Torque

[bob]

Hi Marke ,

 

Previously our Feeder Table was driven by a 43 k W 400 V switch reluctance drive. We had practically no problem to start the conveyor fully loaded. However, due to the scarcity of spare sparts we replaced the drive and motor with a 55 k W VSD and a standard squirrel cage motor. We could not manage to start the conveyor and had to upgrade to a 75 k W size.Even though, the VSD used to trip on overload during start. The VSD is set for sensorless flux vector.

How are VSD compared to SR Drives ?

 

Thanks

 

Bob

[marke]

Hello Bob

 

The torque available from the SR motor is very dependant on the motor and controller, but it does not suffer the same flux constraints as a standard induction motor. One of the problems associated with SR motors, is the high level of torque ripple, but that would not be an issue in this case.

 

I think that the proof of the pudding..... In you case, you cold have used an inverter with a higher output capacity and stayed with the 55KW motor. There are some inverters that are capable of more than the 150% limit imposed by most manufacturers. For example, with the Emotron VFX, it is possible to get up to 400% torque if the inverter is selected correctly.

 

Additionally, the closed loop vector will commonly give beter results at close to zero speed than the sensorless vector. There are quite big differences between the open loop (sensorless) vector drives below 5 Hz.

 

Best regards,

[anu_rags]

Hello Mr. Marke,

 

As you say VFX drives can go upto 400% starting torque, but to my knowledge standard AC motors are designed to give maximum 150% torque for short time. What I can understand is that the drive will not trip even if motor demands starting current upto 400% to start the conveyor. But here my question is that the IGBTs availble in market are for max. 200% current for short time, so what IGBTs are used by Emotron?

 

Regards,

 

[marke]

Hello Anu_Rags

 

Welcome to the forum.

 

Motors are capable of more than 150% torque for a short period of time. The maximum torque is dependent on the flux in the iron and most motors can develop 250 - 300% torque without increased flux.

 

Inverters have a maximum short term output current that is a function of the IGBT devices and other components. The continuous current is a function of the thermal characteristics of the cooling system. If we take a drive with a maximum output current of 100 amps and a maximum continuous current of 67 Amps, we describe this as having an overload capacity of 150% torque.

If we now rerate the drive from 67 amps to 33 amps, it will still have a maximum current of 100 amps and so it is now able to provide 300% torque. At the lower running current, the temperature of the heatsink will be lower and so the maximum current will actually increase a small amount, increasing the maximum torque capacity beyond the 300%.

 

With many drives, you can oversize the drive, but the maximum torque setting is 150%. You can fool the drive by telling it that there is a larger motor connected, but this defeats any motor protection and is not a desirable option. It is more normal to upsize the drive and the motor to get more NMs out the motor shaft. With the VFX, you upsize the drive, but not the motor to get more NM out the motor shaft.

 

Another major difference between many drives is the lowest frequency at which they can deliver maximum torque. This has a major bearing on their ability to "break away" sticky loads. It is not just the maximum torque, but also the maximum torque at xero shaft speed.

 

Best regards,

[mariomaggi]

Dear all,

peak-torque, accelerating torque, starting torque, maximum torque, pull-up torque are different items!

 

I'm convinced that it is possible to obtain a peak torque of 400% only if the motor is under-fluxed in nominal conditions. When the motor is saturated, an increasing of current cannot create a proportional torque.

 

Marke never wrote that it is possible to obtain 400% of starting torque!

 

Regards

Mario

 

 

 

[jOmega]

Mario,

 

how do you define or differentiate the difference between Peak Torque ... and Maximum Torque.

 

Looking at typical motor design curves for IEC and NEMA asychronous (induction) motors, one finds ....

 

 

LRT : Locked Rotor Torque (sometimes referred to as starting torque)

 

PUT : Pull-Up Torque

 

BDT : Breakdown Torque (sometimes referred to 'Peak' torque)

 

 

...and of course, there is Rated Torque (100% value) ..... of which the above three torques are often specified as a percentage of Rated Torque

 

 

It is common to see that the LRT is less than the BDT.

 

LRT is limited by the saturation of the iron of the motor.

 

BDT is more a function of SLIP as opposed to saturation of the iron ..

 

 

A PWM inverter, properly rated so as to be able to source the energy required by the motor to produce BDT, will allow the motor to develop BDT for starting. That is why an inverter will allow an asychronous motor to develop more starting torque than that same motor would develop when DOL started.

 

jO

 

 

[mariomaggi]

Dear jOmega,

 

I'm not the right person to reply, I try:

 

how do you define or differentiate the difference between Peak Torque ... and Maximum Torque.

 

BDT is a minimum torque that all motors of that series must warranty.

Peak torque is a possible potential peak torque that we could find on few motors of that series (this value is important during selection of a precision torque-sensor for test rooms).

 

LRT : Locked Rotor Torque (sometimes referred to as starting torque)

LRT is one thing, starting torque is another thing (average torque during the acceleration phase).

 

Regards, if I'm wrong please make corrections

 

Mario