"Inverter Rated" & what that means

[Bühlmann]

I have an option to pick up a new Marathon N611A washdown motor for the cost of a cheap chinese motor. 2hp, 3ph, 220v, TEFC, 56C, all the fixin's of a good grinder motor. The spec sheet doesn't specifically say that it is "inverter rated" and has a two specs that make me question whether it is safe to run with an inverter:




I'm pretty dumb in general, but when it comes to motors, electricity, electronics, etc. my ignorance is really at it's peak. Are all 3 phase motors "inverter safe" and that's just a new marketing thing, or is there something else I need to know?

Here's a link to the full spec sheet if that helps:

https://documents.mrosupply.com/product_documents/58/10/5810674/Marathon_056T17W99020_Specification_Sheet_k4lBE0i.pdf

​

 

[Stacy E. Apelt - Bladesmith]

Inverter rated means the motor winding is robust enough to run at slow speeds with an inverter/VFD. It is just a higher grade motor for speed control.

 

[ferider]

Basically you need an inverter rated motor, if you use a VFD, and either want to run the motor very slow and/or have long cables (many feet) between VFD and motor. If not, you’ll be fine.

 

[SirSpice]

That looks like a great motor. Something worth noting is that it's 1800 RPM (4 pole). That's a bit slow for a knife grinder. You'll want a large drive wheel or to overclock the speed using the VFD for maximum grinding efficiency.

 

[Bühlmann]

So I’ve read that it’s the type & duty of the insulation that makes for the inverter rating. Not sure how to identify that, so I reached out to Marathon, which is here in Wisconsin. We’ll see what they say. I really want it to work, because that motor has a 10:1 CT ratio vs the standard 4:1 for cheaper motors.

That looks like a great motor. Something worth noting is that it's 1800 RPM (4 pole). That's a bit slow for a knife grinder. You'll want a large drive wheel or to overclock the speed using the VFD for maximum grinding efficiency.

Pretty sure if I understand it correctly that an 1800 rpm motor can effectively be doubled to 3600 with VFD programming. That and a 6” drive wheel gets north of 5600 lfpm if my math is right. Please correct me if my head ain’t right on that.

 

[ferider]

So I’ve read that it’s the type & duty of the insulation that makes for the inverter rating.

That's only half of it - to protect from high Voltage spikes caused by long cables at high currents (see my post above).

The other is high current support (thicker wires and/or better cooling, etc.) for high power, very slow runs.

It goes like this:

- just like in your car, outgoing power is proportional to Torque x RPM
- Therefore, if the VFD feeds constant power, the slower you go, the more Torque there is
- Torque is roughly proportional to current, meaning the slower, the more current
- Voltage spikes caused by larger inductances (long cables) increase with larger current

Roland.

 

[Bühlmann]

I don’t have long runs of wiring, nor am I going to be pushing high torque/low speeds for hours on end since I’m just a muddling doufus in his garage. But I’m not a guy that likes to use an adjustable wrench as a hammer and I’m not interested in buying another motor in a few years because I was tempted by a good price on a motor that’s not the proper service for my application today.

I bet it’d be fine even if it’s not VFD rated for as much as I will use it, so I’m interested to see what Marathon replies with. I know there has to be a bunch of makers on here that happily grind away with a VFD setup on whatever scrap 3ph motor they found for cheap and never give “VFD rated” a thought.

 

[ferider]

I bet it’d be fine even if it’s not VFD rated for as much as I will use it.

Agree.

 

[Natlek]

So I’ve read that it’s the type & duty of the insulation that makes for the inverter rating. Not sure how to identify that, so I reached out to Marathon, which is here in Wisconsin. We’ll see what they say. I really want it to work, because that motor has a 10:1 CT ratio vs the standard 4:1 for cheaper motors.

Pretty sure if I understand it correctly that an 1800 rpm motor can effectively be doubled to 3600 with VFD programming. That and a 6” drive wheel gets north of 5600 rpm if my math is right. Please correct me if my head ain’t right on that.

On 3600 RPM with 6” drive wheel you will get 28 m/s belt speed and that is right on money for grinding with coarse belts . It is much more important how much you can slow down the grinder if you want to use other belts for finishing bevels .
I edited post , first I make math with 5600 RPM ....

 

[Bühlmann]

On 3600 RPM with 6” drive wheel you will get 28 m/s belt speed and that is right on money for grinding with coarse belts . It is much more important how much you can slow down the grinder if you want to use other belts for finishing bevels .
I edited post , first I make math with 5600 RPM ....

Right, which is why I like the idea of a 4-pole motor with a higher CT ratio for getting max performance when going slow. At 10:1 that motor is at 180 rpm with theoretical full torque values. I think I prefer that over the alternative 3600 rpm motor for how I plan to use it.

My intent is not to rehash the 1800/3600rpm discussion, but rather identify whether or not a high CT ratio (in this case 10:1) is indicative of VFD compatibility as opposed to a lower CT ratio (4:1 or 5:1) on a motor that carries a "VFD compatible/ready/rated" label.

 

[SirSpice]

It goes like this:

- just like in your car, outgoing power is proportional to Torque x RPM
- Therefore, if the VFD feeds constant power, the slower you go, the more Torque there is
- Torque is roughly proportional to current, meaning the slower, the more current

Are you sure about this? (Genuine question because I'm not an electrician)
From my understanding the VFDs we are talking about, like the KBAC 27D:
1. Take in single phase AC power, 120/240.
2. Convert it to DC power, which has no phase.
3. Output three phase 240.

VFDs try to deliver constant torque as they approach the motors rated frequency, which is 60hz (1800 rpm). When you increase the frequency past that, you start losing torque.

We might be saying the same thing, but I think it's worth noting that you don't get more torque at lower speed like you would with pulleys (and DC motors, I think).

In my anecdotal experience as someone who has two grinders, one 2 pole (3600) and one 4 pole (1800) with the frequency doubled from 60hz to 120hz on the latter. They mostly function the same.

The 4 pole runs a bit better at mid speeds and I think it's because that's closer to 60hz. When I put a 14 inch wheel and try to run it at full speeds, it stutters. I think it's because it's losing half it's torque at 120hz.

The 2 pole runs great at all but the lowest speeds, like under 10hz. I don't think that any abrasive needs to run at that speed, so it's not a big issue. I mostly use the low speeds to check that the belt is positioned correctly.

I'm honestly not trying to derail this thread, just giving a few pros/cons of him getting a 1800 rpm motor.

Wash down motors are pretty neat because if you ever want to install a coolant misting system on your grinder, you won't have to worry about the motor rusting.

 

[Bühlmann]

Are you sure about this? (Genuine question because I'm not an electrician)
From my understanding the VFDs we are talking about, like the KBAC 27D:
1. Take in single phase AC power, 120/240.
2. Convert it to DC power, which has no phase.
3. Output three phase 240.

VFDs try to deliver constant torque as they approach the motors rated frequency, which is 60hz (1800 rpm). When you increase the frequency past that, you start losing torque.

We might be saying the same thing, but I think it's worth noting that you don't get more torque at lower speed like you would with pulleys (and DC motors, I think).

In my anecdotal experience as someone who has two grinders, one 2 pole (3600) and one 4 pole (1800) with the frequency doubled from 60hz to 120hz on the latter. They mostly function the same.

The 4 pole runs a bit better at mid speeds and I think it's because that's closer to 60hz. When I put a 14 inch wheel and try to run it at full speeds, it stutters. I think it's because it's losing half it's torque at 120hz.

The 2 pole runs great at all but the lowest speeds, like under 10hz. I don't think that any abrasive needs to run at that speed, so it's not a big issue. I mostly use the low speeds to check that the belt is positioned correctly.

I'm honestly not trying to derail this thread, just giving a few pros/cons of him getting a 1800 rpm motor.

Wash down motors are pretty neat because if you ever want to install a coolant misting system on your grinder, you won't have to worry about the motor rusting.

This is how I understand it, too. Good point on the big wheel. I don't have one, but that gives me something else to think about. I'm sure I'm overthinking all of this because that's what I do.....

 

[ferider]

Are you sure about this? (Genuine question because I'm not an electrician)
From my understanding the VFDs we are talking about, like the KBAC 27D:
1. Take in single phase AC power, 120/240.
2. Convert it to DC power, which has no phase.
3. Output three phase 240.

VFDs try to deliver constant torque as they approach the motors rated frequency, which is 60hz (1800 rpm). When you increase the frequency past that, you start losing torque.

We might be saying the same thing, but I think it's worth noting that you don't get more torque at lower speed like you would with pulleys (and DC motors, I think).

I was talking about the flat blue part of the curve (constant power), where the torque (and current) goes up almost linearly as you slow down. The question is how much current do you need below 60Hz in your example, and can the motor take it, plus can it take the Voltage spikes from long cables between VFD and motor.

Anyways, I think the OP will be OK

 

[Stacy E. Apelt - Bladesmith]

Just to make all this clearer to the non-engineering folks ... it really doesn't matter to most hobby knifemaker users.
An inverter rated motor is better, yes. But a regular 3-phase motor will work just fine on a grinder. Very few of us grind at low speed for hours on end.

Where inverter rated motors are needed is things like production line belts in factories and large industrial fans, which are speed controlled by the VFD. These VFD's are often in a control room far away from the motor and run 24/7 in some cases. Where these parameters are found insulation, cooling capacity, and wire gauge are important.