# Thread: VFD - V/Hz High Breakaway Load

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## VFD - V/Hz High Breakaway Load

Hello,

Here are the facts:

VFD Drive
No Encoder
V/Hz Control mode
Load is Constant Torque, with high starting torque required
Motor = 1800RPM rated with 3% slip, 460V rated

V/Hz typically limited to 150% FLT at 3 Hz. Below 3Hz it get wishy washy and you need all sorts of voltage boost which you don't know if you will have enough torque to start the high inertia load.

Question:

I know the load is is at 0RPM (0Hz) at starting, but why can't I just have the drive in V/Hz send the signal to start the motor at 3Hz (87.5RPM). I am not too concerned with current inrush etc. I just need to make sure the load starts.This will put me in the portion of the Torque/speed curve for the capabilities of V/Hz that would allow 150% FLT. It seems like the drive will try to start at 0.001Hz (something arbitrarily close to 0 Hz) and work its way up with a possibility of the load not starting, but if the drive continues to increase the voltage and current without caring if the motor actually started or not eventually it would get to 100%Voltage (460V)and 60Hz at that point the motor would be able to be started at Full Load Torque and the 150% FLT.

What am I missing? Why is there such concern of using V/Hz at low speeds with high starting torque requirements.

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Can you set your min HZ to 3 @ 0 ramp time and second ramp could be whatever?

3. When a VFD is in Scalar control mode (V/Hz), the VFD puts out a PWM pattern to the motor with the required Voltage and Frequency that it WANTS the motor to turn at, but has no way of knowing if the motor is actually doing that. So if the motor "equivalent circuit" is not exactly what the VFD expects, the motor performance will be unexpected as well. At low speeds, this becomes more evident because the cable has capacitance, the motor stator/rotor relationship has capacitance and the entire setup has other variables like leakage reactance that become a larger part of the overall circuit when the speed is low. So as a general rule, standard motors and VFDs using Scalar control are good for at best, a "turn down ratio" of 4:1, meaning 1/4th of the base motor speed or from a Hz standpoint, 15Hz. You can go lower with the VFD, but that's where you will start finding a lot of instability and the motor problems get compounded. "Torque Boost" is a manual adjustment in the VFD that tweaks the V/Hz pattern coming from the drive that increases the torque, at the cost of starting to saturate the windings. So that feature generally has a point at which it reverts back to the proper V/Hz ratio to avoid overheating the motor.

If your VFD is capable of it, I suggest changing to what's called "Sensorless Vector Control" or SVC instead of V/Hz. That adds the ability for the VFD to "see" what's happening in the motor and automatically adjust the output to get you the desired performance on the fly, down to 1Hz or even lower (depending on the quality of the VFD). Your motor may still not be able to run at 3Hz without overheating, unless it was designed to do so or has a separately powered blower keeping it cool, but that's a separate issue. If your drive is capable of SVC, you will need to do what's called an "autotune" procedure that lets the VFD measure all of those variables in the circuit. Without that, you will not get the desired performance.

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With an induction motor you need slip in order to generate torque, and so if you can set it to start at 3Hz or higher (as I just saw was mentioned) then that should help start the motor...unless you need precise control of the speed as it ramps up at very low RPM. But in that case you should be using a synchronous motor.

The voltage has to be boosted above the constant V/Hz target at low RPM because although the inductive reactanced goes down as 1/f (which requires the voltage to be lowered to maintain the same current), the winding's resistance does not. So you need extra voltage at low frequencies just to maintain the current at a target value. And it is often boosted as you say at startup to provide some extra torque, although the magnetics will saturate if this is done too agressively.

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Originally Posted by Jraef
When a VFD is in Scalar control mode (V/Hz), the VFD puts out a PWM pattern to the motor with the required Voltage and Frequency that it WANTS the motor to turn at, but has no way of knowing if the motor is actually doing that. So if the motor "equivalent circuit" is not exactly what the VFD expects, the motor performance will be unexpected as well. At low speeds, this becomes more evident because the cable has capacitance, the motor stator/rotor relationship has capacitance and the entire setup has other variables like leakage reactance that become a larger part of the overall circuit when the speed is low. So as a general rule, standard motors and VFDs using Scalar control are good for at best, a "turn down ratio" of 4:1, meaning 1/4th of the base motor speed or from a Hz standpoint, 15Hz. You can go lower with the VFD, but that's where you will start finding a lot of instability and the motor problems get compounded. "Torque Boost" is a manual adjustment in the VFD that tweaks the V/Hz pattern coming from the drive that increases the torque, at the cost of starting to saturate the windings. So that feature generally has a point at which it reverts back to the proper V/Hz ratio to avoid overheating the motor.

If your VFD is capable of it, I suggest changing to what's called "Sensorless Vector Control" or SVC instead of V/Hz. That adds the ability for the VFD to "see" what's happening in the motor and automatically adjust the output to get you the desired performance on the fly, down to 1Hz or even lower (depending on the quality of the VFD). Your motor may still not be able to run at 3Hz without overheating, unless it was designed to do so or has a separately powered blower keeping it cool, but that's a separate issue. If your drive is capable of SVC, you will need to do what's called an "autotune" procedure that lets the VFD measure all of those variables in the circuit. Without that, you will not get the desired performance.
yes, I was going to suggest open loop control but I just wanted to get the theory if I could have the drive in V/Hz mode start outside of the “muddy” low range. Have it start at 3Hz, 10Hz, 15Hz what ever it takes to still be able to use V/Hz but still get the overload capacity needed to get the load going. Is that a possibility? If the drive doesn’t have feedback and just tells the motor what to do without checking in on it, what is stopping it from sending it a 15Hz signal with the appropriate voltage to be able to start the load? I mean if I didn’t have the drive and across the line started the motor it would be at 60Hz, 460V, and be able to develop ~ 160%FLT for the starting torque .

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Originally Posted by synchro
With an induction motor you need slip in order to generate torque, and so if you can set it to start at 3Hz or higher (as I just saw was mentioned) then that should help start the motor...unless you need precise control of the speed as it ramps up at very low RPM. But in that case you should be using a synchronous motor.

The voltage has to be boosted above the constant V/Hz target at low RPM because although the inductive reactanced goes down as 1/f (which requires the voltage to be lowered to maintain the same current), the winding's resistance does not. So you need extra voltage at low frequencies just to maintain the current at a target value. And it is often boosted as you say at startup to provide some extra torque, although the magnetics will saturate if this is done too agressively.
yeah that is the question. What’s stopping the drive from starting at 3hz or whatever larger frequency so that high starting loads can be started with V/Hz mode.

7. Originally Posted by spraymax6
yeah that is the question. What’s stopping the drive from starting at 3hz or whatever larger frequency so that high starting loads can be started with V/Hz mode.
A drive always starts from zero speed and ramps. All you can do is set a minimum speed, but from a dead stop, it still ramps to that point.

The drive’s ability to create torque in the motor is better than across the line, the drive is capable of a short burst (2-3seconds) of Break Down Torque, which is 200-220% of FLT. The problem is that in V/Hz mode, that ability is limited by the inherent inaccuracy of V/Hz control. With SVC you can have all the torque the motor is capable of at almost zero speed.

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What comes out of a drive in 'sensorless vector mode' is simply PWM synthesizing a given frequency and voltage for the desired torque and the rotor speed. Of course these values are constantly changing as the motor accelerates. So if you know the correct frequency and voltage to use to start the motor, you could just directly send those values, but without actually knowing the motor speed it is pretty hard to adjust the values as the motor accelerates.

As you note, you _could_ simply start the motor across the line, and would see a torque at zero speed that was set by the torque/speed curve of the motor, which is probably greater than 150% of FLT, but it really depends on the particular motor. Of course, when you do this the motor will want to draw high current depending on the motor characteristics, often on the order of 5x nominal full load current. If you wanted to use a VFD in this fashion it would need to be hugely oversized to supply the excess current at 0 speed.

You could do the same thing at _any_ frequency and supply voltage; for any giving electrical input parameters the motor will have its torque/speed curve, and will draw current and accelerate the load depending on that curve. If you pick a combination of voltage and frequency which, given the motor characteristics, will produce a torque/speed curve and a current/speed curve which are both acceptable from 0 speed on up, then you could use this programmed value to accelerate the load.

Doing this would almost certainly operate the motor less efficiently and require higher current than adjusting the drive frequency and voltage to match the rotor speed and required torque, but it clearly has to work in the same way that across the line starting works.

-Jon

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Originally Posted by Jraef
A drive always starts from zero speed and ramps. All you can do is set a minimum speed, but from a dead stop, it still ramps to that point.
So that is the question right there. Suppose you set the min speed. It is at a dead still. The motor cannot move at 0Hz, 1Hz, or 2Hz. The drive still keeps ramping to the 3Hz mark (it doesn't care that the motor didn't start spinning, it is simple V/Hz, no feedback). It now reaches 3Hz, has enough torque to move/start the load. Why is this scenario not possible? Is it that the drive would overload/fault out before it reaches the 3Hz if the motor didn't start turning.

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Originally Posted by winnie
What comes out of a drive in 'sensorless vector mode' is simply PWM synthesizing a given frequency and voltage for the desired torque and the rotor speed. Of course these values are constantly changing as the motor accelerates. So if you know the correct frequency and voltage to use to start the motor, you could just directly send those values, but without actually knowing the motor speed it is pretty hard to adjust the values as the motor accelerates.

As you note, you _could_ simply start the motor across the line, and would see a torque at zero speed that was set by the torque/speed curve of the motor, which is probably greater than 150% of FLT, but it really depends on the particular motor. Of course, when you do this the motor will want to draw high current depending on the motor characteristics, often on the order of 5x nominal full load current. If you wanted to use a VFD in this fashion it would need to be hugely oversized to supply the excess current at 0 speed.

You could do the same thing at _any_ frequency and supply voltage; for any giving electrical input parameters the motor will have its torque/speed curve, and will draw current and accelerate the load depending on that curve. If you pick a combination of voltage and frequency which, given the motor characteristics, will produce a torque/speed curve and a current/speed curve which are both acceptable from 0 speed on up, then you could use this programmed value to accelerate the load.

Doing this would almost certainly operate the motor less efficiently and require higher current than adjusting the drive frequency and voltage to match the rotor speed and required torque, but it clearly has to work in the same way that across the line starting works.

-Jon
From Jraefs post above the drive always seems to have to start at 0 and ramp up. I would ultimately would like to send 50% voltage and 50% speed command initially to the motor. This will guarantee that the motor will be able to start. But from Jarefs post. It looks like the drive in V/Hz will need to ramp up to get to that point, and I am assuming the issue is that it will fault out by the time it gets to a point that can move the load at starting. The thing about across the line starting it is that you hit it with the 100% voltage and 100% frequency right off the bat. And the motor doesn't have time to overload/fault out. It rips through its locked rotor torque position and keeps moving along its curve.

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