105 Derated motor of main 132 exd Motor Suitable for VFD

[Bmw318be]

I had a doubt for derated motor of 105 KW, with 132 kw.

5-50 Hz, constant torque application.

Q1 would the motor at 50 Hz producing 132 KW and as the frequency lower it goes lower KW

Or

At full load frequency it only has 105 KW here ?


It specify that the motor would be having sf of 1 and does it mean we would not be be able to run higher than 105 KW here

 

[Jraef]

I had a doubt for derated motor of 105 KW, with 132 kw.

5-50 Hz, constant torque application.

Q1 would the motor at 50 Hz producing 132 KW and as the frequency lower it goes lower KW

Or

At full load frequency it only has 105 KW here ?


It specify that the motor would be having sf of 1 and does it mean we would not be be able to run higher than 105 KW here

It's difficult to understand what you are asking, but I'll give it a try.

Seems as though maybe someone told you to use a motor rated for 132kW on an application that required 105kW and is using a VFD. You seem to be asking will the motor kW will drop with speed and maybe you want to know if the 132kW motor will only consume 105kW at full speed? Then you seem concerned about the reference to Service Factor. One at a time:

1. Motor rated for 132kW on an application that required 105kW and is using a VFD. Given that you are in Singapore and using kW instead of HP, I'll assume you are going to use IEC motors. I'm guessing that this over sizing is specified because in IEC motors, running from a VFD can have negative thermal effects on the motor. IEC motors do NOT employ a "Service Factor", so in effect, they are ALL 1.0 SF (see #3 below). So if you use them on a VFD, it's sometimes a good idea to use the next larger size above the actual load requirement, the motor will last longer.
2. Will the motor kW will drop with speed? Yes. kW and HP are a shorthand expression of "X amount of torque at Y speed". So if you retain the same amount of torque as you lower the speed, which is EXACTLY what a VFD is doing, then by definition you are dropping the kW output of the motor. But do not confuse kW with torque, they are not the same thing. As I said, that is what is behind the magic of a VFD, it is allowing the motor to produce the SAME torque regardless of speed. A corollary to this that I think may be behind your question is, will my 132kW motor consume 132kW of power at full speed? The answer is no. A motor consumes power commensurate with the LOAD you attach to the motor (plus any losses inside of the motor.) If your LOAD is only requiring 105 kW at full speed, that is all your motor will consume, plus a tiny bit more losses commensurate with the added iron in the larger motor frame, likely less than 1%. But by the same token if you used a 105kW motor on a VFD with a 105kW load, the motor would run hotter because of the VFD, so would have added losses anyway and the difference would be a wash, but the motor life would suffer.
3. You seem concerned about the reference to Service Factor. Here in North America, we use motors that are designed to NEMA standards. NEMA motor designs can include what we call a "Service Factor" (SF), which is an amount of extra thermal capacity designed into the motor that is kept in reserve of the actual nameplate rating in order to handle unforeseen issues that might adversely affect the motor thermally. A typical SF is 1.15, which essentially means you may overload the motor by 115% without loss of performance (but an expected loss of service life). But when you run a SF rated motor from a VFD, you CANNOT ALSO use the SF, or looked at another way, your SF becomes 1.0 regardless of the nameplate SF rating. However this is not a "law", this is just an accepted norm, which sometimes gets ignored by people trying to cut costs by not using the next size up motor. So you will often see Engineers specify that if you use a NEMA design motor, you MUST use it at a 1.0 SF, that way the low ball players can't cut corners and meet the spec. Basically this is essentially the same thing that the specifying engineer was telling you in the statement about using a 132kW motor for a 105kW load, don't skimp on the motor sizing to save money, because he knows the motor will be running warmer on a VFD. He likely added this just in case someone uses a NEMA motor and attempts to use the SF as a different way around his desire for the motor to last longer. This is often the result of a previous bad experience. "Note to self: Don't let THAT happen again!"

 

[Bmw318be]

Hi

Hi

Hi,

Let me rephrase my question for clarity.

My application require a constant torque motor at 700 Nm and this operation require VFD .

The motor vendor is proposing using 105 KW motor derated from 132 KW so in te motor has 2 name plates.

Q My question when I drive using VFD at 50 hz, would the maximum KW is 132 KW here ? .

I am encountering various power demand in applicqtion ranging fron 20 KW to 95 KW.

Q Would at various frequency the Kw is constant under some load

Eg running at 25 hz, my power is half the motor rated KW ?

 

[Jraef]

Hi,

Let me rephrase my question for clarity.

My application require a constant torque motor at 700 Nm and this operation require VFD .

The motor vendor is proposing using 105 KW motor derated from 132 KW so in te motor has 2 name plates.

Q My question when I drive using VFD at 50 hz, would the maximum KW is 132 KW here ? .

It will still be theoretically capable of 132kW, but you should not use it at that level if running from a VFD unless the motor specifically says it is "inverter rated" because of thermal issues. So the proposal to use this motor for a 105kW load is a prudent choice from that standpoint. There are other issues with running motors from inverters that should be addressed, but you do not mention them.

I am encountering various power demand in applicqtion ranging fron 20 KW to 95 KW.

Q Would at various frequency the Kw is constant under some load

Eg running at 25 hz, my power is half the motor rated KW ?

Yes, as I said, the kW is a function of torque and speed, so as you lower the speed and maintain the torque, which is exactly what a VFD does, your kW will drop at the same rate. So on a motor designed for 50Hz, if you run it from a VFD at 25Hz, the motor will be capable of 1/2 of the kW. Again, that does not mean it will CONSUME that much, the consumption is still based on the demands of the LOAD connected to the motor. But the motor capacity to handle an amount of kW at that speed is reduced.

 

[Open Neutral]

A typical SF is 1.15, which essentially means you may overload the motor by 115% without loss of performance (but an expected loss of service life).

Minor rant. Overload TO 115% not by.

Somewhere an English prof wrote a great article on use and misuse of numeric comparisons. I wish I still had a link.
("This post is 150% better than his last one...")

 

[Bmw318be]

Hi

Hi

Hi,

It is not inverter motor but a standard derated 105 kW motor. Theoritically it is capable of 132 KW, at what frequency the KW would be 132 kw if this is 4 pole 50 Hz motor ?

How long, the motor could be run at higher than 105 kw ?


Is getting an inverter duty the motor does not need to derate for vfd

It will still be theoretically capable of 132kW, but you should not use it
at that level if running from a VFD unless the motor specifically says it is "inverter rated" because of thermal issues. So the proposal to use this motor for a 105kW load is a prudent choice from that standpoint. There are other issues with running motors from inverters that should be addressed, but you do not mention them.


Yes, as I said, the kW is a function of torque and speed, so as you lower the speed and maintain the torque, which is exactly what a VFD does, your kW will drop at the same rate. So on a motor designed for 50Hz, if you run it from a VFD at 25Hz, the motor will be capable of 1/2 of the kW. Again, that does not mean it will CONSUME that much, the consumption is still based on the demands of the LOAD connected to the motor. But the motor capacity to handle an amount of kW at that speed is reduced.

 

[Jraef]

Minor rant. Overload TO 115% not by.

Somewhere an English prof wrote a great article on use and misuse of numeric comparisons. I wish I still had a link.
("This post is 150% better than his last one...")

You are correct, point taken.
I think my problem was switching units.

Overload BY A FACTOR OF 1.15 would have been in keeping to my previously established unit convention...

Or

Overload TO 115% once I changed from factors to percentages.

 

[Jraef]

Hi,

It is not inverter motor but a standard derated 105 kW motor. Theoritically it is capable of 132 KW, at what frequency the KW would be 132 kw if this is 4 pole 50 Hz motor ?

How long, the motor could be run at higher than 105 kw ?


Is getting an inverter duty the motor does not need to derate for vfd

If the motor nameplate says it is a 50Hz motor rated at 132kW, then that is what it is when it gets 50Hz. If the motor does not say it is rated for use with an inverter drive, you are on your own with regard to capabilities if you apply it thusly (watching my grammar closely now that I know I am being judged). So how long can you demand more than 105kW? No way to know, but I would avoid it altogether, that is the safest approach.

Using a motor designed for inverter duty would mean that they have already factored in the added thermal stress the motor will experience, so yes, a motor rated 105kW "inverter duty" can be safely run at 105kW without de-rating. Usually, "inverter duty" also means the motor winding insulation is also able to withstand the higher voltage pulses associated with what is called the "standing wave phenomenon" that is an unfortunate side effect of Voltage Source PWM drives (which constitutes all low voltage VFDs sold now). Standing waves form in the conductors going from the VFD to the motor and pulses can reach levels of as much as 3x the line voltage. Standard motor insulation is typically 2x the line voltage, so these pulses will "punch through" the insulation inside the windings and cause turn-to-turn shorts, which eventually lead to total failure.

Inverter rated motors will also often have special bearings with brushes that allow shaft currents to flow safely to ground rather than through the bearing races, causing EDM damage and premature failure. The shaft current is created by the high speed DC pulses in the PWM coupling capacitively to the rotor steel. If no alternate flow path is given, the current flows through the bearings themselves, acting like a microscopic arc welder so to speak.

These other issues can be mitigated externally of the motor as opposed to buying an inverter rated motor, but likely cost more than the added cost of upgrading.

 

[Open Neutral]

Inverter rated motors will also often have special bearings with brushes that allow shaft currents to flow safely to ground rather than through the bearing races, causing EDM damage and premature failure. The shaft current is created by the high speed DC pulses in the PWM coupling capacitively to the rotor steel. If no alternate flow path is given, the current flows through the bearings themselves, acting like a microscopic arc welder so to speak.

<Johnny Carson voice> I didn't know that....</Johnny Carson voice>

There's something new to learn every day.

 

[Besoeker]

Hi,

It is not inverter motor but a standard derated 105 kW motor. Theoritically it is capable of 132 KW, at what frequency the KW would be 132 kw if this is 4 pole 50 Hz motor ?

The motor can deliver 132kW at 50Hz. Whether it does depends on the load. And less at other speeds.

How long, the motor could be run at higher than 105 kw ?

The motor thermal limit is 132kW at 50Hz.

Is getting an inverter duty the motor does not need to derate for vfd

I wouldn't normally de-rate a motor for inverter duty. But we usually fit winding and bearing thermistors for protection.
But, as Jraef has noted there are other points you need to take care with.



Bearing currents. Besides earthing (grounding) the shaft, good cabling techniques minimise the potential for bearing currents. Normal practice here (UK) is to use steel wire armoured cable. The capacitance to the armour shunts the currents resulting from the fast switching edges away from the bearings. According to Gambica, good installation practice is is the most important element of achieving good bearing life.
I know of the bearing failure phenomenon but have not experienced it personally and that's after decades in this field. Maybe that attests to the installation and corroborates the Gambica stance.

FWIW, this what it can look like:



Insulation stresses. This is caused by the vary fast switching edges produced by modern PWM inverters. In contrast to the bearing failures, I have seen this all to often. It usually isn't immediate. It's cumulative over a period of maybe a few weeks to a few months eventually leading to a catastrophic insulation failure. One solution that I've used is to fit reactors at the VFD output. Or, as the other guy said, get the right motor in the first place.

Thermal rating. You mentioned that the load is constant torque. If the motor is to run at reduced speeds for significant periods of time cooling could be a problem. Most standard cage motors that I've come across are TEFC (totally enclosed fan cooled). The fan is shaft mounted and the cooling air directed across the cooling fins on the outside of the motor. Reduce the motor shaft speed and you reduce the effective cooling - dramatically. On the last constant torque application we did, we had a separately powered constant speed cooling fan.

Incidentally, the 700Nm is more than 105kW at rated speed.

Apologies for such a long post - I don't usually ramble on this long!

 

[Besoeker]

Overload TO 115% once I changed from factors to percentages.

Just to add my two pence (3.2 cents!) worth, the OP says that the application is 50Hz.
That would suggest either a European country of maybe an ex-British colony.

The 1.15 service seems to be a mostly North American requirement.
Here we generally rate motors as S1 which is CMR - continuously maximum rated.