motor starter: a device that starts and stops a motor. A motor starter is a combination of a switching device and an Over Load protection method. Can be manually operated or electrically operated. With a manual motor starter, the switching device remains it it's previous state if power fails, so when power is restored the motor can re-start immediately without warning.

Magnetic motor starter: a version of the electrically operated motor starter, called such because it uses an electromagnetic coil to close the contacts in a "contactor". The advantage over a manual motor starter is that if power fails, the coil drops out and opens the contactor, allowing the control circuit designer to employ safety protocols to restart the motor. The term "magnetic" was also necessary back in the beginnings of electric motor use because there was an alternative; a motorized automatic motor starter. They are no longer in existence other than museums, although the same concept is still occasionally used with large medium voltage motors where vacuum breakers are used as the control device. Still, the use of "Magnetic Motor Starter" persists.

"Starting" Current: The high surge of current that a motor will draw during acceleration when the inductance is low due to the rotor not yet fully interacting with the stator fields because it is not at speed. Starting current on a common "Design B" motor is typically 500-600% of the rated FLA, and persists until the motor reaches around 80% speed. On lightly loaded motors with low inertia, that may be only a second or two; on high inertia, high friction loads it may be a matter of minutes.

"Inrush" Current": The instantaneous current drawn by the stator winding when there is NO inductance to limit it, only the stator winding wire resistance. Inrush current persists for only a fraction of a second (less than 2 cycles) as the magnetic fields in the stator windings create magnetic flux in the steel core. Inrush current can be anywhere from 1300 to 2200% of the motor FLA , depending on design of the motor windings and the point of the sine wave at which the contactor closes. Unfortunately the term "Inrush Current" is often mistakenly used to describe "Starting Current"; they are NOT the same.

Reduced Voltage Motor Starter: A method of starting a motor by using a lower voltage (or lower effective voltage) in a stepped or ramped manner to decrease the voltage and thus the current going to the motor for the purpose of reducing the Starting Current (not "Inrush") drawn by the motor, and thereby reducing the torque that the motor produces during that time. RV motor starters can again me manual or electromechanical, but can also be solid state. Manual RV starters are all but forgotten now too.

Magnetic Reduced Voltage Motor Starter: any one of a variety of RV motor starters using magnetic coils in contactors to operate. There are several sub-methods of reducing the effective voltage in that system, all with different pros and cons, but all involve at least 2 steps and EACH step will have a spike of current and torque associated with it, albeit smaller and/or of shorter duration. So I like to describe them in terms of something a bully once offered to be: Do you want one hard punch to the stomach or two softer ones to your face? It's still going to hurt...

Solid State Reduced Voltage Motor Starter: , a.k.a. "Soft Starter", an SCR based RV starter method that gradually ramps the voltage going to the motor, so there is no transition spike of current or torque.

Over Load Relay: a device added to a contactor that senses motor current and interrupts the control circuit voltage (or in the case of a manual starter, releases the holding latch) so as to stop the flow of current to the motor. Can be Eutectic Melting Alloy (solder), Bi-metal, or now Solid State. ALL versions are designed so that the amount of time it takes to reset it matches the expected cool-down rate of a standard motor operating at 40 degrees C (104F) in an ambient environment of 25 degrees C (77F). None are "easier" to reset that the other. Melting alloy OL relays are more accurate and repeatable, but not able to be compensated automatically for changes in ambient operating temperature. Bi-metal can be made that way, but sacrifice accuracy and can "wear out" over repeated trips by building up "thermal memory", meaning they start to trip faster (not fail to trip). Solid State are now much better than both and can add effective phase loss, phase imbalance and arcing ground fault protection.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Although the purpose of a reduced voltage starter is to reduce the starting current, as a side effect it will also reduce the inrush current, yes?

Tapatalk!
In an Autotransformer starter or reactor, no, because you have inrush on the transformer or reactor too. In a Part Winding starter, there is inrush at full levels for the part of the winding you energize, but the second winding already has flux around it by then. On a Wye Delta starter there is a reduction based on the lower effective voltage in Wye, but the transition spike when changing from Wye to Delta can be just as bad or WORSE than the inrush spike. The only type that truly is capable of reducing inrush current is solid state. But Soft Starters often come with a "feature" called Kick Start (or Pedestall Start) that allows users to goose the motor at full voltage for a second or two in the very beginning to overcome friction, icing or cold bearing grease and people who use it often forget that it will also eliminate the reduction of inrush as well.
 
Awesome posting Jraef. I think I might copy both of your posts and put them in my archive of technical references. Are you in the starter business or just extremely knowledgeable about starters and motors?

w piper
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Awesome posting Jraef. I think I might copy both of your posts and put them in my archive of technical references. Are you in the starter business or just extremely knowledgeable about starters and motors?

w piper
After changing careers due to injuries (both physical and financial), I have spent the last 20+ years (oh lord...) working for manufacturers in various roles involving all aspects of motor control, drives and soft starters, both LV and MV. I take it very seriously because when I started out as an industrial electrician at a steel mill, hardly anybody knew anything about controls, but I noticed that the guys who did were not busting their knuckles on pipe and wire as often as I was. At heart I'm a lazy slob, so hard work makes me look elsewhere...
 
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