Bench grinder electrical specs

[robsims]

Hi, the Palmgren bench grinder    https://www.palmgren.com/product/348/Bench-Grinders

is rated  115V/230V. The amps it draws for 115V is 3.5 Amps.  For 230V it draws 1.75 Amps. So this bench grinder is a 402.5 Watt grinder.
My question is will this bench grinder get hot quicker and much more when running at 115V because of the higher amps it draws? 

[james_s]

It draws 402.5VA, the wattage depends on the power factor and may be around half that.

It will get slightly hotter on 120V due to greater I^2R losses, but it should not make a huge difference.

[Benta]

First, it's NOT 1 402.5 W grinder. That's the apparent power. You need to remember power factor.
But it'll probably run hot whatever you do. Cheap Chinese grinders (and other tools) are skimped on copper and iron and run to the limit.
Series resistance is not a big issue, so I wouldn't expect a big difference.

[jpanhalt]

That's an ordinary "off-hand" grinder.  You can roughly shape cutters for single point cutters  (e.g., lathe work) and even dills, but it is not particularly accurate. As the adage says, a drill "follows it point." If you want to sharpen drills, then to drill accurately, you need an accurate point.  That is only one example. 

What do you want the grinder to do?  Attached is what I use for more accurate grinding.  One of the wheels is diamond impregnated for hard stuff, like carbide.  For sharpening drills, I use something else to ensure both cutting lips are equal with the same relief. Now, I have seen expert machinists who can off-hand grind a drill fairly accurately, but I am not one of them. 

The second attachment is an SRD grinder that I use for drills.  Milling bits need something else.  There are several types of grinders for drills, I liked the SRD as it is easy to change to tip's included angle.

[robsims]

I am asking because  i have a chinese 6" bench grinder. It's rated 120 volts 150 watts. After 5 minutes of continuous use it gets very hot. Some reviewers on youtube with that same build grinder, but theirs is a 230 volts 150 watt version, don't have that problem. So what i think is that the chinese manufacturers reduce the copper winding turns to meet the 120 volt requirement but they don't increase the diameter of the wire to acommodate the higher current. If so can i rewind the motor?

[jpanhalt]

Why do you care how hot the motor gets?  Some motors are designed to run at 65°C continuous. I was surprised the first time I came across one in 1975 running a vacuum roughing pump.

[thm_w]

Can you tell which part gets hot first? I'm assuming its the motor area and not the bearings.
and measure the actual temperature after 5mins as jpanhalt says.

[robsims]

The area that is getting hot is the iron core. Temperature rises to 122 degrees celcius. I am afraid the isolation of the windings will burn if i let it run longer. And this is with no load. How did they design a motor that is getting hotter with no load on it

[james_s]

That is hotter than I'd like to see, the copper, if it even is, it may be aluminum, is probably even hotter than that. It's entirely possible the grinder is only intended for intermittent use. I typically run mine for maybe 30 seconds to a minute at a time.

[robsims]

Thanks James_s. I assume you have a chinese bench grinder too. I'm looking for a used washing machine motor to make a bench grinder which i can run longer. Although washing machine motors get hot too, they can run longer

[james_s]

I do, although mine is pushing on 20 years old so it may be considerably different than what you have. Mine doesn't get noticeably hot but I've never tried running it for more than maybe 2 minutes.

[robsims]

Thanks James_s. I will try to run it for not longer than two minutes. If it holds up 2 years it will be fine. In the meantime i'm going to build myself a sturdy bench grinder

[Old Printer]

Electric motors heat up from doing work. Without a load a decent quality motor run on the proper voltage should run indefinitely without over heating. If you want/need a grinder to do a job you need to buy a quality tool specked for that job. You still have not said what you are doing with it. Sharpening 1/4” drills vs a large axe blade are very different. Scrounging unknown motors is a recipe for a lot of wasted time. Washing machines have changed a lot in the last 40 years 

[themadhippy]

Some reviewers on youtube with that same build grinder, but theirs is a 230 volts 150 watt version, don't have that problem.

could it be down to frequency? Generally places with 110v are 60Hz whilst 230 land is mostly 50Hz and running a 50Hz motor at 60Hz tends to make things a bit warmer

[james_s]

Some reviewers on youtube with that same build grinder, but theirs is a 230 volts 150 watt version, don't have that problem.

could it be down to frequency? Generally places with 110v are 60Hz whilst 230 land is mostly 50Hz and running a 50Hz motor at 60Hz tends to make things a bit warmer

It's the other way around, running a 60Hz motor on 50Hz can result in overheating, a 50Hz motor will be fine on 60Hz though. The lower the frequency, the more iron you need.

[robsims]

Old printer, I'll be using the grinder for sharpening drills and chisels. Also once in a while for sharpening  large axe blades and for rounding metal. If i'm busy for an hour or two, i don't want to waste time in between turning the machine off to let it cool down. I won't be busy with the machine everyday, but when i'm busy the machine has to run for at least one hour continuously. So which motor can i use to make a grinder? Can i use a washing machine motor?

[jpanhalt]

I would not use a washing machine motor as direct drive.  First, check the RPM.  Second, in a grinder, the shaft bearings are important and take considerable load.  I doubt a washing machine motor meets either need.  A high speed grinder needs to run smoothly, not like the old, hand driven stones used to sharpen axes many years ago.

I would get a continuous duty motor and a separate belt driven arbor like the attachment.  A washing machine motor, if continuous duty, may actually work to drive that belt assuming you can attach a pulley to its shaft.

[EPAIII]

Dual Voltage electric motors have two coils or sets of coils. Each coil is rated for 115 VAC and the SMALLER of the two current ratings on the name plate. Yes, I said the SMALLER current.

When you connect it for 230 VAC, the two coils (or sets of coils) are in series so each coil sees 115 VAC and that smaller current (1.75 A) because the same current flows in both of the series connected elements. And the (apparent) power is the product of 230 V x 1.75 A = 402.5 W, as previously stated.

When you connect it for 115 VAC operation the two coils (or sets of coils) are connected in parallel. They both see 115 VAC again and each coil once again has the smaller amount of current (1.75 A) flowing in it. But, since they are in parallel, the overall motor draws the sum of the two currents or twice the smallest current on the name plate. I =  2 x 1.75 A = 3.5 A. And the power now is 115 V x 3.5 A = 402.5 W.

This should be no surprise as each coil sees the exact same Voltage and passes the exact same current in both wiring configurations. So the total power, being the sum of the power in all the coils, is exactly the same.

Now, since the current flowing in each coil is exactly the same and the the resistance (impedance) of each coil is exactly the same, the heat generated will also be exactly the same. This is given by the power loss in a resistance which is I^2 x R. This lost power is in the form of heat.

As for the name brand motor vs a cheap Chinese one I would expect the name brand to run cooler. This is probably due to the Chinese ones cutting corners with things like smaller wire gauge which would have more resistance.

There is a practical advantage in using the higher Voltage configuration. Since the lower Voltage connection has a higher current draw, the wire/cable that runs between the pole transformer and the motor would either need to be a heavier gauge or it will dissipate more energy as heat. This is given by the same power loss (as heat) equation as I show above P = I^2 x R. So with the higher Voltage connection you either get a greater amount of power delivered to the motor or you pay less for the copper in the wire/cable.

[Zero999]

Some reviewers on youtube with that same build grinder, but theirs is a 230 volts 150 watt version, don't have that problem.

could it be down to frequency? Generally places with 110v are 60Hz whilst 230 land is mostly 50Hz and running a 50Hz motor at 60Hz tends to make things a bit warmer

It's the other way around, running a 60Hz motor on 50Hz can result in overheating, a 50Hz motor will be fine on 60Hz though. The lower the frequency, the more iron you need.

Not necessarily. That's true in this case, but if it's driving a fan, it will run faster and draw more power because it's a cube law.

The area that is getting hot is the iron core. Temperature rises to 122 degrees celcius. I am afraid the isolation of the windings will burn if i let it run longer. And this is with no load. How did they design a motor that is getting hotter with no load on it

Buy a more expensive grinder, with a bigger motor.

[rstofer]

Buy a more expensive grinder, with a bigger motor.

Absolutely!  There are a lot of good quality grinders around.

Of the grinders that showed up on a quick Amazon search, I would consider the Jet or the DeWalt.  Porter Cable or Delta are also good quality.  None of the other for me...

Be careful where you source your grinding wheels.  There was an incident a long time back where a fellow bought a wheel at a flea market and it exploded when he started the grinder, killing him.  The seller had bought the wheel new but dropped it causing a crack.  Then, rather than trash it, he sold it.  I don't remember the exact charges but negligent homicide comes to mind.

I never like starting a bench grinder.

[Gyro]

When fitting a new wheel, you should always thread a support through the centre hole (like an upturned screwdriver) and then tap the side of the rim with something non-metalic. It should audibly ring - if it doesn't then it's cracked! This doesn't work when the wheel is mounted of course because it is damped by the mounting flanges and card washers.

Never try to DIY a bench grinder, it needs the proper (probably sacrificial - I've never had one shatter) wheel guards.

[robsims]

I totally agree with EPAIII. You got it right. For 120 volts with the same wattage the current will be bigger and so the the diameter of the windings must be bigger too. What i think is that the Chinese use the same gauge wire (same diameter wire) to wire the 230 volt and the 120 volt version of the grinder. No doubt about that. I did a test on the motor core. I connected my DC bench powersupply to the main winding. At 30 volts DC the current flowing through the main winding was 1.7 amps. Withing 3 minutes the core was so hot i could not leave my hands on it for 2 seconds. Question is can i rewind this chinese grinder i have with a heavier gauge wire?. 

[james_s]

I totally agree with EPAIII. You got it right. For 120 volts with the same wattage the current will be bigger and so the the diameter of the windings must be bigger too. What i think is that the Chinese use the same gauge wire (same diameter wire) to wire the 230 volt and the 120 volt version of the grinder. No doubt about that. I did a test on the motor core. I connected my DC bench powersupply to the main winding. At 30 volts DC the current flowing through the main winding was 1.7 amps. Withing 3 minutes the core was so hot i could not leave my hands on it for 2 seconds. Question is can i rewind this chinese grinder i have with a heavier gauge wire?.

But if you have two sets of windings for dual voltage, they get wired in parallel for the lower voltage and series for the higher voltage. When they are in parallel, the current carrying capability is double that of one winding.

Putting DC though ANY winding will make it get hot, in fact that technique is sometimes used for curing varnish in motor stators and transformers. With DC the current is limited only by the resistance of the wire. With AC inductive reactance comes into play and the impedance is much higher.

There is probably not space for heavier wire but you can look. It's also possible you have a shorted turn, that can result in overheating. You can try rewinding but the problem may be too little iron in the core, or some other issue.

[robsims]

Yes you're right james_s. No space for heavy wire and it has a crappy small core. Those are the problems. The grinder is useless for me. I'm a mechanical engineer, so i'm going to make one myself from an old pump or washing machine motor. Only thing i need  for the grinder to do is to run for an hour continuously because  i don't want to interrupt my work to let it cool down. I will trash that Chinese garbage, because it is useless for me or i'll let it run untill it burns.  Yes, i got what i paid for. Wasted $39 on that crappy Chinese shit. I'm going to use the wheel guards on my self made grinder. Question: which motors are best to use? In my country the voltage is 120 and the motor has to run for an hour continuously. I'm going to use 6" wheels

[mag_therm]

Palmgren has been selling hobby grade machine shop tools for about 20 years at least I think.

The surface temperature of the motor housing depends on the insulation temperature class. It is probably class B  limit 130 C or class F  limit  155 C because class H uses more costly insulation like nomex.
Palmgren site says it is capacitor run motor. The current draw on these small motors might not vary much from no load to full load.
The surface temperature of a non fan cooled class B motor might reach 100 C at 20 C Ambient. However there are rules for human contact, I am not what or whether that applies to hobby machine tools.
If you have worked in a place with lots of industrial squirrel cage motors eg paper mill, steel plant etc you will become accustomed to non- fan motors with case temp around 100C, but often they are TEFC with fins, so run a bit cooler on the surface.