Can VFD be set for 3 different speeds with 3 different time & load cycles in a full duty cycle? 6

[edison123]

Can a VFD be set for 3 different speeds and 3 different time & load cycles of

x1 RPM/t1 sec/a1 amps

x2 RPM/t2 sec/a2 amps

x3 RPM/t3 sec/a3 amps in a full duty cycle?

Motor capacity is 5 KW, 6 pole, 50 Hz with external blower for cooling.

Muthu

http://www.edison.co.in

 

[waross]

Use a PLC or other micro computer.
Change speeds by switching in resistors in the speed control circuit.
If the external speed control is a current loop, you may have to add a small power source to generate a current.
There may be some VFDs with computing power built in, but external control gives you a much wider choice of VFDs.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[iop95]

Yes, there are VFDs that allow such setting.
Usually it use 2-3-4 digital inputs to setup 4-8-16 speeds.
Some allow to configure different acc/dec time for each speed.

 

[jraef]

Speeds, yes, there are several VFDs that can do this. Often up to 7 “steps” of speed/time. Not all VFDs are created equal though, so it’s something you need to discuss with your supplier ahead of time.

Amps? I don’t think so unless you can divine a speed that will correlate to a specific speed current. Many drives can be set up for “current limit” at a specific value, several can have two different current limits. I’ve never seen more than two.

But yes, with a PLC and a communication system, you can often send a current limit value into the appropriate register in a drive to accomplish this.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[edison123]

Thanks for your tips.

The present motor is 3 windings 3 speeds (6, 10 and 32 poles) and they are all squeezed in the same slot space and they fail often due to frequent starts. My idea was to have to one winding with generous copper and do the VFD do the required speeds and cycle times. Accel/Decel times option for each speed would be great too.

Of course, the load current is determined by the load and not by VFD at each speed. Sorry for not being clear about that.

So, any run of the mill VFD won't do and I need to spec these different speeds and times at the time of inquiry?

Muthu

http://www.edison.co.in

 

[zlatkodo]

Hi, edison,
I'm just curious. Do you have a photo of the motors's nameplate?
How many slots does it have?
ACW

 

[edison123]

Stator/rotor = 48/65

Muthu

http://www.edison.co.in

 

[LionelHutz]

The VFD doesn't need 3 current setpoints on a single winding motor. Just use digital inputs to select fixed speeds which most VFDs are capable of doing.

The VFD will cause the motor to have a constant torque as you reduce speed, not an increasing torque like changing the poles can provide. This means the power capability of the motor reduces linearly with speed. A 6 pole motor operating at 32 pole speed would be capable of 6/32 = 18.75% of rated power.

 

[zlatkodo]

Wow, even 25 Amps at 0.42 kW and 13 Amps at 2.2 kW and 15 Amps at 3.7 kW!!!
It's obvious that the motor is intended for some short-term duty-cycle.
I believe rewinding it won't be easy either.
Attempting a redesign with 6 poles and supplying power through a VFD is a very clever idea.
Notice: consider redesign to 8 poles in order to avoid 48 slots/6 poles winding that is always more or less unbalanced.
ACW

 

[edison123]

Lionel

Yes, the power will go down with the speed as the nameplate shows.

Quick question - Is it possible to set individual V/F for each of the 3 speeds?

zlatkodo

Thanks. Yes, it is a short time but severe duty application in tyre industry. In the past 3 years, I have rewound many motors from different clients with my own tweaked redesign. While none of them failed so far, I expect their life to be short given the duty severity and hence my thought to rewind it for one speed with generous copper and use the VFD. Present 6 pole winding is 332 332. For 8 pole, I have to go for field weakening to get 1000 RPM which reduces the torque.

Muthu

http://www.edison.co.in

 

[waross]

Can you design the windings for less than grid voltage?
The VFD will supply lower PWM voltages for the slower speeds and you can then, possibly, apply full voltage and get full torque when needed.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[zlatkodo]

Would the current remain the same (25 Amps) at 165 rpm, and how powerful should the VFD be?

 

[edison123]

Bill
That's a good tip. Rewind for a lower voltage and increase V/F (to the possible max input V) when torque boost is needed. It's a possibility. Thanks.

zlatkodo
Current during slow speed (165 RPM) operation is 22 to 23.5 Amps.
Slow speed operating cycle is a 10 minute cycle with total of 64 seconds ON in 10 minutes.
But in 10 minutes, it is switched on almost every minute. (10 starts)
So in one hour, load duration for slow speed operation is 384 seconds (6 min, 24 sec).
This operation repeats 43 times in 8 hours.
In 8 hours, slow speed operates for 43*64 =2752 sec (46 min) with 43*10 = 430 starts

Rest of the duty cycle is operated in other two speeds about which I don't have the data. A whirlwind of action in tyre making.

I guess a VFD rated for 30 Amps minimum is needed.

Or the VFD won't work at all given the widely varying load cycle. What do you drive gurus think?

Muthu

http://www.edison.co.in

 

[LionelHutz]

You could consider re-winding it as a 12-pole, 208V, 50Hz motor. Set the VFD to maintain constant V/Hz up to 120Hz where it reaches 415V. That way, you can spread the 3 speeds over 120Hz. You get double the 12-pole design power at the 6-pole, 120Hz speed and 38% of the design power at the 32-pole speed.

Quick question - Is it possible to set individual V/F for each of the 3 speeds?

You don't set V/Hz, or the current rating, for each speed on a VFD. You set the V/Hz ratio of the motor and the FLA of the motor and then VFD takes care of running it at whatever speed you need.

 

[edison123]

That is an excellent idea, Lionel. Thanks. I can do 200 V, 12 pole VFD duty winding and get the 3 speeds (or any other speed the client wants) with the same V/Hz up to 6 pole speed for 400 V.

As you can see, in 10 minutes, the cycle goes from low speed, mid speed and high speed with varying time cycles for each speed. Can I use a standard VFD to do those time cycles? Or I need a custom built VFD? Is 30 Amps rating for VFD enough for such a harsh duty cycle?

Again, it's a brilliant idea. Thanks.

Muthu

http://www.edison.co.in

 

[LionelHutz]

The motor is going to end up rated somewhere in the 30-40A range when re-wound at the lower voltage. The drive would have to be 400V rated for the FLA of the 200V winding. You need a VFD rated somewhere around 1.5x the FLA of the motor. At minimum, a VFD rated constant torque that matches the FLA of the motor.

 

[iop95]

On original motor, nominal torque at first and second speed is about 37Nm and 24Nm for third speed.
Actual winding of first speed (960rpm) is enough to cover all 3 speeds on VFD.
Sure, may change to a new winding with various voltage/frequency.
I think best choice is 6 poles, 400-415V nominal voltage and 50Hz base frequency. So a 16-20A VFD will be ok.
Without VFD may test/use motor DOL.

 

[edison123]

Lionel - Springing for higher 50 Amps VFD for one time shouldn't be a problem for the rich clients who are losing these motors often with much higher production loss costs.

iop - Thanks. Lionel's idea of 12 pole 200 V gives a wider choice of V/Hz (if required for a torque boost), don't you think?

Still not sure if the VFD can be programmed for such widely varying time cycles.

Muthu

http://www.edison.co.in

 

[zlatkodo]

Hi, edison,
It seems to me that a lot of things need to be done to achieve that.
Perhaps it would be more practical to leave the winding as it is and change the cooling method by adding strong external forced ventilation, if feasible.
ACW

 

[LionelHutz]

The 12-pole is an option to consider if that makes a better motor design compared to the 6-pole. My simplistic view is that the motor operates as a constant torque device with the VFD and a 12-pole produces more torque than a 6-pole so at least on paper the 12-pole, 120Hz method should be better. But, you really need to consider your designed motor output for both options.

As for the VFD, you can get the same speed range with a 6-pole running to 60Hz or a 12-pole running to 120Hz. The turn-down at the lowest speed might be better and the torque/power available when running through the speed range might be better with 12 poles.