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Old 12-27-2017, 03:26 PM   #1
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Default Motor stalls after vfd install

Hi, I installed a Delta c2000 15kw 460v. vfd on a surface grinder because the boss wanted to be able to run the head faster for certain jobs. Original motor was run thru a softstart and was 1750 rpm, 20 hp, 460v. I bypassed the softstart and just used the drive. After programimg and testing, when above 60 hz, the motor just didnt have any torque. I figured it was because it was not an inverter duty motor and i was running it more than the rated 60 hz. We had another 20 hp motor with the same frame but 3500 rpm. So i installed it so i could run lower hz. I changed the motor parameters, 2 pole instead of 4, 460 volt, 23 amps, 3500 rpm. But still the motor stalls when put under heavy load. Amps are low with no load, but when pressure is applied, amps go up over the rated 23, and motor starts to stall. The drive is a used drive they had in stock, but seems to function ok otherwise.
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Old 12-27-2017, 04:16 PM   #2
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It may be efficiency losses, 20hp drive may not give out 20hp
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Old 12-27-2017, 04:34 PM   #3
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Hi, I installed a Delta c2000 15kw 460v. vfd on a surface grinder because the boss wanted to be able to run the head faster for certain jobs. Original motor was run thru a softstart and was 1750 rpm, 20 hp, 460v. I bypassed the softstart and just used the drive. After programimg and testing, when above 60 hz, the motor just didnt have any torque. I figured it was because it was not an inverter duty motor and i was running it more than the rated 60 hz. We had another 20 hp motor with the same frame but 3500 rpm. So i installed it so i could run lower hz. I changed the motor parameters, 2 pole instead of 4, 460 volt, 23 amps, 3500 rpm. But still the motor stalls when put under heavy load. Amps are low with no load, but when pressure is applied, amps go up over the rated 23, and motor starts to stall. The drive is a used drive they had in stock, but seems to function ok otherwise.
Two things. First roughly speaking Volts is proportional to Hertz. Second, current is due to two loads. Part is magnetization aka flux. This does no work. The rest is proportional to the torque (load). As you go above nameplate speed, Volts is limited by nameplate so instead of staying at constant torque with higher speed, you get constant power and trade speed for torque...So this explains the first case.

Second issue is that your load is probably variable torque...in fact wear is proportional to the 5th power of speed so even a small increase in speed is going to require an enormous increase in current to compensate. It might be only proportional to the square of speed but regardless, the effect is the same.

Using a higher speed motor makes this situation even worse. Horsepower = RPM x Troque / 5252 so doubling the RPM but keeping the HP the same cut your torque in half.

No way really around the fact here that the only real practical way to get where you want to go is with more horsepower, not less. Of course this has consequences too.

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Old 12-27-2017, 04:36 PM   #4
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Welcome to the reality of VFDs, as opposed to the fantasy created around them.

Yes, a VFD CAN make the motor go faster by increasing the Hz. But... and this is often NOT in the manuals, the VFD cannot make the motor actually do more WORK than it is designed to do. Here's hopw that works:
  1. A motor supplies torque at a given speed, the shorthand expression of which we call "horsepower", using the formula HP = Torque (ft. lbs.) x RPM / 5250 (a mathematical constant).
  2. Torque in the motor is a product of the current that flows, but is tied to the design of the motor in that it is proportional to the ratio of voltage and frequency applied to it, the "V/Hz ratio". So a motor designed for 460V 60Hz is designed around a V/Hz ratio of 7.67:1 (460 divided by 60). As long as you apply 7.67 V/Hz to that motor, it produces rated torque.
  3. A VFD is specifically designed to MAINTAIN that V/Hz ratio at all times, allowing the motor to develop its full torque at any speed, UP TO its design speed. So if I want to run at 50% speed, the VFD gives the motor 230V 30Hz, and 230/30 = 7.67.
  4. If however you want to run it at 90Hz, in order to maintain a 7.67 V/Hz ratio, you would need to give the motor 690V. But if you have a 480V system, the VFD cannot create voltage that is not there, you are limited by the 480V input. You cannot then maintain the V/Hz ratio above what the motor was designed for, 460V 60Hz.
  5. So what happens if you just increase the frequency is that the motor runs in CONSTANT HP mode, meaning it still follows the same HP formula, but the V/Hz ratio will drop. When that drops, the TORQUE output drops at the SQUARE of the change in V/Hz ratio. So at 90Hz, the V/Hz ratio becomes 460/90 or 5.11, which is 67% of the motor design ratio. Therefore the torque drops by .67 squared, meaning the motor now produces only 44% of its design torque. If you push it to 120Hz, the motor is only producing 1/4th of it's design torque. I once proved this to a class by running a 1HP motor up to 400Hz with the VFD and stopping the shaft with my hand (wearing a welder's glove) because there was almost zero torque left.
Now, there is a possible trick to getting what you want.
IF your motor is designed as 230/460V, AND you have 480V power for the VFD, there IS a way to get higher speed with full torque. You wire up the motor for 230V, then program the VFD to put out 230V at 60Hz (3.83 V/Hz). That allows you to run it up to 120Hz at 460V, because that is still 3.83 V/Hz, which is what the motor wants.


But BEFORE you attempt that, you MUST check with the motor manufacturer (and in your case the grinding wheel) to make sure the system is CAPABLE of those speeds without becoming shrapnel!
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Last edited by JRaef; 12-27-2017 at 04:41 PM.
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Old 12-27-2017, 07:33 PM   #5
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But BEFORE you attempt that, you MUST check with the motor manufacturer (and in your case the grinding wheel) to make sure the system is CAPABLE of those speeds without becoming shrapnel!

This part is very important especially if you going over the stock speed setting and you have to double check the specs how high ya can spin it.

I have see aftermath of mess when someone find a way to overdrive it.
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Old 12-27-2017, 08:11 PM   #6
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Me too. It was ugly in one case, involving plastic surgery. Pillow block bearing race shattered and came to pieces. The motor was rated for 4000RPM, they forgot to check the other parts.
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Old 12-27-2017, 11:29 PM   #7
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Me too. It was ugly in one case, involving plastic surgery. Pillow block bearing race shattered and came to pieces. The motor was rated for 4000RPM, they forgot to check the other parts.
That is one serious crap goof up if they not checking the rest of system to handle that kind of speed.

I just have one last year a large fan just actually blew apart I did see aftermath but that was kinda spooky when that went out. the curpit was someone took the AC motor which it was rated for 1200 RPM and put in the DC motor that rated for 2500 RPM
and blew the fan apart even with 3/8 steel plate around the fan shround just blew apart.,
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Old 12-28-2017, 10:34 AM   #8
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Thanks for everyones input.
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Old 12-29-2017, 12:24 AM   #9
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Originally Posted by JRaef View Post
Now, there is a possible trick to getting what you want.
IF your motor is designed as 230/460V, AND you have 480V power for the VFD, there IS a way to get higher speed with full torque. You wire up the motor for 230V, then program the VFD to put out 230V at 60Hz (3.83 V/Hz). That allows you to run it up to 120Hz at 460V, because that is still 3.83 V/Hz, which is what the motor wants.


But BEFORE you attempt that, you MUST check with the motor manufacturer (and in your case the grinding wheel) to make sure the system is CAPABLE of those speeds without becoming shrapnel!
This doesn't make sense to me. If you were to wire the motor in low voltage and run 480 through the windings they burn up rather quickly.

I would think you would set the nameplate freq to 120 htz, voltage at 480, motor high volt connected. That way at 120 htz you have 480 output.

Yes? No?
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Old 12-29-2017, 08:47 AM   #10
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This doesn't make sense to me. If you were to wire the motor in low voltage and run 480 through the windings they burn up rather quickly.

I would think you would set the nameplate freq to 120 htz, voltage at 480, motor high volt connected. That way at 120 htz you have 480 output.

Yes? No?
A dual voltage motor is capable of 480 on the windings whether it is wired for LV or HV...The insulation does not change.

If you wire for 480 and base speed set to 120 Hz you get 60 Hz and 230 V at 50% of speed. Torque will be 50% since it is underfluxed for that speed and basically all speeds...wrong direction.

Wired for 230 V it gets the proper flux and frequency at 230 V so you get nameplate HP. As frequency and voltage continue to 100% you get double everything. Speed doubles and flux keeps up, as long as amps don't get past name plate since we can't violate I2R. BUT be aware especially of your bearing ratings (2500 RPM is typical for a 4 pole motor) and if say load torque is the square of RPM you will possibly get more out but it won't be 200%. So it's a trick to make a motor potentially double speed and HP rating but physics still applies.

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Old 12-29-2017, 10:01 AM   #11
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ok makes way more sense. I have an application where I want to hit 70htz or 2059rpm at the motor. The motor is a weg sc-00718et3e215tyz, according to tech support an over speed of 70htz would be fine but he seemed lost when I explained how I was going to avoid the torque loss. This motor turn a vickers hydraulic pump rated for 2800rpm.

One of the experts care top chime in?

thanks, sorry for hi jacking the thread.
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Old 12-29-2017, 10:45 AM   #12
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ok makes way more sense. I have an application where I want to hit 70htz or 2059rpm at the motor. The motor is a weg sc-00718et3e215tyz, according to tech support an over speed of 70htz would be fine but he seemed lost when I explained how I was going to avoid the torque loss. This motor turn a vickers hydraulic pump rated for 2800rpm.

One of the experts care top chime in?

thanks, sorry for hi jacking the thread.
That's where the LV wiring trick works. Probably not even necessary but it's there if you get torque (current) limited at 70 Hz.

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Old 12-29-2017, 04:58 PM   #13
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That's where the LV wiring trick works. Probably not even necessary but it's there if you get torque (current) limited at 70 Hz.

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Correct. Remember I said that if your load doesn't NEED any more than the torque available at 70Hz, the fact that it is not there becomes irrelevant.
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Old 12-29-2017, 06:58 PM   #14
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Thanks guys! always learning something new
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