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Member postings for Steviegtr

Here is a list of all the postings Steviegtr has made in our forums. Click on a thread name to jump to the thread.

Thread: Electric motor ratings
10/01/2020 01:59:18

Reading reading reading . If you could have seen the size of the capacitor banks we fitted to Kirkstall forge. they were huge to correct the power factor back to 0.99 as 1 is not possible. The high inrush currents at start up on machinery meant we had to, years ago fit HRC fuses. More recently it was circuit breakers of the D type to stop them tripping on start up. That is why the motor control circuits used to have oil filled dashpots, so the pull on the dashpots was controlled to stop the overloads tripping. An interesting anecdote was when we did lighting installations in workshops where turning machines were used. The rows of lighting had to be on different phases so that the stroboscopic effect did not see the rotation of the machines appear to be stood still. From an ex Electrician with experience & not much reading. Too busy earning a large crust. With pork dripping on. Regards to all of you & a very prosperous new year. If you are retired like me then enjoy. Steve. 

Edited By Steviegtr on 10/01/2020 02:01:39

Thread: VFD Question
10/01/2020 01:42:45

Too many people reading too many books. Not many that have actually worked on these. All theory & no experience. I Spent many weeks just setting up a control circuits to control the speed of motors on machines like. Dosing pumps for Brazilian mint. (What you may say). This mint costs £3500 a barrel & one touch on your tongue would burn a hole in it. You may have guessed but it is one of the ingredients in a after 8 mint. I designed & built the panels that did the dosing. A while ago Nestle decided to make a Baileys after 8 mint & asked me to design a dosing system for that too. It was very delicate as the mint was not meant to contain much alcohol. Also a orange version. We also designed the control of dosing for the various Kit Kat brands. All I said at the beginning of my posts was that a motor driven by a 240v inverter would have to be connected in Delta as you already know. Also that a motor connected to a single phase inverter that is 3 phase, would not have a high starting torque. We proved this many times by R&D work. Some of you think this is bull. It is not it is fact. A 3 phase motor with 240v windings which it must have to work on single phase will not have the same torque as a 3 phase motor connected to a 3phase supplied inverter. I give up. Going to move to a desert island with no roads or electricity & catch fish with a hook.

Thread: The blind leading the blind
09/01/2020 17:26:10
Posted by Brian G on 09/01/2020 14:25:21:

Not the first time, and certainly not the last, but I suppose we should be glad the subject of this famous photo is wearing safety glasses.

Brian G

Brilliant. laugh

Thread: Electric motor ratings
09/01/2020 17:20:45

Well that is the motor I have taken off the Super 7 . To then fit a new when arrives, 1.1kw 1.5hp 3phase motor. Just looking at the manual for the inverter 167 pages. Ouch. Quite a lot of settings to program in.

09/01/2020 15:53:00

It may be that the motor as in this picture. Is power rated at 550w = 0.75hp. But takes 5.1A of power because of the losses discussed by some above.

myford motor

Thread: Great experience buying a dehumidifier
09/01/2020 15:34:20

Dehumidifier 2This is the one I have. Bought it years ago for a boat I had. Still good after many years use.

Dehumidifier

Thread: First Myford question. hopefully not too stupid
09/01/2020 15:18:16

Just looked at mine too & agree with above. It looks like a press fit. Maybe it got hit somehow & elongated the hole.

Thread: soldering stainless steel
09/01/2020 15:08:13

Is this what you are looking for. Bought this on e-bay a few weeks ago to repair a antique coal scuttle. It was made of spun steel though. Bakers fluid

Thread: VFD Question
09/01/2020 14:58:27

Another little ditty when using an inverter is, it is advisable to use screened cable to the motor. The screen ideally should then be connected to earth only at one end only. This is to shield the inverter output from emitting noise. Interference is caused by inverters so any delicate electronic equipment nearby would be effected. Does. not matter if you only use plain cable. Just advisable. Not sure if it would interfere with a CNC setup. I cannot get any deeper into the if's & but's that are being bounced around. I spent most of my career working in industry as an electrician. But will not argue with any comments said.

09/01/2020 12:33:19
Posted by Gerard O'Toole on 09/01/2020 11:27:15:

Thanks Andrew, John, Dave , MArtin and Steves for the clear explanations.

Just one other question.(Sorry!)

Frequency. I have been told that the maximum frequency must be reduced in proportion to the voltage. Currently my VFD is set at maximum 31 Hz.. Is this correct or can I increase it to 50Hz.?I note that 'Old mart' runs his VFD up to 75Hz.

regards

gerard

You can run it as high as you want within reason. 75Hz is not that high. If fitted to a lathe or a milling machine just make sure you do not go over the manufacturers top speed. Steviegtr

09/01/2020 12:30:40
Posted by Andrew Johnston on 09/01/2020 11:24:54:
Posted by Steviegtr on 08/01/2020 23:58:28:
The problem with Delta connected motors is that the starting torque is reduced by route 3.

I don't understand why that would be? If I've got a dual voltage motor then in both star and delta the power available (from voltage/current values on the plate) is the same. In both cases the speed is also the same. Since power is torque time angular velocity, and power and angular velocity are same for delta and star, surely the torque is also the same in star and delta?

The issue of stalling a motor running at low speed is simply explained. Below base speed an induction motor runs at constant torque (*). Whereas using pulleys or gears to reduce speed has a constant power characteristic. So using the formula above running at slow speeds, through a mechanical reduction, results in an increase in torque in proportion to the decrease in speed.

Andrew

(*) Before any smartypants points it out I know VFDs can be programmed to output more than normal current at slow speeds. smile But it's not a good thing in the long term as it results in significantly more losses in the windings.

Rather than me ranting on. Look up the function of a star delta starter.

Thread: Great experience buying a dehumidifier
09/01/2020 12:25:58

I have one running in my fully insulated & thought dry garage. Every day I get at least a pint of water out of it. Brilliant device.

Thread: VFD Question
09/01/2020 08:44:52

It was a project when I was an apprentice to design a building with 3 phase power supply . You got marks for balancing the load. Don't know if they still have but 3 phase polymeters had a out of balance dial on them. The lower you could keep the dial the cheaper your electric was.

09/01/2020 00:16:06

The star point of the generated power from the power station is grounded to earth. It takes some understanding but earth is earth. Neutral is earth. The Pilons only carry 3 phases, usually up to 133,000 volts then reduced to what is needed. That is another subject.

Thread: Looking to pay for someone to finish an engine
09/01/2020 00:09:08

Yes definitely look for the nearest club to you. There are plenty up & down the country. I am sure there will be plenty of offers to assist you.

Thread: VFD Question
08/01/2020 23:58:28

Hello all. at last a subject I know plenty about. I was an electrical engineer & contractor of sorts for 35 years, in that I designed , manufactured & installed control panels to machinery. Mainly in blue chip food factories. We installed many inverters in different machines. If 3 phase was available then of course we fitted 3 phase inverters, usually imo Jaguars. If however only single phase was available & speed control was required then we would use a single phase inverter. Usually Imo Jaguar Cubs. 2 things you need to know is that a motor suitable to this application has to be what is called dual voltage. That is the windings are each actually 240volt wound. If they are 415v then it is no good. So always make sure the 3 ph motor has 240v windings. Most new motors are. The 2nd thing is that a motor used with a 240 single phase to 3ph inverter, must be connected in delta. The problem with Delta connected motors is that the starting torque is reduced by route 3. An example once was an engineer came to me with a box of bits he had purchased for the speed control of a Glucose pump where the only supply available was single phase. I advised the engineer that it would not work because pumping thick liquid would require a high starting torque & that the motor would have to be connected in star. He was adamant that it would work so we built a control panel with the parts supplied. Outcome was that as you guessed the pump once primed with glucose & stopped, would not restart because of the lack of torque connected in DELTA. The reason I say this & I myself are building up a Myford super 7 with a 3 phase motor is when you fit the system & set it up you may find that the best set up is to say use a low gear on your pulleys as you may find that if you run the motor slow & plunge a cutting tool into the work that is may stall. This is normal. So use a lower gear so the motor is running faster & you will not have a problem. I myself have probably gone for a overkill as the single phase motor was 3/4 hp & I have gone for a 1.1 kw 1 1/2 hp 3ph motor & Omron 1.1kw inverter. Also running the inverter at around 50Hz will give you 1425rpm. the same as your single phase motor. The inverters you will be fitting can be turned up to pretty high Hz settings. The one I have will run up to 400 HZ. Just be careful, you will be able to get some silly speeds from your inverter but if run too high 2 things may happen. The 1st is the motor may come apart, explode if you like. The other is the noise will be bad. If you buy a good quality inverter then you can tune within the parameters to stop the high pitched noise associated with these drives. We would regularly overdrive a motor to say 75Hz with no problems so happy speed control. God I can prattle on. I am usually the one asking questions But anything electrical is steviegtr's speciality.....

Thread: Ebay being clogged up by certain sellers
06/01/2020 22:46:07

Sometimes I wonder how they can do it. But they do.

Thread: Anyone with a surface grinder near Leeds?
06/01/2020 22:43:10

Don't think it's a problem doing that but a bit heavy to be lifting onto a machine table. My back has just gone thinking about it.

Thread: Hello from Norfolk
06/01/2020 22:30:08

Welcome, I am new on here myself. If your like me you'll be asking lots of questions that most on here know as second nature.

Thread: Anyone with a surface grinder near Leeds?
06/01/2020 22:26:16

Most car engine refurb places have them. Lockside engineering in Castleford do head skimming so guess they have one.

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