Application assistance: stall torque?

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Application assistance: stall torque?

Postby om_tech_support_JT » Wed Aug 14, 2013 5:22 pm

Q: I'm working on an application where the motor will need to open and close a door/gate vertically. It will be inside a bread baking machine meant to be placed in supermarkets. The door only weighs 1 lb, but when the door closes, there's a rubber seal that the door needs to compress to seal the door. The door won't take much torque at all. When the motor applys enough pressure on the rubber seal, the door will be clamped/locked down. My requirements are 3.5 lb-in from the gearhead shaft at 500~600 RPM. The motor will be mounted on a rack and pinion which pushes the door closed. I only need about 10 oz-in of torque to compress/seal the door properly. I'm looking at your World K series 115 VAC induction motors and gearheads as a possible solution. While the high torque V-series is attractive, I prefer to use a 3/8" diameter shaft this time; not metric.

A: Stall torque is not included in our World K series AC motor specifications. Our literature shows starting torque and rated torque. When you first supply power to the motor, the motor will generate starting torque, and when the speed is accelerated up to its rated speed, the motor will produce rated torque. When a motor stalls, it will draw more current (3~10x; depending on load conditions) thus increasing its torque. This would be the stall torque you're looking for. It is difficult to determine how much stall torque you'll have exactly. Testing is definitely the best way to find out. The safe value to use would be starting torque to calculate your stall torque. We offer gear efficiency in our literature so we can use that along with the gear ratio to calculate the stall torque.

A possible concern is when you run a motor into a hard stop, the impact force introduces excessive fatigue and stress on the gear teeth or shaft. It is definitely not recommended, but if you absolutely have to, try to minimize the impact to maintain a longer life. Another possible concern is the additional heat generated from the motor drawing more than rated current during a stall. Our AC motors with a frame size of 70 mm (2.76") or bigger, or 15W or higher output power, contain a built-in automatic return type thermal protector which will shut off the motor automatically if the operating temperature exceeds a certain level. Once the motor cools down to an acceptable level, the motor will turn on again.

Let's first take a look at a 25W reversible gearmotor with a 25:1 gear ratio.

LINK: ... u-4gn3-6sa

This motor/gearhead combination will output a rated torque of 4.4 lb-in at a synchronous speed of 500 RPM at 60 Hz. It's a reversible AC motor with higher starting torque than the inducton motors you're interested in, and it has a built-in friction brake for instantaneous stops and reverse of direction. The friction brake will also provide some holding force that you may need. As long as your duty cycle is not too high, this type of motor will work.

starting torque is 19.8 oz-in
gear ratio is 3.6:1
gear efficiency is 81%

19.8 oz-in x 3.6 x 0.81 = 57.7368 oz-in = 3.6 lb-in

Since we need 3.5 lb-in and we have a safe estimated stall torque of 3.6 lb-in, I think we're OK. The motor should draw 3~10x more current, and that should increase the torque more.

If you would like more safety factor for torque, I would recommend a bigger gearmotor.

LINK: ... u-5gn3-6sa

This gearmotor has a starting torque of 36 oz-in. 36 oz-in x 3.6 x 0.81 = 104.976 oz-in = 6.561 lb-in (estimation of stall torque).

We also have AC electromagnetic brake motors which could potentially eliminate the clamps you use to secure the door. The electromagnetic brakes on these motors would engage/lock when the power is removed (power-off activated). We also show the brake holding torque in our literature.

LINK: ... u-5gn3-6sa

You are welcome to discuss your application further with our technical support group @ 800-468-3982 or to see if another product could work. We also offer motor systems with torque-limiting functions which are designed to stall with a preset force.

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