4 Advice to Choose a medium voltage electric motor supplier

If your company is considering purchasing Medium Voltage Variable Frequency Drives, there are multiple factors to consider besides upfront cost. We cannot cover all of these in this short article, but these are a few to think about during the decision-making process.

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1. Application

Unlike low voltage drives, where there are two very distinct markets for two very distinct load types: Constant and Variable Torque, most medium voltage drives are built for variable torque applications such as pumps and fans since the majority of motors that size utilizing VFDs are running variable torque applications. There are, however, many constant torque applications such as extruders, grinders, and conveyors that utilize medium voltage variable frequency drives. Variable torque drives are designed to provide full load torque at high speeds, which is in line with how a variable torque application works. If your application will require full load torque throughout the speed range, you’ll need to purchase a medium voltage VFD that is rated for constant torque. EMA can help size this appropriately.

2. Capacitor Type

There is no universal capacitor specification for medium voltage drives that calls for what type of capacitor needs to be used on the DC Bus. The three most common are electrolytic, film-type, and oil-filled. The overall cost of ownership varies greatly depending on what type of capacitor your medium voltage variable frequency drive utilizes because the lifespan of each type is different. The overall lifespan of the three most common capacitor types are:

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• Electrolytic: 7-10 years
• Film-Type: 15 years
• Oil-Filled: 20-30 years

3. Pre-Charge Circuitry

For medium voltage drives, different manufacturers utilize different methods of dealing with pre-charging or “softly” charging the capacitors in the DC Bus bank. For more on the purpose of pre-charge circuitry, see here. Toshiba and TMEIC are two manufacturers (there are others) who utilize pre-charge circuity to reduce the effects of inrush current on the drive’s components and transformers. Siemens and Rockwell utilize the impedance of the phase-shifting transformer in lieu of an actual pre-charge circuitry, which can be problematic because the initial inrush can cause significant stress on the transformer and cause eventual damage

4. Maintenance Time and Cost

Medium voltage variable frequency drives are often on critical applications where downtime is extremely costly. We recently talked to a cement plant that told us that downtime costs them $300,000 per day! When every second costs money, having a drive that is easily repairable can save thousands of dollars. Many MV drives use power cells aka power arms on the output which are modular and can be swapped in 30 minutes or less. It is highly suggested to purchase at least 1 spare power cell during the initial drive purchase and to rotate the spare into the drive during scheduled preventative maintenance. Other spare parts such as power supplies, control cards, and fuses should also be shelved because you simply cannot count on the manufacturer to have these parts when you need them; it is not uncommon for there to be very long lead times on very common medium voltage VFD parts. EMA stocks many spare parts for medium voltage variable frequency drives, so if you’re in that bind, please contact us. 

5. Output Topology

All variable frequency drives create an output to the motor by switching the DC bus into a variable voltage output. “Topology” refers to how the VFD goes about creating that output and this varies from manufacturer to manufacturer. Some medium voltage variable frequency drives are more “motor friendly” than others because their output is closer to a pure sine wave. The tradeoff to that is it utilizes more components which adds more points of failure. The amount of distance between the motor and the drive and the construction of the motor plays a major factor in this as well.

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I am aware that there is really no specified starts/hour with medium voltage motors (4.16kV), and that most manufacturers use the NEMA MG-1 standard that says the given motor can have two consecutive cold starts and one hot start for a given load intertia.

Although its not specifically spelled out, I believe it is a standard approach to limit starts to 2 starts per hour, and we somewhat follow this philosophy when setting up our relays.

What if however the driven load has an inertia much less then specified on the datasheet, can you then achieve more consecutive starts, or more starts per hour? For instance what if your relay curve is set below the motor cold rotor thermal damage curve to protect the motor, and when starting you only use up 20% of the thermal capacity? At first glance this would tell me that I would be capable of ahieving much more than 2 consecutive starts (maybe 5) before I reach 100% thermal capacity. My question is should we allow more consecutive starts in this case or should we still limit to the 2 consecutive starts? This would kind of go hand in hand with the number of starts per hour as well.

You can start anytime the temperature has stabilized to normal running temperature. You can do two starts anytime the temperature is within 5C of ambient.

If the motor can cool back down to a stabilized normal temperature 10 times an hour, you can start 10 times. It is all about the internal temperature, not necessarily time. The time becomes a factor only because of how long it takes to cool down the innards.


Add: MG-1 has more info than just the 2-1 data. You could always ask the motor manufacturer for more detail on increasing the "default" number of starts.

MG-1 is a minimum standard. Keep in mind though that there is no, or very limited, punitive recourse if a manufacturer says that their motors are constructed to MG-1, but in actuality they are not meeting the requirements of MG-1. Since there is no third party independent testing practiced, it remains a wild-west territory.

MG-1 is clear that the restart restriction refers to 100% inertia. The motor algorithm in most SS motor protection relays calculates the safe restart temperature and based on that may give you the safe restart time. When, in addition to the current, RTD's are available and connected, this number becomes less conservative. The relay usually initialized, that occurs at the first start where it profiles the start-up characteristics T/C and builds a mathematical model for the motor/load combination. Each manufacturer of relay has their motor algorithm. Obviously those who have access to motor manufacturing data would have a better chance to build a more realistic model. These would be Siemens, GE, ABB, etc. that builds both relays AND motors.

While Mivey is correct that the determination is temperature based, but the safe restart temperature varies with the inertia and specific motor configuration. Eg. the larger the motor mass is the lower the required ' 'safe' temperature will be. The same is true inversely with the driven mass. The ambient temperature will also have effect.

MG-1 gives no other guidance than to talk to the motor manufacturer. The other warning it states that: 'It should be recognized that the number of starts should be kept to a minimum since the life of the motor is affected by the number of starts.'

So talk to the manufacturer AND add a protective relay that will help you determine the safe restart.
Good advice above.

The difference in running and stopped cool down times surprised me when I first saw it on a relay's motor winding temperature recording. A HP motor drove a pump through a hydraulic variable speed coupling so it could start and run essentially unloaded. When the motor started and ran unloaded for 10 minutes, the temperature was much less than after an aborted start when the motor hit full speed, tripped, coasted to a stop and sat for 10 minutes.

When it was running at light load, the cooling fan cooled the motor. The only cooling when stopped was by radiation and conduction so it took a lot longer to cool down.

The number of starts depends on what the motor is doing between the starts and the nature of the start.

These motors hit full speed so fast (< 2 .5 seconds) that it was possible to do 5-6 starts an hour after we verified that the cool down times and other values in the protection relay matched the actual motor characteristics. The motor vendor was involved.

One concern was rotor heating during acceleration. Rotor temperature is not measured by the RTD's. It has to be estimated by the relay software. Just depending on a winding RTD to tell if its OK to start may lead to an overheated rotor with melted bar joints.

It is not that simple. They need your data in order to calcualte it and it is a case-by-case data. I guess they should be able to supply you with a curve, but on most applications this seem to be woring well. It is perhaps also why MG-1 does not give requirements at any other loads.

Off subject. I hate it that we don't have a spell-checker on this dang machine. I often just cut and paste it into MSWords, let it catch the errors and paste it pack. AT home I sue Firefox on Vista and it DOES spell-check right in the forum as I type.

I 'learned' typing in the air force with two fingers, hunt and peck. Now I advanced to four fingers, but still don't look at the screen when I am typing. When I went to school we did not have keyboarding. Does that sound ancient to some of ya?

Anyways, this is the only public apology I offer for my typing, but I am always regretful when some of that slips through. Don?t cut me slack though for bad English, nor should you expect the same from me.

Off subject. I hate it that we don't have a spell-checker on this dang machine. I often just cut and paste it into MSWords, let it catch the errors and paste it pack. AT home I sue Firefox on Vista and it DOES spell-check right in the forum as I type.

I 'learned' typing in the air force with two fingers, hunt and peck. Now I advanced to four fingers, but still don't look at the screen when I am typing. When I went to school we did not have keyboarding. Does that sound ancient to some of ya?

Anyways, this is the only public apology I offer for my typing, but I am always regretful when some of that slips through. Don?t cut me slack though for bad English, nor should you expect the same from me.

You can get a spell check that runs in IE if the IS department will let you download it (I can't imagine why they would think it was an issue). There may be a grammar checker as well but I have not looked.

I often just cut and paste it into MSWords, let it catch the errors and paste it pack. AT home I sue Firefox on Vista and it DOES spell-check right in the forum as I type.
Make sure you have plenty of money before you start that action........:grin:
Struck me as funny given the context. And it's also an example of something that MSWord wouldn't pick up. I've also had a few odd "corrections" offered by Word. In a quotation I was preparing (in an old version) it wanted to change "Borehole Pumpset" into "Brothel Dump Site". I was vaguely tempted to leave it but, regrettably, common sense kicked in.......

Like you, I also use Firefox. It's on my laptop which is mostly what I use everywhere. The spell check is handy. My one reservation is that I can't find an option for British English. Obviously US English is fine here but, if I used it in other places, I might have to apologize for spelling apologise incorrectly.