Electric motor speed

For many years I have had a Clarke "woodworker" circular saw. Recently,while cutting some MDF,the blade decided to free wheel. A strip down showed that the final drive gear key had sheared. Fortunatly I had a spare gear box taken from a previous motor that burnt out,some years ago.. What I have noticed is that the motor speed is reduced 2 to 1 by the gearing. The blade speed is 5000 RPM, so that means the motor is running at 10000 RPM. It looks like any normal brushed mains motor. No sign of any electronics to change frequency, so how is it done?

Keith.

Put simply, if it is an induction motor the speed depends on the mains frequency and the number of poles, but a brushed motor does not work that way.

Commonly called a universal motor, not to be run without load

Universal motors speed up until the load matches the drive power of the motor.

If run without load they just go faster and faster until they either seize or grenade.

Generally universal motors carry a cooling fan which presents enough load under overspeed conditions to prevent grenade. Usually.

Clive

Redsetter,

I did say it was a Brushed motor/

Mikelkie,

In what way does a Universal motor differ from a brushed mains type?

Keith

As there is nothing about not running it without a sawblade attached,,in the handbook, I assume the fan and gearbox provides sufficient load to avoid a runaway. When I reassembled the machine I did run it without a blade fitted, to check all was ok. I have a slow start unit fitted that takes 10 seconds to reach top speed and it did not continue to speed up after 10 seconds.

Thanks for the information.

Keith.

Yes I heard you the first time that is why I replied as I did! A universal motor is another name for a brushed mains motor. An induction motor is a totally different thing.

I think , ' universal' means it will also run on a dc supply , of the correct voltage.

Hi,Why doesn’t a mains electric drill with no load go faster and faster and seize ? These have brushes.

Maurice

Maurice A mains electric drill will have gears which will load it

Maurice

The common mains electric drill with brushed universal motor always has a cooling fan on the shaft which presents enough load to stop over speeding. Many have gearboxes too which also load things up.

For various reasons these motors have very little acceleration torque capability under overspeed condition.

If spinning up unloaded to grenade speed they will just keep accelerating but the actual acceleration curve has to fall into a fairly narrow band for it to keep going up. With no load the motor balances itself in the band and keeps going faster.

It is possible to make universal motors that are inherently incapable of overspeeding but, so far as I'm aware no one has bothered for many, many years. The price, performance, application ratios for such don't make sense. The whole point of modern universal motors is being cheap to make with lots of power in a compact, lightweight machine.

Clive

Hi,Clive. Thank you for your explanation .

Maurice

Clive ,

Thanks from me.too

Question asked and anserwed !

Keith

The blade speed will be a maximum for that blade, and must not be exceeded. Presumably because there is a danger of it breaking up. It is very unlikely that the motor will be running at 10,000rpm

Peter G. Shaw

Peter,

I checked the RPM of the saw at full speed and got 4970 RPM. With a 2 to 1 reduction that means the motor is running at 10000. RPM, Hence my original question.

Keith

Fair enough Keith, you didn't say that and I assumed that your 5000 rpm was printed on the blade as a maximum speed.

Just out of interest I've had a quick look at my mains powered hand held tools and found that the rpm as stated on the labels vary from 3000rpm (an aging B&D pistol drill) to 10,500 rpm (a 5" angle grinder bought from Aldi). Plus the max rpm for the 5" angle grinder disks is shown as 12,200 rpm. So maybe your 10,000 rpm is indeed possible.

Peter.

A brushed motor can be series or shunt wound also compound wound, the series motor is the one to be careful about running without a load as it is possible to reach speeds that destroy the motor armature or bearings. The series motor will also run on an AC orDC supply hence the universal name.

Mike

It would be common for the brushed motor of a router to run at 25000 rpm.

Think routers here? Even Dremels.

The speed of DC or universal motor does not increase to infinity without load even without friction and windage. It increases unti the armature back EMF, which is proportional to the speed, equals the applied voltage. For some series motors this speed exceeds the mechnical rating of the parts unless a load is applied.

In theory both shunt and series wound brushed DC motor will run on AC or DC. However the inductance of the field winding on a practical shunt motor is too high to allow enough current to flow even at 50Hz.

Robert G8RPI.