maximum frequency on a 50Hz rated motor

[TF and the wire]

I have an application where I want to fit an invertor to a 3 phase 50Hz squirrel cage motor in order to increase the speed.

Is there a rule of thumb that determines the maximum frequency before problems occur with the motor-overheating etc.

 

[Darkwood]

Is the motor old or new and what frequency have you in mind?

Try this link

 

[odinseymour]

I have an application where I want to fit an invertor to a 3 phase 50Hz squirrel cage motor in order to increase the speed.

Is there a rule of thumb that determines the maximum frequency before problems occur with the motor-overheating etc.


I assume you are wanting to control the speed or torque to match the process requirement, or save energy, VFD applications are ideal for this, however it is an extremely complex subject. It would depend on what process you are wanting to control?
I am no expert on the matter, but there are numerous factors that will affect the maximum frequency you can apply before problems start to occur. One will be the motor insulation class, the cooling method used in conjunction with the motor, the type of VFD you are to use, whether it be PWM type etc, the ambient temperature of the surrounding environment, altitude. Have you more detailed information on what process you are controlling?

 

[TF and the wire]

The 1.5kW motor drives a set of roller conveyors on the outfeed from a palletiser. At present it is fed via a contactor at 50Hz factory supply.

I need to increase the cycle time of the palletiser and evacuate the stack faster than at present.

I am using an Allen-Bradley Powerflex 40 invertor. It will be at constant frequency, with just a two wire control, ie a set of run relay contacts.

In answer to your questions

• I don't know off hand the motor insulation class. I can find out on Monday.
• Cooling is by an impellor on the end of the motor shaft, non-drive end. This will rotate at motor speed.
• Not sure about PWM, maybe someone in the know with AB gear can tell us.
• Ambient temperature is what you would expect in a cardboard plant in northern England, 8 degrees C in winter to 30 degrees C in summer.
• Altitude, about 30m above sea level.
• Its probably about 5 years old.

I'll be fitting it this weekend and have it set at 50Hz for the start up on Monday. During production, I'll increase the frequency gradually to see if it serve its purpose.

The stack is evacuated around every 10 minutes and only runs for about 10 seconds so I'll have plenty of time to change parameter A069 for the frequency.

I have used this type of invertor many times before with no problems, usuallynot much either side of its rated frequency. But I was wondering if there is a rule of thumb max and min limit.

 

[Darkwood]

A general rule of thumb is you can increase 30% to 50% above base frequency depending on the motor and job, because what you are doing is increasing the voltage to the windings proportional to the frequency but the consequence of going too high above base frequency is the torque will reduce now if your running a drum etc with high inertia then you can go quite high but a motor under load at high speed may stall, if you require to exceed 30% then refer to manufacturer of motor for advice to see if the winding insulation can withstand the increased voltage.

Due to the short run period i think you can be cheeky and get away with a little higher but this depends on how often the motor is run in a set period of time.

 

[silva.foxx]

From a well respected member (a drives guru) of a US plc/drive site I use:

"Since the motor field rotation speed is determined by frequency only, running a motor above its base speed will cause the rotor to try to spin up to the higher speed. Clearly, it will need to have adequate torque to get to the higher speed.

That indicates that we need to focus on the motor torque curve over the motor base speed and compare it to the load torque curve at these same speeds. If, at any point, the motor matches or falls below load speed, further acceleration is impossible and the speed will not increase beyond that point.

It is true that virtually all motors operate with a constant torque characteristic from zero to nameplate base speed but, above nameplate speed, the torque will be reduced by the inverse of the overspeed ratio. So, for example, if you are up to 70 Hz on a 60 Hz motor, the speed will be 70/60 times nameplate but the torque will be 60/70 times normal rated torque. This behavior continues up to a point where the motor internals no longer can deal with the higher frequencies. At that point, the torque starts to fall off even faster and stalling follows soon after. Each motor design is different in this respect--some inverter duty four pole motors are guaranteed to keep torque in this proportion (constant hp) up to double base speed while many others, especially slow speed motors and early designs, will not hold this proportional relationship up to even 80hz on a 60hz motor.

A further complication is that, while continuous rated motor torque follows the above inverse ratio rule, the short term overload torque available from the motor comes down as the SQUARE of the overspeed ratio. So, at double base speed on most motors, there is no overload capacity left at all.

As to mechanical issues, any NEMA or IEC motor with a cast aluminum rotor excluding two pole designs can be taken to at least 90 Hz without any real balance or bearing concerns. On two pole motors, I quit at 4500rpm mainly for cooling fan concerns.

The bottom line is that using the motor overspeed capabilities is good design practice as long as the limitations are taken into account. For myself, under 100hp and excluding two pole motors and increasing torque loads, I try to design the power train so full speed on the load is 90Hz on the motor. Lots of good things happen when you follow that rule."

- Up to 80Hz on 50Hz motor

- Up to 3750rpm on 2 pole motor

Or upgrade gearbox / drive sprocket to enable motor to be driven within the 50Hz base speed.

.

 

[TF and the wire]

Excellent response Silva Foxx. As I said, I'll take it up gradually and see what happens. I was hoping for 100Hz but that might be a little too high. Perhaps a combination of gearbox and VFD might be needed.

 

[silva.foxx]

This has gotten me more interested than desired!

Some further reading got me to this:

"There are at least three reasons why it is desireable to run a motor over its base speed. First, by increasing the reduction ratio in the power train to the load thereby shifting the motor speed upward, you improve starting torque and running torque at all speeds below motor base speed. From base speed up to maximum speed, the increased mechanical reduction exactly offsets the reduction in motor torque so the load has the same torque available as before. (This is only logical since the motor is capable of constant hp above its base speed up to a point.)

Second, at all operating points, the motor is turning faster than normal for the same load speed due to the increased mechanical reduction so the motor fan cooling is better. This is especially handy if you have a TEFC or ODP motor and need to operate at the low speed end of the motor's thermal capacity.

Third, speed regulation is somewhat better because the motor sees less torque for all load torques. This results is proportionally less motor slip and therefore better speed regulation. (I am assuming an open-loop V/Hz drive here).

In applications requiring snubber or regen braking, there is a fourth benefit of improved braking efficiency due to motor braking torque being multiplied by the higher mechanical ratio. This is offset a bit by the motor mass decelerating from a higher speed.

I can think of two possible disadvantages. First, if the higher mechanical reduction pushes you into a new gearbox or other expensive power train components, it might not be worth it. Second, if fan noise is a problem, higher speeds will naturally make it worse.

There is a general perception that taking a motor over its base speed is not good for the motor. First, bearings are not an issue as long as you are dealing with motors under 200hp and speeds under 4500rpm. The same bearings that are used in 6 pole and 4 pole motors are used in 2 pole motors of different hp ratings. Second, motor balance can be an issue especially for larger sizes. As a rule, you can take motors in 360 frame and smaller with cast rotors up to 90 Hz (again not to exceed 4500rpm) and larger NEMA frames thru 449 to 75-80 Hz. Wound or built up rotors are not included and must be evaluated on an individual basis. Third, some motors are designed to develop constant hp far into the overspeed range (120Hz is not uncommon) but many others start to fall off rather quickly. You need to know what the motor curve looks like before going overspeed very far. Some motors have a maximum safe speed listed on their nameplate. Do not be tempted to violate that limit.

Note that the discussion above is about taking the motor overspeed, not the load. Any attempt to take a load over its design speed is foolhardy and to be avoided unless you are personally in possession of WRITTEN assurances by the manufacturer that it is safe. Seeing a stamping press flywheel explode from overspeed just once is more than any professional career needs!! "

...again this is from where the first load of my info came from!

From what I've read; obtain details of your motor and gearbox, etc and contact manufacturer for confirmation that both can safely be oversped to your desired value for the calculated time period. Good luck. I'm sure what you have in mind will be fine. Make sure your motor is "inverter rated".

Regards
s.f

 

[thepurple1]

If your looking to get more duty out of the motor because someone has done their calculations wrong and undersized the motor the best bet is to fit a bigger one, it sounds like the motor is way of it curve if you need it increase it to 100Hz. If you are increasing the frequency then I would advise you to get written confirmation from the motor manufacturer to guarantee the motor. I have seen motors burn out at 60 odd Hz when consultants have messed up. Also by increasing the frequency does generate a lot more electrical noise and cause interferance with computors and other sensitive equipment. My advice fit a bigger motor and slow it down

 

[Darkwood]

Excellent revision work by silva foxx 9/10 for focus on subject