{"id":4224,"date":"2023-08-07T14:46:26","date_gmt":"2023-08-07T18:46:26","guid":{"rendered":"https:\/\/mrsnyder.com\/?p=4224"},"modified":"2023-09-25T15:00:20","modified_gmt":"2023-09-25T19:00:20","slug":"key-thoughts-on-converting-old-dc-drives-to-ac-drives","status":"publish","type":"post","link":"https:\/\/mrsnyder.com\/key-thoughts-on-converting-old-dc-drives-to-ac-drives\/","title":{"rendered":"Key Thoughts on Converting Old DC Drives to AC Drives"},"content":{"rendered":"
Base Speed and Horsepower<\/strong><\/p>\n You will sometimes hear it said that a DC motor HP is stronger than an AC motor. Strictly speaking, this is not true. Engineers need to be wary of the fact that DC motors often have base speeds lower than 1800 rpm, whereas AC motors are typically 1800. AC motors may also have base speeds lower than 1800; however, this is most common on older DC motors. Engineers sometimes make the mistake of only looking at the HP without consideration for base speed.<\/p>\n If we remember from school, Power = Torque X Speed. A good rule of thumb is that an 1800 rpm motor has approximately 3 ft-lbs. of torque per HP. However, DC motors, particularly on center winder systems (more on that in a minute), often had lower base speeds of 600 rpm or less. A 600 rpm motor will have 3 times as much torque at or below base speed as an 1800 rpm motor of the same horsepower. This is a very important consideration when running low line speeds or sizing a center winder.<\/p>\n Below base speed, motors operate in a constant torque speed range. Torque is constant as speed increases up to the base speed. Above base speed, motors operate in a constant horsepower range where torque decreases as speed increases.<\/p>\n On a center winder, rpms decrease as diameter increases (for a constant line speed). Torque increases, however, since torque = tension X radius.\u00a0 Therefore, low base speed motors offer an advantage in this environment. Using a lower base speed motor, you can often use a variable speed drive that is 1\/2 or 1\/3 the size of what might be required using a standard motor. It should be noted, however, lower base speed motors are much bigger and more expensive than standard base speed motors. The savings typically come in the cost of the variable speed drive used. It\u2019s important to keep that in mind as well.<\/p>\n Practical Center Wind Example AC Motor 1800 RPM Base Speed Motor Sizing<\/strong><\/p>\n DC Motor 600 RPM Base Speed Motor Sizing<\/strong><\/p>\n Two conclusions:<\/strong><\/p>\n Note For Non-Center Wind Machines<\/strong><\/p>\n The example provided was for a center wind application where torque requirement decreases with increasing speeds. However, the concept holds true for machines where multiple, different products might run. On such a machine it would be possible to run higher tensions at lower speeds and lighter tensions at faster speeds, if desired.<\/p>\n Another Important Point for Consideration:<\/strong><\/p>\n One final note in terms of DC vs AC control is the fact that DC drives were inherently good torque regulators. They were good torque regulators whether they had an encoder or not.\u00a0 AC drives are often not nearly as accurate in torque control if they don\u2019t have an encoder. Don\u2019t assume that your open loop DC drive operating in torque limit can be replaced by an open loop AC drive with no encoder. This will often cause a problem. At a minimum, verify with the manufacturer the capabilities of the drive doing the control and make sure you have a good handle on the degree of torque precision required for your application.<\/p>\n Contact us at M.R. Snyder Company to learn more about our drive systems<\/a>. We can help you upgrade your current infrastructure or integrate an all new system.<\/p>\n<\/div>
\n<\/strong>(Ignoring friction considerations and safety factor for simplicity)<\/em><\/p>\n\n
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\nA 3 HP motor only has 9 ft-lbs, so we would go to the next standard size of 5 HP.\u00a0<\/em><\/strong><\/li>\n<\/ul>\n\n
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