Benefits of 4 pole motors on high speed spindles?

[Goemon]

Can anyone tell me what the benefits are of 4 pole 24,000 rpm spindles?

I can understand the benefit of 4 pole low speed milling head motors where the goal is more HP at lower speeds but.... on a 24,000 rpm CNC spindle, it's less obvious. You have the extra hassle of needing an 800hz VFD without any extra speed or torque. Why do they make them?

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[Jim Dawson]

Everything else being equal, the 4 pole will have twice the torque of a 2 pole.

[Goemon]

Everything else being equal, the 4 pole will have twice the torque of a 2 pole.

That's not correct. It only applies to old low speed milling head motors because the base speed at 60hz is 1800rpm instead of 3600 for a 2 pole. A 4 pole 2.2kw spindle with a base speed of 24,000 rpm at 800hz has to have the same torque as a 2 pole 2.2kw 24,000rpm 400hz spindle.

Torque is hp at a specific speed. 1hp at 10,000 rpm is .53 ft lb regardless of how many poles.

[Al_The_Man]

Torque is hp at a specific speed. 1hp at 10,000 rpm is .53 ft lb regardless of how many poles.

HP contains a RPM value, Torque does not have to necessarily.
For an induction motor, Less poles = lower inductance, The H.S, 2 pole spindles posses very low inductance and the inductive reactance at anything lower than 100Hz gets progressively lower as the frequency decreases, hence the overheating..
Al.

[Goemon]

HP contains a RPM value, Torque does not have to necessarily.
For an induction motor, Less poles = lower inductance, The H.S, 2 pole spindles posses very low inductance and the inductive reactance at anything lower than 100Hz gets progressively lower as the frequency decreases, hence the overheating..
Al.

That would explain (one of) the benefits of using 4 pole motors for low speed applications but I am asking what the benefits of using them for 24,000 rpm CNC spindles (I.e. The kind we use on CNC routers).

If you compare a 2.2kw 2 pole spindle with a base speed of 24,000 rpm at 400hz with a 2.2kw 4pole spindle with a base speed of 24,000rpm at 800hz - I want to know why they make the 4 pole kind given that it restricts your choice of VFD? 2.2kw with a base speed of 24k is .66 ft lb of torque regardless of the number of poles.

Torque, hp and rpm are always linked in that if you know any two, you can work out the 3rd and the answer is not adjusted for poles or frequency. So there must be some other benefit of milling at 800hz over 400hzv(at 24,000 rpm).

Does what you said about cooling apply at high speed too? I.e. Does a 4 pole motor run cooler at 24000rpm at 800hz than a 2 pole at 400hz?

[Uguessedit]

Guess I don’t know why vfd choice would matter. My vfd’s will run a 2 pole or a 4 pole. In fact my router has the same vfd as one of my small mills and it is a 2 pole 300hz motor at 18000rpm on the router and the mill is a 4 pole at 3600rpm. With belt drive the 4 pole can run up to a max 10000rpm before I’d have to change the belt to a cog timing. I think I’m general 2 poles tend to be the choice for higher rpm applications especially where it’s direct drive.

That would explain (one of) the benefits of using 4 pole motors for low speed applications but I am asking what the benefits of using them for 24,000 rpm CNC spindles (I.e. The kind we use on CNC routers).

If you compare a 2.2kw 2 pole spindle with a base speed of 24,000 rpm at 400hz with a 2.2kw 4pole spindle with a base speed of 24,000rpm at 800hz - I want to know why they make the 4 pole kind given that it restricts your choice of VFD? 2.2kw with a base speed of 24k is .66 ft lb of torque regardless of the number of poles.

Torque, hp and rpm are always linked in that if you know any two, you can work out the 3rd and the answer is not adjusted for poles or frequency. So there must be some other benefit of milling at 800hz over 400hzv(at 24,000 rpm).

Does what you said about cooling apply at high speed too? I.e. Does a 4 pole motor run cooler at 24000rpm at 800hz than a 2 pole at 400hz?

[Jim Dawson]

Guess I don’t know why vfd choice would matter. My vfd’s will run a 2 pole or a 4 pole. In fact my router has the same vfd as one of my small mills and it is a 2 pole 300hz motor at 18000rpm on the router and the mill is a 4 pole at 3600rpm. With belt drive the 4 pole can run up to a max 10000rpm before I’d have to change the belt to a cog timing. I think I’m general 2 poles tend to be the choice for higher rpm applications especially where it’s direct drive.

If your mill motor is a 3600 RPM, 60Hz base speed motor, then it is a 2 pole motor.

A 4 pole motor base speed @60Hz is 1800 RPM

A 6 pole motor base speed @60Hz is 1200 RPM.

If the motor is designed for high speed, then it's only a matter of applying the correct frequency to achieve the desired RPM. Many standard 4 pole motors are rated to 6000 RPM, Baldor NEMA 56 and 184 frame motors < 5HP for instance. But not all motors are rated this high, so be careful.

A 1 Hp 4 pole motor will develop twice the rated torque as a 2 pole, 1 HP motor.

With the proper VFD a motor will develop about 100% rated torque up to the base speed (constant torque), thus variable HP. Above the base speed it will produce constant HP with torque decreasing as the RPM increases.

Router spindles normally will not run properly below some rated RPM, the minimum speed is dependant on the spindle and manufacturer specs.

[Ritzy321]

That would explain (one of) the benefits of using 4 pole motors for low speed applications but I am asking what the benefits of using them for 24,000 rpm CNC spindles (I.e. The kind we use on CNC routers).

If you compare a 2.2kw 2 pole spindle with a base speed of 24,000 rpm at 400hz with a 2.2kw 4pole spindle with a base speed of 24,000rpm at 800hz - I want to know why they make the 4 pole kind given that it restricts your choice of VFD? 2.2kw with a base speed of 24k is .66 ft lb of torque regardless of the number of poles.

Torque, hp and rpm are always linked in that if you know any two, you can work out the 3rd and the answer is not adjusted for poles or frequency. So there must be some other benefit of milling at 800hz over 400hzv(at 24,000 rpm).

Does what you said about cooling apply at high speed too? I.e. Does a 4 pole motor run cooler at 24000rpm at 800hz than a 2 pole at 400hz?

I purchased one of those 2.2kW 4 pole from BST automation. I had quite a lot of problems with it and the crappy inverter, I don't think it could produce it's anywhere near rated power and had stalled it a few times.

I am comparing this to a teknomotor 2kW i bought after which cost a bit more but does what is advertised and you get a an actual torque curve as well as a PTC thermal overload protection for peace of mind.

It's a 4 pole motor why is it only rated to run at 12,000rpm when my 2 pole motor is rated down to 6,000rpm?

I know for some people these chinese spindles are fine but it ended just costing me more money and a heap of time.

[mactec54]

I purchased one of those 2.2kW 4 pole from BST automation. I had quite a lot of problems with it and the crappy inverter, I don't think it could produce it's anywhere near rated power and had stalled it a few times.

I am comparing this to a teknomotor 2kW i bought after which cost a bit more but does what is advertised and you get a an actual torque curve as well as a PTC thermal overload protection for peace of mind.

It's a 4 pole motor why is it only rated to run at 12,000rpm when my 2 pole motor is rated down to 6,000rpm?

I know for some people these chinese spindles are fine but it ended just costing me more money and a heap of time.

The motor 4 pole 12,000 RPM is 400Hz 24,000 RPM would be 800Hz you brought the wrong spindle if you wanted 24,000 RPM

If you intended it to be 12,000 RPM then it can go down to around 2,000 RPM and still have good torque which your 2 pole spindle won't have much useful torque at 6,000 RPM

[Ritzy321]

The motor 4 pole 12,000 RPM is 400Hz 24,000 RPM would be 800Hz you brought the wrong spindle if you wanted 24,000 RPM

If you intended it to be 12,000 RPM then it can go down to around 2,000 RPM and still have good torque which your 2 pole spindle won't have much useful torque at 6,000 RPM

Sorry I meant minimum speed 6kRpm-24kRpm vs 12kRpm-24kRpm. 12kRpm is what the Chinese state the minimum speed is...

[mactec54]

Sorry I meant minimum speed 6kRpm-24kRpm vs 12kRpm-24kRpm. 12kRpm is what the Chinese state the minimum speed is...

Is it air cooled that would be the difference the 4 Pole motor can run slower than the 2 Pole if it is water cooled I have some 2 Pole that run at 3,000 RPM manufacture spec's that have a higher torque at low RPM due to the Spindle motor design, the design of the Rotor and Windings is going to determine the low speed it can run at

[slodat]

f = np/120 where f=frequency, n=speed, p=number of poles.

My milling machine motor is 8,000 rpm, 4 pole, 5hp continuous/7.5hp for 30 minutes. It's base frequency is 266.6Hz. This particular motor has a flat, full power curve from 1500 - 8000 rpm at the expense of being quite inefficient. The rest of the conversation you guys are having lies in the characteristic curves for the motors in question.

[mactec54]

f = np/120 where f=frequency, n=speed, p=number of poles.

My milling machine motor is 8,000 rpm, 4 pole, 5hp continuous/7.5hp for 30 minutes. It's base frequency is 266.6Hz. This particular motor has a flat, full power curve from 1500 - 8000 rpm at the expense of being quite inefficient. The rest of the conversation you guys are having lies in the characteristic curves for the motors in question.

You are not on the same page your motor is a very different motor to what we are talking about 400Hz and 800Hz 24,000 RPM your motor is a standard that a lot of machining centers use, I have some of those in stock 3Hp 5Hp and 7.5Hp 4 pole 8,000 RPM