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01-02-2012, 06:50 PM #1
Minimum operating Hertz 3 phas motor
I am operating a 3 phase Baldor 5 hp motor 460V. What is the minimum Hertz that I can safely run this motor? It runs fine @ 30 Hertz but am unsure if this might harm the motor?
01-02-2012, 07:32 PM #2
If you can run it that slow you are pretty safe as the drive will protect it from most conditions. The only thing the drive can't see accurately is motor temperature. If it's a typical motor it needs it own fan to cool it. If this slow speed is an ongoing deal you might think about installing some type of aux cooling system for the motor.
01-02-2012, 08:12 PM #3
You can go much lower than that but the voltage has to be reduced proportionately. At least in the drive I have, that can be adjusted, which means it can be adjusted to the wrong ramp. YMMV.
01-02-2012, 10:12 PM #4
I like to use 20hz as a low limit.
With a full featured VFD, properly set up, motor heating is calculated and protection is provided by the VFD. At least this is the case with Toshiba G3 and Siemens Micromaster devices.
My understanding is that less than 5hz is considered "slow" even for a well matched motor drive combination. I would only use that frequency with no load unless you have over-sized the motor enough to do the work.
01-03-2012, 06:08 PM #5
calg I would be mighty interested in info on the siemens or toshiba that states it will protect a motor at low hz from overheating; not sure of anyway that can be so other than watching the motor thermostat since motors are all designed differently - hence the reason I see for a "rule of thumb" answer only. vfds will keep v/hz curve the same all the way to zero hz - or increase it at low hz in something called voltage boost - to get more torque at low speeds - that just heats the motor up MORE, so again, not sure how they can say they will protect the motor from overheating other than again simply watching the thermostat. If u have some info to the contrary please share!
motors that need to produce full rated torque to 0 speed are either oversized mechanically (make it a 5 hp motor and call it a 3 and then the torque output at 3hp is only 3/5ths what it is good for so it can do it - nice cheat or have a separately excited fan in them them so it still cools at the <10hz speeds.
S_W_Bausch liked this post
01-03-2012, 07:08 PM #6
I'm not a VFD sales person or an engineer of any sort but I tend to agree with mike_kilroy...the drive has no ability to see the actual temp of the motor (windings) so cooking the goose could be a possibility.
My other thought is why would you run the motor at 30 HZ? If this is the operating range of whatever the output is, re-belt or re-gear so the motor is operating near nameplate RPM. I don't know your application but mechanically getting in the ball park is always the first step..then tweak the motor speed to fine tune the operation.
01-03-2012, 08:23 PM #7
The thermal "sensor" is a calculation based on entered motor parameters.
Stops/ starts/ load amps etc, all come into the equation. Even the length of the feed wires.
I can't believe it is exact, It may even be so much smoke and mirrors. but certainly it sounds good. Plus, the response on fault can be programmed.
Do Nothing, output signal, shut down, etc.
You could easily download the literature from the Siemens web site. Read through and enjoy ;-) 142 pages in PDF format. I would be happy to send the file to your e-mail.
01-03-2012, 08:32 PM #8
sample text from Siemens literature
P0610 Motor I2t temperature reaction Min: 0
CStat: CT Datatype: U16 Unit: - Def: 2
P-Group: MOTOR Active: first confirm QuickComm. No Max: 2
Defines reaction when motor I2t reaches warning threshold.
0 No reaction, warning only
1 Warning and Imax reduction (results in reduced output frequency)
2 Warning and trip (F0011)
Trip level = P0614 (motor I2t overload warning level) * 110 %
The purpose of motor I²t is to calculate or measure the motor temperature and disable the inverter if the
motor is in danger of overheating.
The motor temperature will be dependent on many factors, including the size of the motor, the ambient
temperature, the previous history of the loading of the motor, and of course, the load current. (The square of
the current actually determines the heating of the motor and the temperature rises with time - hence I²t).
Because most motors are cooled by built in fans running at motor speed, the speed of the motor is also
important. Clearly a motor running at high current (maybe due to boost) and a low speed, will overheat more
quickly than one running at 50 or 60 Hz, full load. The MM4 take account of these factors.
The drives also include inverter I²t protection (i.e. overheating protection, see P0290) in order to protect the
units themselves. This operates independently of the motor I²t, and is not described here.
The measured motor current (r0027) is compared with the rated motor current (P0305), and other motor
parameters (P0304, P0307, etc.), and the temperature of the motor calculated, a calculation which also
includes the output frequency (motor speed) to account for fan cooling. If parameter P0335 is changed to
indicate a forced cooled motor, the calculation is modified accordingly.
Where parameters are not entered by the user, such as P0344 (motor weight), a calculated value will be
used based on a Siemens motor. If required, the motor time constant can be adjusted using P0611, in effect
overwriting the calculated value.
The resulting temperature is displayed in % of maximum temperature in r0034. When this value reaches
the value set in P0614 (default 100%), a warning A0511 occurs. If no action is taken and the temperature
reaches 110%, then the inverter trips, showing F0011. The reaction to the warning can be changed from
this default using P0610; for example, the drive can react as though a current limit has occurred, or a fault
forced immediately. The warning level of P0614 can also be adjusted to raise and lower the warning or trip
level as required.
Parameter r0034 is particularly useful to monitor if the calculated motor temperature is rising excessively.
P0611 Motor I2t time constant Min: 0
CStat: CT Datatype: U16 Unit: s Def: 100
P-Group: MOTOR Active: Immediately QuickComm. No Max: 16000
Defines motor thermal time constant and is calculated automatically from the motor data (see P0340).
A larger number increases the time taken for the calculated motor temperature to change.
Default value is dependant on inverter rated power.
P0614 Motor I2t overload warning level Min: 0.0
CStat: CUT Datatype: Float Unit: % Def: 100.0
P-Group: MOTOR Active: first confirm QuickComm. No Max: 400.0
Defines the [%] value at which alarm A0511 (motor overtemperature) is generated.
Motor I2t calculation is used to estimate a maximum tolerable period (i.e. without overheating) for motor
overload. The I2t calculation value is deemed = 100 % when this maximum tolerable period is reached (see
A motor over-temperature trip (F0011) is produced at 110 % of this level.
P0640 Motor overload factor [%] Min: 10.0
CStat: CUT Datatype: Float Unit: % Def: 150.0
P-Group: MOTOR Active: Immediately QuickComm. Yes Max: 400.0
Defines motor overload current limit in [%] relative to P0305 (rated motor current).
Limited to maximum inverter current or to 400 % of rated motor current (P0305), whichever is the lower.
See function diagram for current limitation.
mike_kilroy liked this post
01-03-2012, 08:42 PM #9
The Toshiba units have similar functionality, and will display the percentage of "thermal overload" on the display. That is, an 80% display says the motor is calculated to be 8/10ths of the way to the thermal set point. 100% would be at the set point, 110% would be over the set point and in fault. But of course, the fault condition can be ignored (time programmable) ignored until the smoke get's out. ;-) I have the set points programmed quite conservatively.
I have had no troubles with my machine tool applications with either of these drive types.
Happy camper! I do use the gear box rather than the VFD for the big jumps.
01-04-2012, 10:54 AM #10
Just to clear up some misconceptions here:
1) the "Thermal" protection being mentioned above is related to OVERLOAD current, not normal operating temperature. That's what they mean by "I²t" thermal calculations. But unless the VFD is somehow told what the motor heating and cooling parameters are as speed is reduced, the VFD has no way of knowing ACTUAL motor temperature and the I²t protection is NOT intended to be what provides that protection. All they are saying there is that the motor overload protection is there, you don't need another external Over Load Relay (as used to be the case years ago before they started including that function). In the case of Siemens drives, they do have the option of inputting a SIEMENS motor catalog number and the drive will know the heating / cooling information of THAT SPECIFIC motor, so they offer an advantage there. Other high end drives will allow you to enter custom heating / cooling curve data, but it's only valuable IF you already have that data from the motor mfr, and 90% of the motors out there will not have that data available.
2) The speed reduction capability has everything to do with the MOTOR design, not really the VFD. Any VFD is capable of telling a motor to run at 1Hz or 1/2Hz or 0.01Hz, but that does not mean the motor will do that without damage. So there are different DESIGNS of motors available, and those that are designed for operation on a VFD are called "inverter duty" (or something to that effect). They will provide you with what is called a "turn down ratio" that indicates the safest low speed it can operate on from a VFD. If your motor does not have a turn down ratio specified, I would never run it lower than a 4:1 ratio. So on a 60Hz motor that's 15Hz (and even that is a stretch depending on what the nature of the load is, see item 3). When you select an "inverter duty" motor, that's where you get the turn down ratio specified, i.e. 6:1, 10:1, 100:1, Baldor even offers one that is 1000:1. The highest ratios are always "blown" motors, meaning they have a separately powered cooling fan on them that runs the same amount of air across them regardless of the motor speed. That of course means a separate power source run out to the motor. In the 6:1 and maybe 10:1 ratios you can still find TENV (Totally Enclosed Non-Ventilated) motors that use only the mass of iron and motor shaft itself to dissipate motor heat without fans. But this is limited, hence the lesser turn down ratios.
3) The LOAD connected to the motor makes a difference too. In centrifugal loads, such as pumps and fans, the load on the motor decreases as the cube of the speed reduction. So if you have a centrifugal pump that needs 10HP at 60Hz, at 1/2 speed it will only be a 1.25HP load on that motor (.5 x .5 x .5). However, most centrifugal loads stop working at less than 50% speed anyway, so motor cooling is a moot point. However if you have a constant torque type of load, like a machine tool cutting head, you want full torque even at low speeds, so the motor load, even though reduced at the speed ratio, is still based on full current (current follows torque), so any reduced cooling capability rears it's ugly head in those applications.
4) When you reduce the speed of a motor with a VFD, the type of VFD makes a difference in the motor's PERFORMANCE, which can have an indirect effect on the motor heat. If you do not have at least a Sensorless Vector Drive (SVC), then running a motor below 1/2 speed runs an ever increasing margin of error in the torque and/or speed response to changes in load. A basic Scalar (V/Hz only) drive does not have the smarts to recalculate what it takes to keep the motor operating at the correct speed or torque when the load causes it to slow down (increased slip), so the motor begins to heat up more as the slip increases and more of the motor power is wasted. It's not a lot, but combined with the other issues can have a cumulative effect. An SVC drive will make the motor do exactly what it needs to do and no more. That's especially important when you get into running at low speeds.
5) The "Full Torque at Zero Speed" issue has to do with a motor / drive combination that would be used on something like a hoist where you want the motor to be capable of holding a load still when the mechanical brakes are released. That only comes with full Closed Loop Vector drives and blown motors, you should never attempt anything like that without that (although some hoist motors are designed to do this without being blown for short duty cycles).
01-04-2012, 02:21 PM #11
Thank you Jraef, I did not have time today to respond in like.
Altho all good drives have I2t protection routines of some sort, I do like the concept of guestimating the motor heat at lower than base speed speed based on current and actual speed.... it would not be so hard to come up with an equation that estimates this for a NOMINAL MOTOR DESIGN since cooling goes down by the 3rd power of speed and I2r loss remains constant..... in fact, one could go a tad further and ask for "no load base speed current" and use that for the magnetizing current in the equation for a particular motor and probably get closer too.... I like the question for weight so they get some concept of the motor thermal time constant.... nice ideas. some engineers were doing good to add into the basic I2t equation <base speed extra heating....
01-04-2012, 03:30 PM #12
Regarding your point 1)
Your understanding of the VFD protection is not how I read the description from the Siemens literature. the Descriptions says 'motor current" Not "motor overload current", and follows with summary of factors that are used to determine motor temperature. Yes, the drive is told what thermal factors are to be considered.
The "protection" is based on this calculated motor temperature, not on "overload current" Even if the feature is called I squared T. (Which IS the source of the heat)
If not entered, the drive defaults to some Siemens motor specification.
Legitimate protection in my view!
Certainly GIGO prevails.
Like any insurance, you need to read the fine print! ;-)