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Posted By: Hesham Braking of single phase induction motor - 02/22/12 10:06 AM
I'm working on a control circuit to brake a single phase induction motor in a washing machine. The motor is (PSC)Permanent split capacitive motor with a 6UF capacitor as in the picture.
[Linked Image from]
I've searched for braking techniques and I found that there is a method for braking using DC circuit, but, unfortunately, I couldn't find more details about that method and how much current needed for such circuit. Please if anybody knows anything about that method please reply
Do an internet search for dc injection braking, or dynamic braking, you should find all the info you need there. Also if you happen to be running the motor with a variable speed drive(VSD) most of those can be set up for dynamic braking with the addition of a braking resistor, check your manual.
Posted By: Hesham Re: Braking of single phase induction motor - 02/22/12 10:33 PM
Thanks for your reply laugh
but the problem is I have got no manual. I've also searched for injection braking I've found many answers, but I couldn't get the formula that decides how many DC volts & DC amperes needed for such braking. The AC current with full load =1.75A. How can I decide the DC voltage needed??
Posted By: Tesla Re: Braking of single phase induction motor - 02/23/12 01:40 AM
In America such calculations are normally done with decade boxes and variable voltage power supplies.

There are simply no end of circuits already solved -- sitting on the shelf for this and that purpose.

You need to hit the books -- at a library dedicated to electrical engineering.

You'll also find amazing amounts of knowledge in basic electrical engineering texts.

As for the principle I think you'll agreed it's obvious: a steady DC induced magnetic field in the MAIN WINDINGS will cause the rotor - -with its induced magnetism to brake to a stop.

One need only figure out how to break the AC connection ( a switch ? ) while making a DC connection. ( the same switch ? )

A decade box ( of resistors ) tapping a DC power supply ( half-wave ) ) would start out at high values and low current -- later being adjusted down to a value that stops rotation without collateral damage...

This test rig would then be followed by a production solution with tweaks for switches and time delay before drop out.


However, it does seem over engineered since a simple pause in power would seem to bring a washing machine to an acceptable halt without further complication -- and it's expense.


More generally, we are electricians who install field wiring/ wire buildings.

As a rule, we don't have bench equipment and are entirely unconcerned about solving engineering problems -- particularly ones that constitute the typical college course.

I'm sure that you'll find other forums much more dedicated to such topics.

They're out there.

It really is a different mind-set.

We tend to be focused on what's going to make our customers and inspectors happy.

We're not allowed to self-engineer -- when it's going into a building.

That's the motivation for the NEC. Only tested and proven devices are deemed worthy - -and only when they've been installed according to manufacturer's instructions.

That turns out to be a lot tougher than one might think. Just that alone can take 10,000 hours of field experience on top of 1,000s of hours of book learning.

I am not sure exactly what kind of induction motor it was (one capacitor and a start relay) but IBM had a fractional HP motor in an old sorter that they did braking by shorting the input leads (big double pole double throw relay). It hauled it down pretty fast.
Posted By: Scott35 Re: Braking of single phase induction motor - 02/26/12 09:20 AM
If I remember correctly, injecting a DC equal to 250% the Induction Motor's FLA will result in a Braking effort of 25% the Starting Torque.
This is for "Standard" Squirrel-Cage Rotor type Induction Motors (not Wound Rotor or Synchronous).

So look at supplying 250% the Motor's rated FLA.

DC Voltage will be what ever is required to drive 2.5x the FLA into the Stator Windings (around 6-12VDC may be ample).

-- Scott (EE)
Can I pose a practical question? Why brake the motor in this application? Most washing machines I've owned in the last 30 years had/have a delay that stops you opening the door for up to a full minute. Is this an industrial washer? A domestic machine's drive train might not stand the stress of braking a full load of wet dhobi.
Most American washing machines are top loaders and there is nothing to keep you from lifting the lid when it is running.
Older ones even let it run on "wash" mode with the cover open. It was only the spin cycle that stopped when you opened the top.
They usually have a drum brake that hauls them down pretty fast. Newer machines won't run at all with the top open but they will fill.
Most UK machines are front loaders. They sell top loaders here in France, but never fancied one because we Brits nearly always fit them under a counter top as part of a kitchen or ute room. The OP's machine must be quite old - modern machines use sophisticated motors you could never mess with because everything is wired though a printed circuit board- and they have been like that for years, albeit with grooved-belt drive in earlier versions. My present machine was bought in the UK 2012 and runs from dead slow to 1200rpm electronically, direct drive. Downside is that they are not economically repairable like our 1960's Hotpoint twintub. That was little more than a motorised paddle in a tub, and took all day Saturday to do the weekly wash, rinse and spin chores aided by a pair of wooden tongs!
The Whirlpool machines I am familiar with (really about 10 brand names, 100 models/same basic machine) use a garden variety induction motor with a direct drive coupler under the transmission. They are actually pretty simple and all the fancy stuff in the different models boils down to different timers and a couple add on gadgets.
Since the same basic motor is so ubiquitous in US washers and they show up at the curb a lot, there are web sites telling you how to wire these for other uses.
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