Bringing 3 phase power onto my property

[Zerk]

We have lots 4 kV motors on VFDs, that run all the time for years. The power savings pays for it for us, because we don't need them to be ran at full speed.

I would have to read further into that warning, to find out if you really need to worry about it. I have never heard it mentioned before.

 

[Mendonsy]

Whats the cost differernce for a VFD for each machine VS one homemade rotary phase converter for the whole shop?

It depends on what you are trying to do. If you want indvidual VFD's for each machine so that you can use them for speed control they typically cost about $100 per hp (a 3hp is about $300 for example). If you just need a 3 phase power source to run several machines there are VFD's made specifically as phase converters that which are slightly less expensive.
The concern about older motors failing when connected to a VFD is definitely a real issue, but it can be resolved by adding a "load side reactor" (special transformer) between the drive and the motor. Almost everyone who sells the VFD's also has the matching reactors.

edit: Most (but not all) VFD's are made to run on 3 phase power. They will run fine on single phase but must be "derated" by a factor of 1.73.
example: If you have a 3hp motor that you want to run on single phase, you'll need at least a 5.25 hp VFD drive to run it. (3*1.73=5.19)

 

[CalG]

Whats the cost differernce for a VFD for each machine VS one homemade rotary phase converter for the whole shop?

I've got two home built RPC a 5hp and a 2 hp. I don't use the 5hp now, having switched to the VFD. The 2 hp unit supplies a small milling machine with two 2 speed motors (H &V spindle) plus the feed and suds pump. The complication of four motors on a single machine makes the RPC a good choice.

RPC can be put together for little money. Any 3ph motor will work, bigger is better, but at least as big as the largest motor you need to start. (think FLA)

I use one of those "static" phase converters to get the RPC spinning. These devices are little more than a starting capacitor and a current sensing relay to kick the cap out.
The spinner motor stays in line once running and generates the third leg. I've been in and out with "balancing capacitors". But now have removed them all. Still works just fine!

 

[techman]

"Motors are rated to withstand voltage magnification caused by harmonic distortion of the motor current. However, when the same motor is connected to VFDs, the motor and its insulation can be subjected to a repetitive stresses from voltage pulses caused by harmonic distortion that can seriously damage the motor or its insulation rating degradation, causing catastrophic failure in a short period of time. Even if such failure does not happen quickly, the pulses will eventually degrade the system and reduce the potential lifespan of the motor. This is due, in part, to the odd-order voltage produced by a VFD, and not the typical sinusoidal wave that is normally provided from a power utility. As a result, when motors are connected to VFDs, they will experience repeating voltage pulses. Those voltage pulses may, under certain circumstances, be magnified as they reach the motor terminals. For example, in tests under actual operating conditions, a 460 volt motor supplied by voltage from a VFD has been empirically measured with pulses as high as 1200 volts, in contrast with the 620-660 volt level measured in the absence of magnification. "

While the statement above is true, it is also not applicable in a number of ways. Small motors (under 10 hp) do not suffer from many of the effects described. Primarily they operate at much lower currents, greatly minimizing the effects described. In particular the high voltage effects are mostly related to the wiring to the motor. If the wiring between the VFD and motor is short, the effects are minimal. See the attached graphic for a 460 volt motor. Again in small motors the causation is also minimal due to low currents.

I work on applications utilizing VFDs of all sizes, and the problems described are virtually non existent with small motors. The biggest problem is when running at reduced speeds, the internal fan does not sufficiently cool the motor if it is at load, which can result in overheating.

paul

 

[CobyRupert]

While the statement above is true, it is also not applicable in a number of ways. Small motors (under 10 hp) do not suffer from many of the effects described. Primarily they operate at much lower currents, greatly minimizing the effects described. In particular the high voltage effects are mostly related to the wiring to the motor. If the wiring between the VFD and motor is short, the effects are minimal. See the attached graphic for a 460 volt motor. Again in small motors the causation is also minimal due to low currents.

I work on applications utilizing VFDs of all sizes, and the problems described are virtually non existent with small motors. The biggest problem is when running at reduced speeds, the internal fan does not sufficiently cool the motor if it is at load, which can result in overheating.


paul

^ What he said!! :thumbsup: My understanding is that it's the first winding of the motor that sees the spike (reflected wave) and is subjected to the risk of insulation breakdown. Besides keeping cable runs short, you can also:
-Use drive rated ("inverter duty") motors. (My understanding is the first motor winding insulation is beefed up, and they can handle the reduced cooling effects better if running at a slower speed) -Or,
Use a special terminator box (e.g. Allen Bradley 1204 terminator) at the motor that protects the motor from seeing this spike .
Use a filter on the output of the drive (check drive manufacturer recommendations / type).

 

[buickanddeere]

"Motors are rated to withstand voltage magnification caused by harmonic distortion of the motor current. However, when the same motor is connected to VFDs, the motor and its insulation can be subjected to a repetitive stresses from voltage pulses caused by harmonic distortion that can seriously damage the motor or its insulation rating degradation, causing catastrophic failure in a short period of time. Even if such failure does not happen quickly, the pulses will eventually degrade the system and reduce the potential lifespan of the motor. This is due, in part, to the odd-order voltage produced by a VFD, and not the typical sinusoidal wave that is normally provided from a power utility. As a result, when motors are connected to VFDs, they will experience repeating voltage pulses. Those voltage pulses may, under certain circumstances, be magnified as they reach the motor terminals. For example, in tests under actual operating conditions, a 460 volt motor supplied by voltage from a VFD has been empirically measured with pulses as high as 1200 volts, in contrast with the 620-660 volt level measured in the absence of magnification. "

This is why!!!

I have 1940's, 1950's, 1960's and newer three phase motors. I can't afford to replace them even if replacements were available. Some may have special frames specifically designed for the particular machine tool. I know it's 2016, again the machines are not even 1980.

That is why we don't purchase an old cheap VFD with a noisy spiked wave form unless inductive filters are installed between the VFD and the motor.
The new VFD's have a clean wave form.
As for motor failures on old VFD's . The motor was already weak and on it's last legs . It was about to fail VFD of not . Ever consider the inrush spike a VFD protects the motor from that an across the line starter causes?

 

[Zerk]

There are alot of VFDs out there.

 

[roadhunter]

I was just looking at the rates for the service. This is the same service rates used for pivots, water tanks, barns, churches, schools, and other non residential applications.
$32 access charge
.21 for the first 500 kwh
.12 for the next 1,000 kwh
.09 anything over 1,500 kwh

 

[Zerk]

It may be a deal considering how much power you use, but around here is around .13/kwh. Meter charge is less then $15 a month.

 

[roadhunter]

It may be a deal considering how much power you use, but around here is around .13/kwh. Meter charge is less then $15 a month.

Access fees are higher because this is a very rural area. There are many miles between houses in most of this region. $28 for residential. .13 for the first 500, .10 over 500 and .07 for off peak.