Recommended starts per hour for motors

I looking at a data table from a motor manufacturer and it looks like 75hp motor should per started only 3 times per hour and have at least a run time of 3 minutes each time. What is your expereince in this area?

I have a system with a 160kW motor with the same restriction. It uses Star/delta starting.

The machine manufacturer helpfully provided a "Manual override" if we exceed this though!!!???

We start it off load but it still sends the ammeter skywards.I would use the manufacturers data as a guide though, and try not to exceed it!

On our larger motors (100hp and above), you get two freebies. After that, wait 1/2 hour. If it fails, wait 1 hour before trying again. If it fails, wait 2 hours. If it fails wait 4 hours... you see a pattern here.

I believe that the starts per hour spec is assuming direct contactor starting. Anything that reduces the inrush current as in wye-delta, part-winding, or electronic softstart would likely permit more starts in the same time period.

If you really want to take the motor to its safe limits, add thermostats in the stator windings and measure the temp rather than estimate it.

Actually, we had tried as an evaluation or case study as part of our cost reduction program, in our 7.5hp airconditioning compressor a 3 mins stop and 12 mins run or 4 times stop per hour hoping that we could reduce energy consumption on it at around 20%. We did it because the compressor was no longer controlled by thermostat because we can not meet the set temperature. A sort of undersized with respect to the room area. We got a significant reduction on energy cost, though actual temperature slightly rose up, but only to find out that we shorten the service life of the compressor. We replace the compressor after two months and our assumption is that it was caused by 4 times stop per hour. The compressor by the way is connected direct on line. Like DickDV said, it might have been different if it were started in reduced voltage or using vfd. But I just dont know if it is ok to use vfd in such kind of refrigerant compressor (copeland sealed type, 7.5hp)

Try a simple softstarter first, James. It's cheap and will give you some insight on why the first compressor failed

I 2nd the softstart. I use to work in a facility that had some rather larger pumps for a washer that during times may restart 15 times an hour. We never had an issue with them once the softstarts were put in.

Haven't tried using softstarter but maybe this time I would. Thanks a lot to all of you guys

I worked at a mine once where the operators would keep on resetting the overloads on bogged 400 HP crusher motors until the rotor bars melted

Soft starts or inverters will give you more starts per hour save energy & reduce wear.
Most modern inverters 5kw > will give good energy saving even at syncronous speed.
If you can reduce the speed by 10% it gives a considerable saving in energy.

Soft starters do not save energy. It takes the same energy to accelerate a load up to speed slowly as it does to do it quickly.

There may be some energy cost savings if you are on peak-demand metering but the kw-hrs will be the same.

Slugging in a refer compressor is liquid hitting the top of the piston if I recall the term correctly. The expansion valve is not set correctly and letting liquid thru to the compressor suction.

Crank case heaters are used to keep the oil warm and ease starting. I suppose in piston type compressors they would also turn any liquid freon in oil to gas (caused by leakage by the rings). A definite need in cold climates.

In compressors - refer or air - they start real hard if the line is not bled off to eliminate high discharge pressure on starting. This is why you do not want to short cycle refer compressors. I would NEVER start and stop intentionally every 15 minute. I would bet he replaced his compressor because he burned out the motor due to short cycling. IF mine I would have a 30 minute restart delay to prevent short cycling.

His problem is he does not have a large enough refer plant. He can do whatever he wants with the motor but if the compressor is not large enough it is futile. If he wanted to save energy that bad he should have only restarted the compressor after a one hour delay BUT that still decreases the amount of heat pumped ie air conditioning or refrigeration efficacy.

VFDs will not save energy just by slowing the motor. By slowing the load yes - especially so in the case of centrifugal pumps or blowers.
EXCEPTION
1 supposedly VFD will have unity power factor so there will be some savings there especially so if you pay power factor charge.
2 They may lower the demand charge also by reducing the high starting current seen with across line starting.

Dan Bentler

VFD's will save energy if slowing the motor, of course this will depend on the load & other factors, we have a number of 5-35 kw motors on one plant, the total energy saving is in the magnitude of 12%.
Another system where we have circulation pumps that we slow to 50% when not directly required & even on 4kw pumps the saving is 10% (note we have about 20 AB powermonitors on various areas of the plant & the logged data has shown a remarkable reduction in consumption.
This all started with ABB retrofitting their VSD's on a refridgeration plant (no not the compressors) these consisted of a 45kw fan & 4 35kw motors.
The energy saving was between 7 & 9%, however I made an even greater saving by simply automatically shutting the plant down when not required.