motor

sujinvipin

Joined Aug 18, 2011
28
One question , i know this is a little childish question, but want to know what if a voltage of 33V is applied on a 24V ac motor? please don't underestimate this question....

SgtWookie

Joined Jul 17, 2007
22,230
If the motor is specifically designed for AC, you will overheat it; if there is no overtemp protection, you will burn it up quickly.

If the motor is a universal type, you'll probably still burn it up; it will take longer though.

tommydyhr

Joined Feb 3, 2009
39
As Wookie said, you will have to somehow limit the voltage (a basic step-down transformer comes to mind).

GetDeviceInfo

Joined Jun 7, 2009
2,198
assuming your 33 volts is AC, your into about 30% overvoltage. Your motor will come up to speed marginally faster, have slightly better speed regulation, and higher power output. Due to temperature rise however, you'll have to derate the rated power output. Much will depend on the magnetics of the motor as to how much derating is appropriate. It may render the motor unusable for it's intended application, or may even, as prior postings suggest, cascade into heat failure.

Bernard

Joined Aug 7, 2008
5,784
I believe it depends on how heavly the motor is loaded. Might monitor temp @ 33V , unloaded, then decide on how much load it can take. A series resistor could be used on a small motor, say 1A draw using 9Ω @ 10W.

sujinvipin

Joined Aug 18, 2011
28
I can know there may be overheating, but i have a doubt in that.

Suppose the motors FLA is 10A for 24Volts ac , now if we apply 33volts ac doesnt it consume the current less compared to 24V, for a particular motor power?So in that case how does overheating take place?

tommydyhr

Joined Feb 3, 2009
39
The higher the voltage, the more likely the core is to become magnetized above what it was originally designed for. Eventually the core may become saturated, which will lead to high energy loss because of the increased eddy currents.

These currents will manifest themselves as an energy loss in the form of heat, which may overheat the motor, or at the very least, decrease it's lifetime substantially.

Since speed is mainly a function of the frequency and the number of poles, you will most likely not experience less current draw (It may even draw more current if it's controlled by a VFD).

sujinvipin

Joined Aug 18, 2011
28
Ok so you are saying that the core saturates earlier than designed state.
Can you tell me the relation of it?

tommydyhr

Joined Feb 3, 2009
39
In order to understand how this saturation occurs, and it's detrimental effects, you first have to think about how an inductor works.

A voltage applied to an air-core inductor in and on itself always acts to oppose the change in current through the inductor. With AC, we have a changing current through this coil, which will develop a changing magnetic field. This magnetic field will induce a voltage in the coil which will change in magnitude at just the right rate to induce a voltage which will balance out the applied voltage.

When you add an iron core to the inductor, you change this relationship between the current through the coil and the developed magnetic flux, but you don't change the fixed relationship between the magnetic flux and the induced voltage. This incidentally means that you will need less current through the inductor in order to develop the same magnetic flux, and you will need less change in current to develop the same voltage. What this means is that the addition of the core actually increases the total inductance of the coil.

When you reach a certain point of magnetic flux, however, the iron core will become saturated. When this happens, the inductor will start to act more like an air core inductor, which in turn means that with the same applied voltage, the current through the coil will increase significantly (due to the reduced inductance).

sujinvipin

Joined Aug 18, 2011
28
I have simple doubt , if we use a VFD of a particular company, and if we connect it to the motor output and supply input, other than giving our necessary settings, does it automatically works at consatnt V/f ratio and thereby limiting the starting current to safe value?

tommydyhr

Joined Feb 3, 2009
39
Most (if not all) VFDs indeed has some kind of soft-start capability. If not built-in, then at least as an option.

In most cases this works by, given some ramp-up time, slowly increasing both the voltage and the frequency to the motor, thus avoiding substantial inrush-current. As you mentioned yourself, the V/f-ratio remains fixed.

strantor

Joined Oct 3, 2010
6,829
I have simple doubt , if we use a VFD of a particular company, and if we connect it to the motor output and supply input, other than giving our necessary settings, does it automatically works at consatnt V/f ratio and thereby limiting the starting current to safe value?
that depends what you define as "necessary settings". If you include entering motor nameplate data and performing an autotune as "necessary settings" then the answer is yes.

If you don't enter the nameplate data and perform an autotune, then the VFD doesn't know what motor is connected to it, and might burn it up.

GetDeviceInfo

Joined Jun 7, 2009
2,198
I have simple doubt , if we use a VFD of a particular company, and if we connect it to the motor output and supply input, other than giving our necessary settings, does it automatically works at consatnt V/f ratio and thereby limiting the starting current to safe value?
VFD's outputs are typically fully scalable, so as mentioned, your motor voltage is a parameter that modifies the output, meaning yes, your VFD will control the output for your motor ratings.

Autotuning is not a standard offering across manufacturers. In ABB drives, the autotune function walks the motor through varying magnetic phases and stores information so that it can maximize efficiencies over the opperating range. It will have no problem running the drive without an autotune.