torque in a 3 phase motor

ganutenator · Aug 7, 2017

trying to understand torque.

thought it was the vfd. it was a two speed motor. torque seems to be more a product of the motor instead of the vfd.

When I hooked it up to low speed, it didn't stall at 22rpm at the saw blade. 30:1 gear box ratio. aka 44hz.

wired at high speed, 22rpm at saw blade is 22hz and stalls.

customer wants to run saw at 10rpm, but it stalls. He still thinks it the vfd. I think it the motor, but think he may be partially correct.

the vfd set to constant torque. the vfd is a 10hp vfd. the motor is 3.5hp. so the vfd should be more than capable. have the cli turn up to 20amps. and the torque boost maxed out.

jraef · Aug 7, 2017

HP = Torque in Ft=lbs x RPM / 5250

There are different types of 3 phase 2 speed motors, 2 speed 2 winding, and 2 speed 1 winding, then in the 2S1W, there are Constant Torque, Variable Torque and Constant HP variations.

The trick to knowing is in the nameplate: If the nameplate shows 2 speeds but only one HP value, that's a Constant HP version. If it shows 2 speeds and two HP values, and the lower HP value is is the same ratio as the speed ratio, then it's Constant Torque. If it's two speeds and two HP values, but the HP of the lower speed is 1/4th that of the higher, then it's a Variable Torque. If it's two speed two winding, you would have 6 wires going to it and the speeds are generally exactly a 2:1 ratio, because one is 4 pole, the other is 2 pole.

All the VFD does is to change the speed without changing the torque. So being that you are apparently stalling when on the high speed, that indicates it's likely a Constant HP version, at the LOWER speed you would have MORE torque than at the higher speed to get the same net HP.

ganutenator · Aug 7, 2017

jraef said:
HP = Torque in Ft=lbs x RPM / 5250

There are different types of 3 phase 2 speed motors, 2 speed 2 winding, and 2 speed 1 winding, then in the 2S1W, there are Constant Torque, Variable Torque and Constant HP variations.

The trick to knowing is in the nameplate: If the nameplate shows 2 speeds but only one HP value, that's a Constant HP version. If it shows 2 speeds and two HP values, and the lower HP value is is the same ratio as the speed ratio, then it's Constant Torque. If it's two speeds and two HP values, but the HP of the lower speed is 1/4th that of the higher, then it's a Variable Torque. If it's two speed two winding, you would have 6 wires going to it and the speeds are generally exactly a 2:1 ratio, because one is 4 pole, the other is 2 pole.

All the VFD does is to change the speed without changing the torque. So being that you are apparently stalling when on the high speed, that indicates it's likely a Constant HP version, at the LOWER speed you would have MORE torque than at the higher speed to get the same net HP.

I left the motor name plate data in the office. dang. well i know that there is 6 motor leads. which speed has 4 poles and which speed has 2 poles? and do poles have to do w/ torque?

Gene Bond · Aug 7, 2017

3 ftlb / HP @ 1800 rpm (round numbers)
So, 1.5 ft/lb / HP @ 3600, 4.5flb / HP @ 1200, 6ftlb / HP @ 900...

so, 4 pole, 3.5hp = 3x3.5 = 10.5 fltlb
2 pole, 3.5hp = 2.5x3.5 = 5.25 ftlb

Same HP, 1/2 speed = 2x torque

jraef · Aug 7, 2017

ganutenator said:
I left the motor name plate data in the office. dang. well i know that there is 6 motor leads. which speed has 4 poles and which speed has 2 poles? and do poles have to do w/ torque?

As the number of poles goes up, the speed goes down and the torque increases, assuming the HP stays the same. But Gene Bond's post is what you need to know.

I sort of misled you by the way, ALL of the 2 speed motors will have 6 leads going to them... It's the configuration inside of the motor that is the difference.

ganutenator · Aug 7, 2017

customer: the variable frequency drive the problem. i'm going to just hook it up across the line direct.
me: if you do that, you will have no speed control and the motor will run at 22 rpm. it runs at 22 rpm right now no problem right? customer: yes
me: so how would hooking it up direct solve your problem?
customer: i need it to run at 10 rpm w/o stalling.
me: so how will hooking it up direct solve your problem
customer: because the vfd doesn't have enough torque at 10 rpm.
me: but i just proved that it the motor by wiring it up to the low speed
customer: it the vfd.
me: shoots self.

torque in a 3 phase motor

ganutenator

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jraef

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ganutenator

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Gene Bond

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jraef

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ganutenator

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