Motor Gurus

When a, let's use 1800 rpm motor, is started across the line the field is rotating 1800 rpm faster than the rotor at start up. Because the frequency is high (2pi FL) the current is limited. This reduces starting torque to less than breakdown.
What would be the frequency required to start that motor with maximum torque or current?
How would you calculate that number? Would you use the difference between full load and breakdown? As the motor came up to speed would you continue that same speed difference?
I would expect heating and a big reduction in starts per hour.


I should get some interesting comments from the motor guys out there.

the current is not limited and is high.
look at a VFD.

The motor torque production can be though of as having a direct relationship to the ratio of voltage and frequency that the motor was designed for. So if you maintain that ratio, you can create full load torque that the motor was designed to deliver. So for example a 460V 60Hz designed motor will produce FLT at any point where the ratio of V/Hz is 7.67:1, which is 460/60. So if you want FLT from a motor that is turning at 1Hz, you give it 7.67V, at 10Hz you give it 76.67V. This is essentially what a VFD does for you.

A little higher than that ratio and you can get Break Down Torque, based on the ratio of the current expected at BDT, which because once you are at that point, is going to be synonymous with current. So if your motor is capable of 225% FLT as BDT, then the current to get to BDT will be 225% of what is would be at FLT, with commensurate heating effects. That means the V/Hz ratio will be 17.25 V/Hz, so again at 1hz you give it 17.25V, at 5Hz you give it 86.25V etc. etc. At low speeds, the ability to do this accurately with all of the varying electrical equivalent circuit interactions going on gets more and more difficult with simplistic V/Hz drives, but that's where a "Vector Control" capable drive comes in to play. By being able to slice the "pie" of current going to the motor into smaller and smaller units of measure, a Vector Drive can effectively separate out the current vectors that produce torque vs the current vectors that produce flux, and maximize the torque production by only fluxing the motor as much as is absolutely necessary. So in essence, more of the total current going to the motor is producing torque when a Vector Control algorithm is used.

At that extreme low end there are also losses to contend with based on that issue of flux producing current as well, because although the majority of the circuit losses are based on the current through the windings or the load on them, there are some, about 25% of the total losses, that are fixed. As the overall power consumed by the motor and load gets smaller and smaller, those fixed losses begin to represent a larger PORTION of the energy consumed, which does show up as current, and therefor additional heat (in relation to the work performed). How much that is depends a lot on the motor design, but is usually reflected in the efficiency rating of the motor. This is why, to be accurate, a Vector Drive must establish and maintain a mathematical equivalency circuit for the motor connected to it, which is what is done in a "tuning" procedure at commissioning.

Thank you.
So if I set the VFD to ramp at a modest pace with torque boost that is all I'm going to get from that motor?

We try to use motors rated for use with a vfd.
the motor is more rugged than a standard motor.
Where I am now, everything has to be more sturdy due to the nature of the business.

janes

gas said:

Thank you.
So if I set the VFD to ramp at a modest pace with torque boost that is all I'm going to get from that motor?

Click to expand...

If you have only a basic V/Hz VFD and use torque boost at low speeds, you might get close to BDT, but there is no way to know for sure. A basic V/Hz VFD has no feedback loop to tell the drive how the motor is responding to what the drive is giving it, so the drive just puts it out there and you hope for the best.

A Vector Drive, even one with so-called "Sensorless Vector Control" (SVC), is looking at the motor model it created, comparing that to what it sees the motor doing in the feedback loop, and tweaks the output of the drive to maximize the motor performance. So if you NEED the motor to accelerate in a specific time frame, and it will require BDT from that motor to accomplish that, the SVC drive will make the motor deliver it, within the limits of the capabilities of the components (usually around 3 seconds at BDT). It cannot do miracles or magic, it cannot make the motor put out MORE than its rated Break Down Torque, but it can give it to you whenever you need it, at whatever speed.