Alkin W31 Compressor question

[mattjm]

Why do we want to rely on the house breaker to protect the motor? Breakers are there to protect the house wiring from a device (usually multiple devices) drawing too much power and overheating the wires. The motor should have it's own fuse or breaker to shut itself down if it draws too much current.

By the time a 12 amp device is drawing 20 amps and trips the house breaker, it may already be severely damaged.

Matt

 

[skippy31]

The house breaker is there to protect the wires and the load. For something like a compressor that will be dedicated on that circuit we use the breaker to protect both the wires and the device.

You do often see a fuse/overtemp fuse added to many small or electronic appliances that normally do not draw anywhere close to the maximum available from the circuit.

Many electric motors have an overtemp fuse built in as well.

Mat.

 

[Scuba Duck]

If your compressor has a latching motor starter (sometimes called an inductive or magnetic starter), it probably also has the corresponding thermal overloads. In the event the motor draws too much current, the overloads heat up and trip, much like a circuit breaker. Commonly found on ebay if your compressor didn't come with one. The latching/magnetic feature prevents your compressor from restarting in the event it trips off for some reason.

eBay Store - ElectricMotorSite: Electrical Controls: NEW Siemens Three Phase 7.5 HP Magnetic Starter NEMA 1

 

[cool_hardware52]

If your compressor has a latching motor starter (sometimes called an inductive or magnetic starter), it probably also has the corresponding thermal overloads. In the event the motor draws too much current, the overloads heat up and trip, much like a circuit breaker. Commonly found on ebay if your compressor didn't come with one. The latching/magnetic feature prevents your compressor from restarting in the event it trips off for some reason.

eBay Store - ElectricMotorSite: Electrical Controls: NEW Siemens Three Phase 7.5 HP Magnetic Starter NEMA 1

This is huge overkill for a small single phase motor. Some, not all, single phase motors have an internal "overtemp" switch that will interrupt the motor if the temperature exceeds some value. Most of these over temp switches are "auto reset". i.e. they close when the temperature drops. Such motors will indicate that they are "thermally protected" on the motor name plate.

The reason why 3 Phase motors need an overload relay is that the motor will still run on two of three legs. In other words if you have an overload on one of the three legs, and only have a fuses, the fuse on the over loaded leg blows, the motor "single phases" of tries to operate on the remaining two legs and burns up.

A 3 pole overload relay will open all three legs if any one leg sees an over current condition.

With a single phase motor if you "open" either leg it will stop. That means a fuse or circuit breaker is sufficient to protect a single phase motor.

Latching starters or relays or "mag starters" have a place on single phase motors. They are useful if you need ensure the motor won't restart after a power failure. Continuous blending of nitrox is a classic example of an application where you do not want the compressor to "auto restart" after a power failure.

You might also have other interlocks that can control the motor. For example if you have a low oil pressure switch that can stop the motor, High cylinder head temp switch switch that can stop the motor etc. These "control switches" usually don't have the current capacity to directly switch the motor load, but they can operate the coil in the "mag starter"

Motor controls are a specialty area. If you want to add / modify motor controls you need a qualified motor control technician. This is outside the norm for many general electricians.

Tobin

 

[Scuba Duck]

My appologies for not going into detail regarding the intricasies of motor controllers. For those of us who want to be able to plug the air compressor into the dryer outlet, and not change out the breaker, (I still need to wash and dry clothes sometimes) the use of an appropriately sized motor controller with appropriate thermal overloads is wise. The use of the magnetic coil in the controller also affords many options as you mentioned, i.e. various temp and pressure shut downs. The operation of the motor controller is not that complicated, the devices (temp or pressure switches such as barksdale Barksdale Inc | Series 96200) are wired into current path for the coil such that they open at desired temp or pressure to interrupt the current to the coil, the coil unlatches, and the motor stops. The only way for the motor to restart, is for those conditions that caused the corresponding safety action to clear (temp or pressure to drop below level at which the switch changes state) so that the current path for the coil is restored. And for the magnetic starting sequency (ie push the start button) to be initiated. The start button is put in parallel with the ouput of the main contacts. The start button is also of a momentary type, when you push the start button, current runs through the coil to close the main contacts. When you release the start button, the power to maintain the coil energized comes from the load side of the main contacts. You can shut down the motor by pressing the stop button, which interupts current through the coil, or if any of the safety devices (temp or pressure switches) changes states and interrupts current through the coil.

This type of motor controller can be used with single or three phased motors of any size.

 

[cool_hardware52]

My appologies for not going into detail regarding the intricasies of motor controllers. For those of us who want to be able to plug the air compressor into the dryer outlet, and not change out the breaker, (I still need to wash and dry clothes sometimes) the use of an appropriately sized motor controller with appropriate thermal overloads is wise.

This thread concerns a small compressor powered by a single phase motor. Many single phase motors have internal thermal overloads, including the 3 hp single phase motor furnished on the W31 compressor that this thread concerns. Adding a mag starter / overload relay to such a motor adds nothing in terms of protection for the motor against overload. If you want over current protection for a single phase motor a fuse or circuit breaker can provide it at much lower cost.

The use of the magnetic coil in the controller also affords many options as you mentioned, i.e. various temp and pressure shut downs. The operation of the motor controller is not that complicated, the devices (temp or pressure switches such as barksdale Barksdale Inc | Series 96200) are wired into current path for the coil such that they open at desired temp or pressure to interrupt the current to the coil, the coil unlatches, and the motor stops. The only way for the motor to restart, is for those conditions that caused the corresponding safety action to clear (temp or pressure to drop below level at which the switch changes state) so that the current path for the coil is restored. And for the magnetic starting sequency (ie push the start button) to be initiated. The start button is put in parallel with the ouput of the main contacts. The start button is also of a momentary type, when you push the start button, current runs through the coil to close the main contacts. When you release the start button, the power to maintain the coil energized comes from the load side of the main contacts. You can shut down the motor by pressing the stop button, which interupts current through the coil, or if any of the safety devices (temp or pressure switches) changes states and interrupts current through the coil.

This type of motor controller can be used with single or three phased motors of any size.

A mag starter needs to sized for the motor in question. It should have the correct number of poles, any required axillary contacts, and a coil voltage appropriate for the application.

If the application requires an overload relay, (a single phase 220 volt 3 hp compressor with a thermally protected motor does not) the availability of heater coils of the correct current rating should also be verified.

Tobin

 

[Scuba Duck]

The link below is to a 3 HP, single phase, magnetic motor starter/controller with overload protection. The start/stop switch is additional. You can get a start/stop switch the that mounts directly on the controller enclosure or one to mount in a remote location, depending on your application. It is true, you should obtain the motor controller that is of the correct size and type (single vs 3 phase) for your motor, I would advocate nothing less.

NEW Siemens Single Phase 2 HP or 3 HP Magnetic Starter - eBay (item 230097714423 end time Feb-21-08 11:28:48 PST)

 

[CraigAClark]

I use mag starters (contactor + overload relay) in most of the machines we assemble, with the exception of those where a frequency drive (device is programed with motor information and will respond as overload protection) is installed. Besides allowing for control circuit switches and motor overload protection, any "users" (starter coil, clock, TDRs, CRs, repeat cycle timers, drain solenoids, etc.) can have one leg run (neutral on 115v or 115v leg on 220v) through an aux relay on the overload to protect these devices. This of course is in addition to the supply power being fuse protected.

Craig