Trying to interpret the speed control features of inverter drives for induction motor. One of the specs is "speed control range" and is given in a ratio format (1:1500). If I plan to use the motor in the 900 to 1800RPM range, how the "speed control range" spec applies?

 

What this means is if the base speed of your motor is 1800 RPM the drive will allow you to do a ratio of 1:1500 of that or down to somewhere between 1 and 2 RPM. Usually motors also have ratio as well so on the motor you may see it is 1:1000 or 1:2000... so effectively you could go down to 1 to 3 rpm with either of those ratios.

long story short take the base speed divide by that ratio and bam that is the SLOWEST you can run and maintain control and NOT overheat the motor with a full load.

 

By motor "base" speed do you mean the speed at which the motor is rated to run if connected directly to the stated supply (typ. 1800RPM for 3ph-60Hz)?

Taking aside the motor ratios for a moment, the calculation you did based on my stated problem (1800RPM/1500) yields a result of 1.2RPM. Does this result indicates the resolution (minimum adjustment step) to which I can expect to dial the speed of the motor?

I just reviewed the specs of some motors I have been considering but did not found any spec in ratio format associated with speed. Any more info you could provide? Assuming I find them, do I use the smaller ratio of the two (driver/motor)?

Thanks for taking the time to share your knowledge. Look forward to your response.

 

The speed range indicates min possible driver/motor speed but is related to driver's parameters mostly. Also min speed must be kept with nominal motor moment as well. But min speed for your application maybe recommended 2...3 times higher than min value as "rule of thumb". Cheaper V/F frequency inverters have significantly lower speed range - app.1:10...15.

 

<i>By motor "base" speed do you mean the speed at which the motor is rated to run if connected directly to the stated supply (typ. 1800RPM for 3ph-60Hz)? </i>

That is correct. you reach base speed at nameplate voltage and HZ such as 460VAC and 60Hz... nameplate RPM is considered base speed.

<i>Taking aside the motor ratios for a moment, the calculation you did based on my stated problem (1800RPM/1500) yields a result of 1.2RPM. Does this result indicates the resolution (minimum adjustment step) to which I can expect to dial the speed of the motor?</i>

NO, what this means is you CAN NOT run the motor below 1.2 RPM. however, in my opinion when you have a ratio that large i.e. 2000:1 if you can run at that low of an RPM (in closed-loop vector mode for the VFD) you can pretty much run full torque at zero speed.. i.e. at a standstill. This ratio has nothing to do with resolution of speed adjustments. It is simply letting you know how slow you can run and still pull full torque.

YOU CAN vary the speed of the motor from 1.2 RPM (or zero RPM in my opinion) to 1800 RPM (or base speed or even more) to the resolution and accuracy of the speed setpoint... i.e. your potentiometer and ananlog input resolution... or maybe you set over a comms bus etc. so 16 or 32 bit resolution etc.

<i>I just reviewed the specs of some motors I have been considering but did not found any spec in ratio format associated with speed. Any more info you could provide? Assuming I find them, do I use the smaller ratio of the two (driver/motor)?</i>

ONLY use a smaller ratio if you don't want to run slow...that will save $ if that is ok. just make sure the minimum RPM based off the ratio is happy with what you want to run for a speed range. If you buy a 1800 RPM 20:1 motor and you run it at 10 RPM you'll burn the motor up...however if the slowest you will run is 100 RPM a 20:1 works great and saves $ on initial purchase.

If you want to run slow consider the highest ratio that fits that slow speed. Also consider a drive that runs sensor-less vector mode or even a closed-loop vector mode if you want to run less than 5-10 RPM... OR if you want better speed control and higher RPM's. all of these suggestions add $ to the system...but may be necessary depending on what you are doing.

<i>Thanks for taking the time to share your knowledge. Look forward to your response.</i>

Sorry for the slow response... been out for a few days.